The First - ERIC · 2014. 3. 18. · Chapters included are: (1) "The Jovian System" (describing the history of astronomy); (2) "Pioneers to Jupiter" ... Eight technical and 12 non-technical

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Morrison, David; Samz, JaneVoyage to Jupiter.National Aeronautics and Space Administration,Washington, DC. Scientific and Technical InformationBranch.NASA-SP-43980

208p.; Colored photographs and drawings may notreproduce well.

Superintendent of Documents, U.S. Government PrintingOffice, Washington, DC 20402 ($9.00).Reports - Descriptive (141)

MF01/PC09 Plus Postage.Aerospace Technology; *Astronomy; Satellites(Aerospace); Science Materials; *Science Programs;*Scientific Research; Scientists; *Space Exploration;*Space Sciences*Jupiter; National Aeronautics and SpaceAdministration; *Voyager Mission

This publication illustrates the features of Jupiterand its family of satellites pictured by the Pioneer and the Voyagermissions. Chapters included are: (1) "The Jovian System" (describingthe history of astronomy); (2) "Pioneers to Jupiter" (outlining thePioneer Mission); (3) "The Voyager Mission"; (4) "Science andScientsts" (listing 11 science investigations and the scientists inthe Voyager Mission);.(5) "The Voyage to Jupiter--Cetting There"(describing the launch and encounter phase); (6) 'The FirstEncounter" (showing pictures of Io and Callisto); (7) "The SecondEncounter: More Surprises from the 'Land' of the Giant" (includingpictures of Ganymede and Europa); (8) "Jupiter--King of the Planets"(describing the weather, magnetosphere, and rings of Jupiter); (9)"Four New Worlds" (discussing the nature of the four satellites); and(10) "Return to Jupiter" (providing future plans for Jupiterexploration). Pictorial maps of the Galilean satellites, a list ofVoyager science teams, and a list of the Voyager management team areappended. Eight technical and 12 non-technical references areprovided as additional readings. (YP)

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For sale by the Superintendent of DocumentsU.S. Government Printing Office, Washington. D.0 20402Library of Congress Catalog Card Number 80-600126

6

FOREWORD

Few missions of planetary exploration have provided such rewards of insightand surprise as the Voyager flybys of Jupiter. Those who were fortunate enough tobe with the science teams during those weeks will long remember the experience; itwas like being in the crow's nest of a ship during landfall and passage through an ar-chipelago of strange islands. We had known that Jupiter would be remarkable, forman had been studying it for centuries, but we were far from prepared for the tor-rent of new information that the Voyagers poured back to Earth.

Some of the spirit of excitement and connection is captured in this volume. Itssenior author was a member of the Imaging Team. It is not common that a personcan both "do science" at the leading edge and also present so vivid an inside pictureof a remarkable moment in the history of space exploration.

April 30, 1980

ri

Thomas A. MutchAssociate AdministratorOffice of Space Science

INTRODUCTION

The two Voyager encounters with Jupiter were periods unparalleled in degreeand diversity of discovery. We had, of course, expected a number of discoveriesbecause we had never before been al e to study in detail the atmospheric motions ona planet that is a giant spinning sphere of hydrogen and helium, not had we everobserved planet-sized objects such as the Jovian satellites Ganymede and Callisto,which are half water-ice. We had never been so close to a Moon-sized satellite suchas lo, which was Isnown to be dispersing sodium throughout its Jovianneighborhood and was thought to be generating a one-million-ampere electrical cur-rent that in some way results in billions of watts of radio emission from Jupiter.

The closer Voyager came to Jupiter the more apparent it became that the scien-tific richness of the Jovian system was going to greatly exceed even our most op-timistic expectations. The growing realization among Voyager scientists of thewealth of discovery is apparent in their comments, discussions, and reports as re-counted by the authors in their descriptions of the two encounters.

Although many of the discoveries occurred in the few weeks around each en-counter, they were, of course, the result of more than those few weeks of effort. Infact, planning started a decade earlier, and the Voyager team of engineers and scien-tists had been designing, building, and planning for the encounters for seven years.The Pioneer spacecraft made the first reconnaissance of Jupiter in 1973-1974, pro-viding key scientific results on which Voyager could build, and discoveries from con-tinuing ground-based observations suggested specific Voyager studies. Voyager isitself just the second phase of exploration of the Jovian system. It will be followedby the Galileo program, which will directly probe Jupiter's atmosphere and providelong-term observations of the Jovian system from an orbiting spacecraft. In themeantime, the Voyager spacecraft will continue their journey to Saturn, andpossibly Uranus and Neptune, planets even more remote from Earth and aboutwhich we know even less than we knew of Jupiter before 1979.

As is clearly illustrated in this recounting of the voyage to Jupiter, scientificendeavors are human endeavors; just as Galileo could not have foreseen theadvancement in our knowledge initiated by his discoveries of the four Jovian moonsin 1610, neither can we fully comprehend the scientific heritage that our explorationof space is providing future generations.

April 1980 Eduard C. StoneVoyager Project Scientist

vii

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ACKNOWLEDGMENT

The authors are grateful to the many members of the Voyager Project teamwho not only made this historic mission of exploration possible but also took timefrom their busy schedules to offer us assistance, information, and encouragement inthe preparation of this book. Among many too numerous to name individually, weparticularly thank E. C. Stone, A. L. Lane, C. H. Stembridge, R. A. Mills,E. Mon-toya, M. A. Mitz, and B. A. Smith, L. Soderblom, and their colleagues on theVoyager Imaging Team. We are grateful to F. E. Bristow, D. L. Bane, L. J. Pieriand especially J. J. Van der Woude for their assistance in obtaining optimum ver-sions of the photographs printed here. C. B. Filcher and I. de Pater kindly madeavailable their groundbased pictures of the Jovian magnetosphere. Many colleagueshave read and provided helpful comments on parts of the manuscript, among themG. A. Briggs, S. A. Collins, S. Cruikshank, J. Doughty, A. L. Guin, A. L. Lane,R. A. Mills, E. Montoya, E. C. Stone, J. L. Ward, and especially C. R. Chapman.

ix

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CONTENTS

Foreword v

Introduction vii

Acknowledgment ix

Chapter 1. The Jovian System 1

Chapter 2. Pioneers to Jupiter 11

Chapter 3. The Voyager Mission 23

Chapter 4. Science and Scientists 33

Chapter 5. The Voyage to JupiterGetting There 47

Chapter 6. The First Encounter 63

Chapter 7. The Second Encounter: More SurprisesFrom the "Land" of the Giant 93

Chapter 8. JupiterKing of the Planets 117

Chapter 9. Four New Worlds 139

Chapter 10. Return to Jupiter 169

Appendix A. Pictorial Maps of the Galilean Satellites 177

Appendix B. Voyager Science Teams 195

Appendix C. Voyager Management Team 197

Additional Reading 199

xi

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Jupiter is the largest planet in the solar systema gaseous world as large as 1300 Earths, marked by alternating bands ofcolored clouds and a dazzling complexity of storm systems. The Voyager mission gave us our first close look at this spec-tacular planet. 1P-210851

11

CHAPTER 1

THE JOVIAN SYSTEM

Introduction

In the Sun's necklace of planets, one gemfar outshines the rest: Jupiter. Larger than allthe other planets and satellites combined,Jupiter is a true giant. If intelligent beings existon planets circling nearby stars, it is probablethat Jupiter is the only member of our plane-tary system they can detect. They can see theSun wobble in its motion with a twelve-yearperiod as Jupiter circles it, pulling first oneway, then the other with the powerful tug of itsgravity. if astronomers on some distant worldsput telescopes in orbit above their atmospheres,they might even be able to detect the sunlightreflected from Jupiter. But all the otherplanetsincluding tiny inconspicuous Earthwould be hopelessly lost in the glare of our star,the Sun.

Jupiter is outstanding among planets notonly for its size, but also for its system of orbi:-ing bodies. With fifteen known satellites, andprobably several more too small to have beendetected, it forms a sort of miniature solarsystem. If we could understand how the Joviansystem formed and evolved, we could unlockvital clues to the beginning and ultimate fate ofthe entire solar system.

Ancient peoples all over the world recog-nized Jupiter as one of the brightest wanderinglights in their skies. Only Venus is brighter, butVenus, always a morning or evening star, neverrules over the dark midnight skies as Jupiteroften does. In Greek and Roman mythology th,planet was identified with the most powerful ofthe gods and lord of the heavensthe GreekZeus; the Roman Jupiter.

As befits the king of the heavens, theplanet Jupiter moves at a slow and stately pace.Twelve years are required for Jupiter to com-plete one orbit around the Sun. For about sixmonths of each year, Jupiter shines down on us

from the night sky, more brightly and steadilythan any star. During the late 1970s it was awinter object, but in 1980 it will dominate thespring skies, becoming a summer "star" about1982.

Early Discoveries

Even seen through a small telescope or pairof binoculars, Jupiter looks like a real world,displaying a faintly banded disk quite unlike thetiny, brilliant image of a star. It also reveals thebrightest members of its satellite family as star-like points spread out along a straight line ex-tended east-west through the planet. There arefour of these planet-sized moons; with theirorbits seen edge-on from Earth, they seem tomove constantly back and forth, changing theirconfiguration hourly.

In January 1610, in his first attempt toapply the newly invented telescope toastronomy, Galileo discovered the four largesatellites of Jupiter. He correctly interpretedtheir motion as being that of objects circlingJupiterestablishing the first clear proof ofcelestial motion around a ceiter other than tneEarth. The discovery of these satellites playedan important role in supporting the Copernicanrevolution that formed the basis for modernastronomy.

A few decades later the satellites of Jupiterwere used to make the first measurement of thespeed of light. Observers following theirmotions had learned that the satellite clockseemed to run slow when Jupiter was far fromEarth and to speed up when the two planetswere closer together. In 1675 the Danishastronomer Ole Roemer explained that thischange was due to the finite velocity of light:The satellites only seemed to run slow at largedistances because the light coming from themtook longer to reach Earth. Knowing the

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Galileo's notes summarizing his first observations of theJovian satellites 10, Europa, Ganymede, and Callisto inJanuary 1610 were made on a piece of scratch paper con-taining the draft of a letter presenting a telescope to theDoge in Venice. These observations were the result ofGalileo's first attempt to apply the telescope to astronomi-cal research.

dimensions of the orbits of Earth and Jupiterand the amount of the delay (about fifteenminutes), Roemer was able to calculate one ofthe most fundamental constants of the physicaluniversethe speed of light (about 300 000kilometers per second).

The four great moons of Jupiter are calledthe Galilean satellites after their discov-rer.Their individual nameslo, Europa, Ga.'y-mede, and Callistowere proposed by SimonMarius, a contemporary and rival of Galileo.(Marius claimed to have discovered thesatellites a few weeks before Galileo did, butmodern scholars tend to discredit his claim.) lo,Europa, Ganymede, and Callisto are names oflovers of the god Jupiter in Greco-Roman my-thology. Since Jupiter was not at all shy abouttaking lovers, there are enough such names forthe other eleven Jovian satellites, as well as forthose yet to be discovered.

In the century following Galileo's death,improvements in telescopes m ,de it possible tomeasure the size of Jupiter and to note that itbulged at the equato,. The equatorial diameter

2

is known today to be 142 800 kilometers, whilefrom pole to poi:. Jupiter measures only133 500 kilometers. For comparison, the diam-eter of the Earth is 12 900 kilometers, onlyabout one-tenth as great, and the flattening ofEarth is also much smaller (less than one per-cer,t). By measuring the orbits of the satellitesand applying the laws of planetat y motion dis-covered by Johannes Kepler and Isaac Newton,astronomers were also able to determine thetotal mass of Jupiterabout 2 x 1024 tons, or318 times th, mass of the Earth.

Once the size and mass are known, it is

possible to calculate another fundamentalproperty of a planetits density. The density,which is the mass divided by the volume, pro-vides important clues to the composition andinterior structure of a planetary body. The den-sity of Earth, a body composed primarily ofrocky and metallic materials, is 5.6 times thedensity of water. The mass of Jupiter is 318times that of Earth; its volume is 1317 timesthat of Earth. Thus Jupiter's density is substan-tially lower than Earth's, amounting to 1.34times the density of water. From this low den-sity, it was evident long ago that Jupiter wasnot just a big brother of Earth ani the otherrocky planets in the inner solar system. Rather,Jupiter is the prototype of the giant, gas-richplanets Jupiter, Saturn, Uranus, and Neptune.These giant planets must, from their low den-sity, have a composition fundamentally differ-ent from that of Mercury, Venus, Earth, Moon,and Mars.

Jupiter Through the Telescope

Jupiter is a beautiful sight seen with thenaked eye on a clear night, but only through atelescope does it begin to reveal its magnifi-cence. The most prominent features are alter-nating light and dark bands, running parallel tothe equator and subtly shaded in tones of blue,yellow, brown, and orange. However, thesebands are not the planet's only conspicuousmarkings. In 1664 the English astronomerRobert Hooke first reported seeing a large ovalspot on Jupiter, and additional spots werenoted as telescopes improved. As the planetrotates on its axis, such spots are carried acrossthe disk and can be used to measure Jupiter'sspeed of rotation. The giant planet spins so fastthat a Jovian day is less han half as long as aday on Earth, averaoin^ just under ten hours.

During the nineteenth century, observersusing increasingly sophisticated telescopes were

able to sec more complex detail in the bandstructure, with wisps, streaks, and festoons thatvaried in intensity and color from year to year.Furthermore, observations revealed the re-markable fact that not all parts of the planetrotate with the same period; near the equatorthe apparent length of a Jovian day is severalminutes shorter than the average day at higherlatitudes. It is thus apparent that Jupiter's sur-face is not solid, and astronomers came to real-ize that they were looking at a turbulent kalei-doscope of shifting clouds.

Although the face of Jupiter is alwayschanging, some spots and other cloud featuressurvive for years at a time, much longer than dothe largest storms on Earth. The record forlongevity goes to the Great Red Spot. Thisgigantic red oval, larger than the planet Earth,was first seen more than three centuries ago.From decade to decade it has changed in sizeand color, and for nearly fifty years in the lateeighteenth century no sightings were reported,but since about 1840 the Great Red Spot hasbeen the most prominent feature on the disk ofJupiter.

It was not until the twentieth century thatthe composition of the atmosphere of Jupitercould be measured. In 1905 spectra of theplanet revealed the presence of gases thatabsorb strongly at red and infrared wave-lengths; thirty years later these were identifiedas ammonia and methane. These two poisonousgases are the simplest chemical compounds ofhydrogen combined with nitrogen and carbon,respectively. In the atmosphere of Earth theyare not stable, because oxygen, which is highlyactive chemically, destroys them. The existenceof methane and ammonia on Jupiter demon-strated that free oxygen could not be presentand that the atmosphere was dominated byhydrogena reducing, rather than oxidizing,condition. Subsequently, hydrogen was iden-tified spectroscopically. Although much moreabundant than methane or ammonia, hydrogenis harder to detect.

In the 1940s and 1950s the German-American astronomer Rupert Wildt used all theavailable data to derive a picture of Jupiter thatis still generally accepted. He noted that boththe low total density and the observed presenceof hydrogen-rich compounds in the atmospherewere consistent with a bulk composition similarto that of the Sun and stars. This "cosmic con/position" is dominated by the two simplestelements, hydrogen and helium, which togethermake up nearly 99 percent of all the material in

This ground-based photograph of Jupiter showing theGreat Red Spot in the southern hemisphere was taken withthe Catalina Observatory's 61-inch telescope in December1966.

the universe. Wildt hypothesized that the giantplanets, because of their large size, had suc-ceeded in retaining this primordial composi-tion, whereas the hydrogen and helium hadescaped from the smaller inner planets. He alsoused his knowledge of the properties ofhydrogen and helium to calculate what the in-terior structure of Jupiter might be like, con-cluding that the planet is mostly liquid or gas.Wildt suggested that there probably was a coreof solid material deep in the interior, but thatmuch of Jupiter i3 fluidextremely viscous andcompressed deep below the visible atmosphere,but still not solid. The atmosphere seen fromabove is just the thin, topmost layer of an oceanof gases thousands of kilometers thick.

Recent Earth-Based Studiesof Jupiter

In the past, a great deal of planetaryresearch was basically descriptive, consisting ofvisual observations and photography. Begin-ning in the 1960s, a new generation of planetary

3

North north north N North polartemperate belt region

North northtemperate belt

North temperate North temperatebelt zone

North northtemperate zone

North equatorialbelt

N. component

S. component

North tropicalzone

Equatorial zoneequatorial band

South equatorialbelt

South temperate South tropical

zone

belt zone

temperate belt m411111111111WSouth south South temperate

South polar South southregion Great Red Spot S temperate zone

The major features of Jupiter are shown in schematic form. The planet is a banded disk of turbulent clouds; all its stripes areparallel to the bulging equator. Large dusky gray regions surround each pole. Darker gray or brown stripes called beltsintermingle with lighter, yellow-white stripes called zones. Many of the belts and zones are permanent features that have beennamed. One feature of particular note is the Great Red Spot, an enigmatic oval larger than the planet Earth, which was firstseen more than three centuries ago. During the years the spot has changed in size and color, and it escaped detection entirelyfor nearly fifty years in the I700s. However, since the mid-nineteenth century the Great Red Spot has been the most promi-nent feature on the face of Jupiter. 129351

scientists began to apply the techniques ofmodern astrophysics and geophysics to thestudy of the solar system. Inspired in part 5ythe developing space programs of the UnitedStates and the Soviet Union, scientists began toask more quantitative questions: What are thesurfaces and atmospheres made of? What arethe temperatures and wind speeds? Exactlywhat quantities of different elements andisotopes are present? And how can these newdata be used to infer the origin and evolution ofthe planets?

Wildt had already suggested the basic gasesin the atmosphere of Jupiter: prig arilyhydrogen and helium, with much smaller quan-tities of ammonia and methane. Undetected butpossibly also present were nitrogen, neon,argon, and water vapor. The abundance ofhelium was particularly a problem; although itwas presumably the second-ranking gas afterhydrogen, it has no spectral features in visiblelight and its presence remained only a

hypothesis, unconfirmed by observation.Although the presence of a gas can usually

be inferred from spectroscopy, solids or liquidscannot normally be detected in this way. Thus

4

the composition of Jupiter's clouds could notbe determined directly. However, the presenceof ammonia gas provided an important clue. Atthe temperatures expected in the upper atmos-phere of the planet, ammonia gas must freezeto form tiny crystals of ammonia ice, just aswater vapor in the Earth's atmosphere freezesto form cirrus clouds. Most investigatorsagreLl that the high clouds covering much ofJupiter must be ammonia cirrus. But ammoniacrystals are white, so the presence of thismaterial provides no explanation for the manycolors seen on Jupiter. Additional materialsmtut be presentperhaps colored organic com-pounds, produced in small amounts by the ac-tion of sunlight on the atmosphere.

Because Jupiter is five times farther fromthe Sun ihan is the Earth, a given area onJupiter ieceives only about four percent asmuch soar heating as does a comparable areaon Eartn. Thus Jupiter is colder than Earth;even though it may be warm deep below itsblanket of clouds, Jupiter presents a frigid face.

The development of a new science, in-frared astronomy, in the 1960s made it possibleto measure these low temperatures directly. In

These blue filter photographs of Jupiter were taken at Mauna Kea Observatory, Hawaii. They show changes on Jupiter'ssurface between 1973 and 1978. rhe dates of the obsellations are (top left) July 25, 1973; (top right) October 5, 1974;(middle left) October 2, 1975; (middle right) November 20, 1976; (bottom left) January 28, 1978; (bottom right) December19, 1978.

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the case of a cloudy planet like Jupiter, the in-frared emission evident at various wavelengthsoriginates at different depths in the atmos-phere. It is a general property of any mixed,convecting atmosphere that the temperaturevaries with depth; the rate of variation dependsonly on the composition of the atmosphere, thegravity of the planet, and the presence orabsence of condensible materials to formclouds. On Jupiter it is about 1.9° C warmerfor each kilometer of descent through theatmosphere. Thus, although the ammoniaclouds are very cold, a little above 173° C, ifone goes deep enough one can reach tempera-tures that are quite comfortable. With a varia-tion of 1.9° C per kilometer, terrestrial "roomtemperature" would be reached about 100

kilometers below the clouds.To measure the total energy radiated by a

planet, it is necessary to utilize infrared radia-tion at wavelengths more than one hundredtimes longer than the wavelengths of visiblelight. Even when detectors were developed thatcould measure such radiation, it was impossibleto observe celestial sources such as Jupiterbecause of the opacity of the terrestrial atmos-phere. Even a tiny amount of water vapor inour own atmosphere can block our view oflong-wave infrared. To make the requiredmeasurements, it is necessary to carry a

telescope to very high altitudes, above all but afraction of a percent of the offending watervapor.

In the late 1960s a Lear-Jet airplane wasequipped with a telescope and made availableby NASA to astronomers to carry out long-wave infrared observaticns from above 99 per-cent of the terrestrial water vapor. In 1969Frank Low of the University of Arizona and hiscolleagues used this system to make a remark-able discovery: Jupiter was radiating more heatthan it received from the Sun! Repeated obser-vations demonstrated that between two andthree times as much energy emanated from theplanet as was absorbed. Thus Jupiter must havean internal heat source; in effect, it shines by itsown power as well as by reflected sunlight.Theoretical studies suggest that the heat isprimordial, the remnant of an incandescentproto-Jupiter that formed four and one-halfbillion years ago.

At the same time that the internal heatsource on Jupiter was being revealed with long-wa ie airborne infrared telescopes, a newdiscovery was being made from short-wave in-frared observations. The clouds of Jupiter aretoo cold to emit any detectable thermal radia-tion at a wavelength of 5 micrometers (aboutten times the wavelength of green lig:It). Never-theless, images of Jupiter at 5 micrometersrevealed a few small spots where large amountsof thermal energy were being emitted. Thesources of the energy appeared to be holes orbreaks in the clouds, where it was possible tosee deeper into hotter regions. The discovery ofthese hot spots opened the possibility of prob-

Images of Jupiter in visible light (right) and five-micrometer infrared light (left) show the planet's characteristic belts andzones. The infrared image reveals areas that emit large amounts of therm' energy. The source of the energy is thought to bebreaks in the Jovian cloud cover, which allow investigators a glimpse of the deep regions of the atmosphere. One of themysteries of Jupiter concerns its heat balance: The planet appears to radiate more heat than it receives from the Sun.(P-20957]

ai

ing deep regions of the Jovian atmosphere thathad previously been beyond the reach of directinvestigation.

The Jovian Magnetosphere

The discovery of planetary magneto-spheres began in 1959 when the first U.S. Ex-plorer satellite detected the radiation beltsaround the Earth. Named for James Van Allenof the University of Iowa, whose geiger-counterinstrument aboard Explorer 1 first measuredthem, these belts are regions in which chargedatomic particlesprimarily electrons and pro-tonsare trapped by the magnetic field of theEarth. They are one manifestation of the ter-restrial magnetospherea large, dynamicregion around the Earth in which the magneticfield of our planet interacts with streams ofcharged particles emanating from the Sun.

At almost the same time that the terrestrialmagnetosphere was being discovered by arti-ficial satellites, astronomers were findingevidence to suggest similar phenomena aroundJupiter. Radio astronomy is a branch of sciencethat measures radiation from celestial bodies atradio frequencies, which correspond to wave-lengths much longer than those of visible or in-frared light. All planets emit weak thermalradio radiation, but in the late 1950s inves-tigators found that Jupiter was a mt.ch strongerlong-wave radio source than would be expectedfor a planet with its temperature. This radiationbore the signature of high n-znergy processes.Physicists had seen similar emissions producedin synchrotron electron accelerators, hugemachines in which electrons are whirled aroundat nearly the speed of light so that they can beused for experiments in nuclear physics. TheRussian theorist I. S. Shklovsky identified theJovian radio radiation as also resulting fromthe synchrotron process, due to spiraling elec-trons trapped in the planet's magnetic field.From the intensity and spectrum of the ob-served synchrotron radiation, it was clear thatboth the magnetic field of the planet and theenergy of charged particles in its Van Allenbelts were much greater than was the case forEarth.

Using radio telescopes of high sensitivity,astronomers determined the approximatestrength and orientation of the magnetic fieldof Jupiter. Although they were able to measuresynchrotron radiation only from the innermostparts of the Jovian magnetosphere, they couldinfer that the total volume occupied by themagnetosphere was enormous. if our eyes were

sensitive to magnetospheric emissions, Jupiterwould look more than twice the diameter of thefull moon in the sky.

All four Galilean satellites orbit within themagnetosphere of Jupiter; in contrast, ourMoon lies well outside the terrestrial magneto-sphere. Striking evidence of the interaction ofthe satellites and the magnetosphere was pro-vided when it was found that the innermostlarge satelliteloactually affects the burstsof radio static produced by Jupiter. Only whento is at certain places in its orbit are these strongbursts detected. Theorists suggested that elec-tric currents flowing between the satellite andthe planet might be responsible for this effect.

The Jovian Satellites

or nearly three centuries after their dis-covery in 1610, the only known moons ofJupiter were the four large Galilean satellites.In 1892 E. E. Barnard, an American astron-omer, found a much smaller fifth satellite orbit-ing very close to the planet, and between 1904and 1974 eight additional satellites were foundfar outside the orbits of the Galilean satellites.The outer satellites are quite faint andpresumably no more than a few tens of kilo-meters in diameter, and all have orbits that aremuch less regular than those of the five innersatellites. Four of them revolve in a retrogradedirection, opposite to that of the inner satellitesand Jupiter itself.

In 1975 the International AstronomicalUnion assumed the responsibility for assigningnames to the non-Galilean satellites of Jupiter.Following tradition, they named the inner satel-lite Amalthea for the she-goat that suckled theyoung god Jupiter. The outer eight were namedfor lovers of Jupiter: Leda, Himalia, Lysithea,Elara, Ananke, Carme, Pasiphae, and Sinope.For the non-Galilean satellites, the "e" endjngis reserved for satellites with retrograde orbits;those with normal orbits have names that end in44a.,,

Because they are so large, the Galileansatellites have attracted the most attention fromastronomers. More than fifty years ago largetelescopes were used to estimate their sizes, anda careful series of measurements of their lightvariation showed that all four always keep thesame face pointed toward Jupiter, just as ourMoon always turns the same face toward Earth.Also, the subtle gravitational perturbationsthey exert on each other were used to determinethe approximate mass of each.

7

The pattern of the Galilean satellites changes from hour tohour, as seen from Earth. Viewed edge-on, the nearly cir-cular orbits produce an apparent back and forth motionwith respect to Jupiter. These images recreate the kinds ofobservations first made by Galileo in 1610.

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Callisto, the outermost Galilean satellite, islarger than the planet Mercury. It also has thelowest reflectivity, or albedo, of the four, sug-gesting that its surface may be composed ofsome rather dark, colorless rock. Callisto takesjust over two weeks to orbit once aroundJupiter.

Ganymede, which requires only seven daysfor one orbit, is the largest satellite in theJovian system, being only slightly smaller thanthe planet Mars. Its albedo is much higher thanthat of Callisto, or of the rocky planets such asMercury, Mars, or the Moon. In 1971 astron-omers first measured the infrared spectrum ofreflected sunlight from Ganymede and foundthe characteristic absorptions of water ice, in-dicating that this satellite is partially coveredwith highly reflective snow or ice.

Europa, which is slightly smaller than theMoon, circles Jupiter in half the time requiredby Ganymede. Its surface reflects about sixtypercent of the incident sunlight, and the in-frared spectrum shows prominent absorptionsdue to water ice; Europa appears to be almostentirely covered with ice. However, its color inthe visible and ultraviolet part of the spectrumis not that of ice, so some other material mustalso be present.

lo, innermost of the Galilean satellites, isthe same size as our Moon. It orbits the planetin 42 hours, half the period of Europa. LikeEuropa, it has a very high reflectivity, but,unlike Europa, it has no spectral absorptionsindicative of water ice. Before Voyager, identi-fication of the surface material on lo presenteda major problem to planetary astronomers.

When the sizes and masses of thesesatellites were measured, astronomers couldcalculate their densities. The inner two, lo andEuropa, both have densities about three timesthat of waternearly the same as the density ofthe Moon, or of rocks in the crust of the Earth.Callisto and Ganymede have densities only halfas large, far too low to be consistent with arocky composition. The most plausible alterna-tive to rock is a composition that includes ice asa major component. Calculations showed thatif these satellites were composed of rock andice, approximately equal quantities of eachwere required to account for the measured den-sity. Thus the two outer Galilean satellites werethought to represent a new kind of solar systemobject, as large as one of the terrestrial planets,but composed in large part of ice.

In 1973 the attention of astronomers wasdramatically drawn to lo when Robert Brown

1

The Galilean satellites in orbit around Jupiter, along with the outer satellites, constitute a miniature solar system. Here theyare shown relative to the size of Mercury and that of the Moon. The portrayal of their internal and external composition isbased on theoretical models that preceded the Voyager flybys. [PC-17054ACI

This image of lo's extended sodium cloud was taken February 19, 1977, at the Jet Propulsion Laboratory's Table MountainObservatory. A picture of Jupiter, drawings of the orbital geometry, and to's disk (the small circle on the left) are includedfor perspective. The sodium cloud image has been processed for removal of sky background, instrumental effects, and thelike. This photograph demonstrates that the cloud is highly elongated and that more sodium precedes lo in its orbit thantrails it. (P-20047)

of Harvard University detected the faint yellowglow of sodium from the region of space sur-rounding it. It seemed that this satellite had anatmosphere, composed of the metal sodium!Continued observations showed, however, thatthis was not an atmosphere in the usual sense ofthe word. The gas atoms were not bound gravi-tationally to lo, but continuously escaped fromit to form a gigantic cloud enveloping the orbitof the satellite. Fraser Fanale and Dennis Mat-son of the Caltech Jet Propulsion Laboratorysuggested that bombardment of to by high-energy particles from the Jovian Van Allenbelts was knocking off atoms of sodium by a

process called sputtering, releasing these atomsand allowing them to expand outward to formthe observed sodium cloud. No one anticipatedthen that powerful volcanic eruptions on 10might also be contributing to this remarkablegas cloud.

This picture of the satellites was developedjust as the first space probe reached the Joviansystem. In the next chapter we describe thePioneer program by which scientists reachedout across nearly a million kilometers of spaceto explore Jupiter, its magnetosphere, and itssystem of satellites.

9

r

CHAPTER 2

PIONEERS TO JUPITER

Reaching for the Outer Planets

Since the beginning of the Space Age, scien-tists had dreamed of sending probes to Jupiterand its family of satellites. Initially, robot space-craft were limited to studying the Earth and itsMoon. In 1962, however, the first true interplan-etary explorer, Mariner 2, succeeded in escapingthe Earth-Moon system and crossing 100 millionkilometers of space to encounter Venus, study-ing Earth's sister planet at close range using halfa dozen scientific instruments. By the mid 1960sa U.S. planetary spacecraft had also flown toMars, there had been a second flyby of Venus,and an ambitious program was under way fortwo more flybys of Mars in 1969, followed by aMars orbiter in 1971. Based on this success withthe inner planets, NASA scientists and engineersbegan to plan seriously to meet the challenge ofthe outer solar system.

Not only Jupiter, but Saturn and evenUranus and Neptune, were considered as pos-sible targets. However, the distances betweenthe outer planets are so vast that many years offlight would be required for a spacecraft to reachthem, even using the most powerful rocketboosters then corlempiatt t: If a cautious ex-ploration program were t °Bowed, investigatingone planet at a time before designing the nextmission, it would be well into the twenty-firstcentury before even a first reconnaissance of thesolar system could be achieved. A way to bridgethe space between planets in a more efficient,economical manner was needed.

In the late 1960s celestial mechaniciansscientists who study the motions of planets andspacecraftbegan to solve problems posed bythe immensity of the outer solar system. If aspacecraft is aimed to fly close to a planet in justthe right way, it can be accelerated by the gravityof the planet to higher speeds than could ever beobtained by direct launch from Earth. If a sec-ond, more distant planet is in the correct align-

ment, the gravity boost given by the first en-counter can speed the craft on to the second.Jupiter, with its huge size and strong gravita-tional pull, could be used as the fulcrum for aseries of missions to Saturn, Uranus, Neptune,and even distant Pluto. In addition, the early1980s would offer an exceptional opportunity,one repeated only about once every two cen-turie5. At that time, all four giant planets wouldbe in approximate alignment, so that gravity-assist maneuvers could be done sequentially. Asingle spacecraft, after being boosted fromJupiter to Saturn, could use the accelerationof Saturn to continue to Uranus, and in turncould be accelerated all the way out to Neptune.Such an ambitious, multiplanet mission wasnamed the Grand Tour.

The first essential step in the Grand Tourwas a flyby of Jupiter. However, this planet isten times farther away from Earth than Venus orMars. In addition, there were two potentiallylethal hazards that had not been faced before ininterplanetary flights: the asteroid belt and theJovian magnetosphere.

The first danger was presented by themany thousands of asteroids that occupy a beltbetween the orbits of Mars and Jupiter. Thelargest asteroid, Ceres, was discovered in 1801and was initially thought to be the "missingplanet" sometimes hypothesized as lying be-tween Jupiter and Mars. However, Ceres isonly 1000 kilometers in diameter, too small todeserve the title of planet. Hundreds more ofthese minor planets were discovered during thenineteenth century, and by the 1960s more than3000 had well- determined orbits. Most wereonly a few tens of kilometers in diameter, andastronomers estimated that 50 000 existedthat were 1 kilometer or more in diameter. Anyspacecraft to Jupiter would have to cross thiscongested region of space.

Even 50 000 minor bodies spread throughthe volume of space occupied by the asteroid

11Pioneer 10 was launched on March 2, 1972, at 8:49 p.m. from Cape Canaveral, Florida. A powerful Atlas-Centaur rocketserved as the launch vehicle, which propelled the space probe to its goal nearly a billion kilometers away. The beauty of thenight launch was enhanced by the rumbling thunder and flashing lightning of a nearby storm.

1)4.2

belt would present little direct danger, althougha chance collision with an uncatalogued objectwas always possible. Much more serious wasthe possibility that these larger objects wereaccompanied by large amounts of debris, fromthe size of boulders down to microscopic dust,that were undetectable from Earth. Collisionswith pebble-sized stones could easily destroy aspacecraft. The only way to evaluate thisdanger was to go there and find out how muchsmall debris was present.

A second danger was posed by Jupiteritself. In order to use the gravity boost ofJupiter to speed on to another planet, a space-craft would have to fly rather close to the giant.But this would mean passing right through theregions of energetic charged particles surround-ing the planet. Some estimates of the numberand energy of these particles indicated that thedelicate electronic brains of a spacecraft wouldbe damaged before it could penetrate thisregion. Again, only by going there could thedanger be evaluated properly.

The Pioneer Jupiter Mission

In 1969 the U.S. Congress approved thePioneer Jupiter Mission to provide a reconnais-sance of interplanetary space between Earthand Jupiter and a first close look at the giantplanet itself. The Project was assigned byNASA to the Ames Research Center in Moun-tain View, California. The primary objectiveswere defined by NASA:

Explore the interplanetary medium beyondthe orbit of Mars.

Investigate the nature of the asteroid belt,assessing possible hazards to missions tothe outer planets.

Explore the environment of Jupiter, includ-ing its inner magnetosphere.

The Pioneer spacecraft was designed foreconomy and reliability, based on previous ex-perience at Ames with Pioneers 6 through 9, allof which had proven themselves by years ofsuccessful measurement of the interplanetarymedium near the Earth. Unlike the Marinerclass of spacecraft being used to investigateVenus and Mars, the Pioneer craft rotated con-tinuously around an axis pointed toward theEarth. This spinning design was extremelystable, like the wheels of a fast-moving bicycle,and required less elaborate guidance than anonspinning craft. In addition, the spin pro-

12

vided an ideal base for measurements ofenergetic particles and magnetic fields in space,since the motion of the spacecraft itself sweptthe viewing direction around the sky and al-lowed data to be acquired rapidly from manydifferent directions. The only major disadvan-tage of a spinning spacecraft is that it does notallow a stabilized platform on which to mountcameras or other instruments that require exactpointing. Thus the spacecraft design was opti-mized for measurements of particles and fieldsin interplanetary space and in the Jovian mag-netosphere, but had limited capability forobservations of the planet and its satellites.

As finally assembled, the Pioneer Jupiterspacecraft had a mass of 258 kilograms. Onehundred forty watts of electrical power atJupiter were supplied by four radioisotope ther-moelectric generators (RTGs), which turnedheat from the radioactive decay of plutoniuminto electricity. The launch vehicle for the flightto Jupiter was an Atlas-Centaur rocket,equipped with an additional solid-propellantthird stage. This powerful rocket could accel-lerate the spacecraft to a speed of 51 500 kilo-meters per hour, sufficient to escape the Earthand make the billion-kilometer trip to Jupiter injust over two years. The specific scientific in-vestigations to be carried out on Pioneer wereselected competitively in 1969 from proposalssubmitted by scientists from U.S. universities,industry, and NASA laboratories, and alsofrom abroad. Eleven separate instrumentswould be flown, in addition to two experimentsthat would make use of the spacecraft itself.

Three complete Pioneer spacecraft, withtheir payloads of 25 kilograms of scientific in-struments, were built: one as a test vehicle andtwo for launch to Jupiter. One of thesethe testvehicleis now on display at the National Airand Space Museum in Washington. The firstopportunity to launchthe opening of the"launch window"was on February 27, 1972.However, it was not until shortly after dark onMarch 2 that all systems were ready, andPioneer 10 began its historic trip to Jupiter.

Pioneer 10 was the first human artifactlaunched with sufficient energy to escape thesolar system entirely. Fittingly, the craft carrieda message designed for any possible alien astro-nauts who might, in the distant future, find thederelict Pioneer in the vastness of interstellarspace. A small plaque fastened to the spacecrafttold the time and planet from which it had beenlaunched, and carried a symbolic greeting fromhumanity to the cosmos.

2

--...mosom

it

Two identical Pioneer spacecraft were designed and fabri-cated by TRW Systems Group at their Redondo Beach.California facility. Each weighed only about 260 kilograms,yet carried eleven highly sophisticated instruments capableof operating unattended for many years in space Datasystems on board controlled the instrumentation, recessedand processed commands, and transmitted informationacross the vast distance to Earth

The second Pioneer was to wait more thana year before launch. By following far behindPioneer 10, its trajectoryin particular howdeeply it penetrated the radiation belts duringJupiter flybycould be modified, dependingon the fate of the first spacecraft. At dusk onApril 5, 1973, Pioneer 11 blasted from thelaunch pad at Cape Canaveral and followed itspredecessor on the long, lonely journey into theouter solar system.

Flight to Jupiter

Within a few hours of launch, each Pioneerspacecraft shed the shroud that had protected itand unfurled booms supporting the scienceinstruments and the RTG power generators.After each craft had been carefully tracked andprecise orbits calculated, small onboard rocketswere commanded to fire to correct its trajectoryfor exactly the desired flyby at Jupiter. Pioneer10 was targeted to fly by the planet at a mini-

mum distance of 3 Jupiter radii (R j) from thecenter, or 2 R j (about 140 000 kilometers)above the clouds. This close passage, inside theorbit of lo, allowed the craft to pass behindboth to and Jupiter as seen from Earth, so thatits radio beam could probe both the planet andits innermost large satellite. Pioneci II was in-tended to fly even closer to Jupiter, but theexact targeting options were held open untilafter the Pioneer 10 encounter.

eli Pioneer 10, all instruments appeared tobe working well as the craft passed the orbit ofMars in June 1972, just 97 days after launch. Atthis point, as it headed into unexplored space, ittruly became a pioneer. In mid-July it began toenter the asteroid belt, and scientists and engi-neers anxiously watched for signs of increasingparticulate matter.

Pioneer 10 carried two instruments designedto measure small particles in space. One, withan effective area of about 0.6 square meters,measured the direct impact of dust grains assmall as one-billionth of a gram. The otherlooked for larger, more distant grains by meas-uring sunlight reflected from them. To the sur-prise of many, there was little increase in therate of dust impacts recorded as the craft pene-trated more and more deeply into the belt. Atabout 400 million kilometers from the Sun,near the middle of the belt, there appeared to bean increase in the number of larger particlesdetected optically, but not to a level that posedany hazard. In February 1973 the spacecraftemerged unscathed from the asteroid belt, hav-ing demonstrated that the much-feared concen-tration of small debris in the belt did not exist.The pathway was open to the outer solarsystem!

On November 26, 1973, the long-awaitedencounter with Jupiter began. On that date, at adistance of 6.4 million kilometers from theplanet, instruments on board Pioneer 10detected a sudden change in the interplanetarymedium as the spacecraft crossed the pointthe bow shockat which the magnetic presenceof Jupiter first becomes evident. At the bowshock, the energetic particles of the solar windare suddenly slowed as they approach Jupiter.At noon the next day, Pioneer 10 entered theJovian magnetosphere at a distance of 96Rj from the planet.

As the spacecraft hurtled inward towardregions of increasing magnetic field strengthand charged plasma particles, the instrumentsdesigned to look at Jupiter began to play theirrole. A simple line-scan camera that could build

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Earthat

launch

Earthat

encounter

100 days200

Asteroid belt

Jupiterat launch

Jupiterat

encounter

The asteroid belt is a region between Mars ar.,i Jupiter that is populated by thousands of minor planets, most only a fewkilometers in diameter. Before 1970, some theorists suggested that large quantities of abrasive dust might damage spacecraftpassing through the asteroid belt. Pioneer 10 proved that this danger was not present, thus opening the way to the outer solarsystem.

up an image from many individual brightnessscans (like a newspaper picture transmitted bywire) obtained its first pictures of the planet,and ultraviolet and infrared photometersprepared to observe it also. By December 2,when the spacecraft had crossed the orbit ofCallisto, the outermost of the large Galileansatellites, the line-scan images were nearly equalin quality to the best telescopic photos takenpreviously, and as each hour passed they im-proved in resolution. Near closest approach,Pioneer 10 transmitted partial frames of Jupiterthat represented a threefold improvement overany Earth-based pictures ever taken.

Tension increased as the spacecraft plungeddeeper into the radiation belts of Jupiter.Would it survive the blast of x-rays and gamma-rays induced in every part of the craft by theelectrons and ions trapped by the magnetic fieldof Jupiter? Several of the instruments measur-ing the charged particles climbed to full scaleand saturated. Others neared their limits but, asanxious scientists watched the data being sentback, the levels flattened off. Meanwhile, the

14

spacecraft itself began to feel the effects of theradiation, and occasional spurious commandswere generated. Several planned high-resolutionimages of Jupiter and its satellites were lostbecause of these false signals. But again the sys-tem stabilized, and no more problems occurredas, just past noon on December 3,1973, Pioneer10 reached its closest point to Jupiter, 130 000kilometers above the Jovian cloud tops.Pioneer had passed its most demanding testwith flying colors, and at a news conference atAlves, NASA Planetary Program DirectorRobert Kraemer pronounced the mission "100percent successful." He added, "We sentPioneer off to tweak a dragon's tail, and it didthat and more. It gave it a really good yank,and it managed to survive." The Project Sci-ence Chief pronounced it "the most excitingday of my life," and most of the hundreds ofscientists and engineers who participated in theencounter probably agreed with him.

Pioneer 11 continued to follow steadily,emerging from the asteroid belt in March 1974.Based on the performance and findings of

Pioneer 10, it was decided to send Pioneer I Istill closer to Jupiter, but on a more inclinedtrajectory. On April 19 thrusters on the space-craft fired to move the Pioneer 11 aimpoint just34 000 kilometers above the clouds of Jupiter.

In using the Pioneer 10 data to assess thehazard to Pioneer 11, scientists had to considerthree aspects of the charged particle environ-ment. First was the energy distribution of theparticles: The most energetic presented themost danger. Second was the flux, the rate atwhich particles struck the craft. Third was thetotal radiation dose. One can make an analogywith a boxing match. The energy distributiontells you how hard the blows nf your opponentare. The flux is a measure of how many times aminute he hits you, and the total dose measureshow many blows land. The spacecraft reactsjust like a boxer; the crucial question is howmuch total dose it absorbs. Enough radiationblows, and the system is knocked out. The tra-jectory chosen for Pioneer 11 resulted in higherflux, since the craft probed more deeply intoJupiter's inner magnetosphere than had Pioneer10. But by moving at a high angle across the

equatorial iegions where the flux is highest, thetotal dose could be kept below that experiencedby Pioneer 10.

Pioneer II entered the Jovian magneto-sphere on November 26, 1974, just a year afterits predecessor. Closest approach took place onDecember 2. As with Pioneer 10, the radiationdose taxed the spacecraft to its limit. Againspurious commands were issued, this timeaffecting the infrared radiometer more than theimaging system. The craft flew at nigh latitudeover the north polar region of Jupiter, an areanever seen from Earth, and returned several ex-cellent high-resolution pictures. Once more thelittle Pioneers had succeeded against the odds inopcaing the way to the giant planets.

Following their encounters with Jupiter,both Pioneer spacecraft returned to their nor-mal routine of measuring the interplanetarymedium. Pioneer 19 had gained speed from thegravity field of Jupiter and became the firstcraft to achieve the velocity needed to escapefrom the solar system. Pioneer II, however,had used the pull of Jupiter to bend its trajec-tory inward, aiming it across the solar system

Pioneer 10 and II encounters ss it h Jupiter ale slims n as s less ed from the Lelestial North Pole Pioneer 10 %%sung around thegiant planet in the counterelockssise dirtLtion, %%He Pioneer 11 tollossed 41 l illek111se approach In this %less , Jupiter rotatescounterclockssise,

15

toward Saturn. Following the successes atJupiter, NASA announced that Pioneer I Iwould be targeted for a close flyby of Saturnfive years later, which was successfully carriedout in September 1979. In early 1980, far be-yond their design lifetimes, both spacecraftwere still performing beautifully.

Jupiter Results

The scientific results of the Pioneer flybysof Jupiter were many and varied. As is alwaysthe case, some old questions were answered andnew problems were raised by the spacecraftdata. Highlights of these results are summar-ized below.

Photographs of Jupiter. The line-scan imagingsystems of Pioneer returned some remarkablepictures of the planet during the two en-counters, showing individual features as small

as 300 kilometers across. In addition, thePioneers were able to look at Jupiter fromangles never observable from Earth.

One of the discoveries made from thesepictures was the great variety of cloud struc-tures near the boundaries between the lightzones and dark belts. Many individual cloudpatterns suggested rising and falling air. Theconvoluted swirls evident in these regions ap-peared to be the result of dynamic motions;unfortunately, with only the few "snapshots"obtained during the hours of the flyby, theactual motions of these clouds could not befollowed.

Thermal Emission. In the year the PioneerProject was begun, astronomers on Earth hadmeasured that Jupiter emitted more heat than itabsorbed f om the Sun. From the Earth thesemeasurements could be made only of the sunlitpart of the planet; neither the night side nor the

In this view of Jupiter, the Great Red Spot is prominent and the shadow of lo traverses the plane ary disk. The gross mor-phology of the belts and zones, with structure:, showing turbulence and convective cells in the middle latitudes, is clearlyseen. The small white spots surrounded by dark rings, seen mainly in the southern hemisphere, indicate regions of intensevertical convective activity, somewhat similar to cumulonimbus or thunderclouds.

ZETILFII"---0t6.1

One of the best Pioneer images of Jupiter was obtained at arange of 545 000 kilometers by Pioneer I I. Structure withinthe Great Red Spot and the surrounding belts and zones canbe seen. There was much less turbulent cloud activityaround the spot at the time of the Pioneer flybys than wasseen five years later by the Voyager cameras.

voles could be seen. One of the main objectivesof the Pioneer flybys was to determine the heatbudget accurately from temperature measure-meats at many points on both the sunlit and thenight sides.

The Pioneer data confirmed the presenceof a heat source in Jupiter and supplied a quan-titative estimate of its magnitude. The globaleffective temperature was found to be 148° C,to a precision of ± 3 degrees. This temperatureimplies that Jupiter radiates 1.9 times as muchheat as it receives from the Sun. The corre-sponding internal heat source is 10" watts. Sur-prisingly, the poles were as warm as theequator; apparently, the atmosphere is veryefficient at transferring solar heat absorbednear the equator up to high latitudes, orperhaps the internal component of the heatcomes preferentially from the polar regions.

Helium in the Atmosphere. The Pioneer in-frared experiment made the first measurementof the amount of helium on Jupiter. The ratioof the number of helium atoms to the numberof hydrogen atoms was found to be He/ H, --0.14 ± 0.08. This is consistent with the knownsolar ratio of He/H2 = 0.11. Measurements ofhelium in the upper atmosphere were also madeby the ultraviolet experiment.

i- :-' os, (.)

Hydrogen gas

Cloud tops

Ammonia crystals

Ammonium hydrosuffide crystal--Ice crystals

Water droplets

///V isible

123°C \\ Transparent //// clouds

Hydrogen/Helium gas

atmosphere /\ . / Distance/(km)/

Transitionzone

1980° C

Liquidhydrogen

11 000° C

3 m Jr1atmospheres

pressure

Liquidmetallic

hydrogen

Possible"sea" ofhelium

Possiblesolidcore

30 000 K

70 000

60 000

50 000Transition

zone

40 000

30 000

20 000

10 000

0Pioneer 10 confirmed theoretical models of Jupiter thatsuggest the planet is nearly all liquid, with a xery small coreand an extremely deep atmosphere The liquid interiorseethe, with internal heat energy, which is transferred fromdeep within the planet to its outer regions. The temperatureat the center may be 30 000 K. Since the temperature at thecloud tops is arouad 121' C', there is a large range oftemperatures within the planet

17

PIONEER SCIENCE INVESTIGATIONSProject Scientist: J. H. Wolfe, NASA Ames

Investigation Principal Investigator Primary Objectives

Magnetic fields E. J. Smith, JPL Measurement of the magnetic field ofJupiter and determination of the struc-ture of the magnetosphere.

Magnetic tields (Pioneer I I on!) I N F. Ness, NASA Goddard Measurement of the magnetic field ofJupiter and determination of the struc-ture of the magnetosphere.

Plasma analyzer J H. Wolfe, NASA Ames Measurement of lowenergy electronsand ions, determination of the struc-ture of the magnetosphere.

Charged particle composition J. A. Simpson, U. Chicago Dcierminatio of the number, energy,and composition of energetic chargedparticles in the Jovian magnetosphere.

Cosmic ray energy spectra F. B. McDonald, Measurement of number and energy ofNASA Goddard very high energy charged particles in

space.

Jovian charged particles I. A Van Allen, U. Iowa Measurement of number and energydistribution of energetic charged par-ticles and determination of magneto-spheric structure.

Jovian trapped radiation R. Walker Filhus, Measurement of number and energyUC San Diego distribution of energetic charged par-

ticles and determination of magneto-spheric structure.

Asteroid-meteoroid astronomy R. K. Soberman, Observation of solid particles (dustGeneral Electric and larger) in the vicinity of the space-

craft.

Meteoroid detection

Celestial mechanics

Ultraviolet photometry

Imaging photopolarimetry

W. H. Kinard, NASA Langley Detection of very small solid particlesthat strike the spacecraft.

J. D. Anderson, JPL Measurement of the masses of Jupiterand the Galilean satellites with highprecision.

D. L. Judge, Measurement of ultraviolet emissionsU. Southern Calitornia of the Jovian atmosphere and from cir-

cumsatellite gas clouds.

T. Gehrels, U. Arizona Reconnaissance imaging of Jupiter andits satellites; study of atmosphericdynamics.

Measurement of Jovian temperatureand heat budget; determination ofhelium to hydrogen ratio.

Probes of structure of Jovian atmos-phere and ionosphere.

Jovian infrared thermal structure G. Mi.inch, Caltech

S-Band occultation A. J. Kltore, JPL

18

2J

Atrognheric Structure. Several Pioneer in-vestiations yielded information on the varia-tion of atmospheric temperature and pressurein the regions above the ammonia clouds. Nearthe equator, at a level where the atmosphecicpressure is the same as that on the surface of theEarth (I bar), the temperature is 108° C.About 150 kilometers higher, wl .;re thepressure drops to 0.1 bar, is the minimum at-mospheric temperature of about 165° C.Above this point the temperature rises again,reaching about 123° C near a pressure levelof 0.03 bar. Presumably this temperature rise isdue to absorption of sunlight by a thin haze ofdust particles in the upper atmosphere ofJupiter.

Internal Structure. The measurements of theamount of helium, of the gravitational field,and of the size of the internal heat source onJupiter greatly clarified scientists' understand-ing of the deep interior of the planet. Calcu-lations showed that the core of Jupiter must beso hot that hydrogen cannot become solid, butmust remain a fluid throughout the interior.Even at great depths, therefore, Jupiter doesnot have a solid surface. The theory that the

Great Red Spot was the result of interactionswith a surface feature below the clouds thusbecame untenable. Whatever its exact nature,the Red Spot must be a strictly atmosphericphenomenon.

Magnetic Field. Pioneer data showed that themagnetic field of Jupiter has a dipolar nature,like that of the Earth, but 2000 times stronger.The calculated surface fields measured about 4gauss, compared to a field of about 0.5 gausson the Earth. The axis of the magnetic field wastilted 11 degrees with respect to the rotationaxis, and it was offset by about 10 000kilometers (0.1 Rj) from the center of theplanet.

Satellite Atmospheres. Two experiments yield-ed exciting new information on possible at-mospheres of the Galilean satellites. First wasan occulation, in which the Pioneer 10spacecraft was targeted to pass behind lo asseen from Earth. At the moments just beforethe spacecraft disappeared, and just afterreemergence from behind the satellite, the radiosignal was influenced by a thin layer of ionizedgas, in which the electrons had been stripped

Pioneers 10 and I I did not obtain very detailed pictures of the satellites of Jupiter. The best view was of Ganymede (left),which showed a surf: cc of contrasting light and dark spots of unknown nature. A less detailed image of Europa (right)clearly reveals the illuminated crescent but supplies little information about the surface of this ice-covered satellite.

from the atoms by absorbed sunlight or byother processes, The ionosphere thus dis-covered had a peak density of about 60 000electrons per cubic centimeter. In addition, avery extended far-ultraviolet glow, probablydue to atomic hydrogen, was found near the or-bit of to by the ultraviolet photometer.

Masses of Jupiter and its Satellites. Preciseradio tracking of the Pioneers as they coastedpast Jupiter and its satellites revealed thatJupiter is about one percent heavier than hadbeen anticipated, and several satellites werefound to have masses that differ by more thanten percent from values determined previously.These improvements in knowledge of themasses were required to achieve the closesatellite flybys being planned for later missions.

The Inner Magnetosphere. A great deal of thescientific emphasis of the Pioneer missions wasdirected at characterizing the particles andfields in the inner magnetosphere, the region inwhich charged particles are trapped in stable or-bits. The Pioneers found that it extended toabout 25 Rj, well beyond the orbit of Callisto.Within this region, instruments on the space-craft recorded the numbers and energy of elec-trons, protons, and ions. The electrons reacheda maximum concentration near 3 Rj, and theirnumbers remained almost constant from therein toward the planet. The maximum concentra-tion of protons observed by Pioneer 10 was at3.4 R J ' a little inside the orbit of to. Pioneer 11penetrated deeper and found another maximum,about twenty times higher, at 1.9 Rj; at thisdistance, 10 million energetic protons hit each

T ac Pioneer spacecratt carried this plaque on the Immo, beyond ihr solar ssstem, bearing data that tal ss here and ss hen thehuman species Ii ed and that cons eN &talk of our biological form \\ hen Pioneer 10 Hess bs lupuer it acquired sulticientkinet,e energy to carry it completely out of the solar system. Some time betssecn one and ten billion Nears Irom nosy, theprobe may pass through the planciars sNstelll of a remote stellar neighbor, one of ss hose planets may hale L olsi..d intelligentlife. It the spacecrat I is detected and !hell InSpelied. l'ioncer's message call real] across the eons to Lommunicate itsgreeting.

20

square centimeter of the spacecraft every sec-ond. It was believed that the gap between thesetwo peaks was due to tiny Amalthea, the inner-most satellite, which orbits at 2.5 R./. Apparent-ly this satellite sweeps up the particles as itcircles Jupiter. Another large dip in the protondistribution was attributed to sweeping by lo,with smaller effects seen near the orbits ofEuropa and Ganymede. There was an addi-tional small effect at 1.8 RJ' later found byVoyager to be due to Jupiter's ring and its four-teenth satellite.

The Outer Magnetosphere. From about 25 Rjoutward to its boundary near 100 Ri, the Jo-vian magnetosphere is a complex and dynamicplace. Beyond about 60 Ili, both Pioneersfound the boundary to be highly unstable, ap-parently blown in and out by variations in thepressure of the solar wind, which consists ofcharged particles flowing outward from theSun. In this region concentrations of Jovianparticles are sometimes seen that rival the inner

magnetosphere in intensity. Between 60 R./ and25 RJ' a region sometimes called the middlemagnetosphere, the particle motions are moreordered, and for the most part electrons andprotons are carried along with the planet's rota-tion by its magnetic field. Near the equatorialplane, the flow of these particles produces anelectric current circling the planet, and this cur-rent in turn generates its own magnetic field,which approaches in strength that of Jupiteritself. Occasionally the outer magnetospherecollapses down to about 60 Ri, and energeticparticles are squirted from the middle region in-to space; these bursts of Jovian particles cansometimes be detected as far away as Earth.

At the same time that Pioneer scientistswere analyzing their results and developing newconcepts of the Jupiter system, a new team ofinvestigators had been selected for the next mis-sion to Jupiter. From about 1975 on, attentionshifted from Pioneer to its successorVoyager.

21

CY,CY)

CHAPTER 3

THE VOYAGER MISSION

Genesis of Voyager

Voyager had its origin in the Outer PlanetsGr, nd Tour, a plan to send spacecraft to all theplanets of the outer solar system. In 1969, thesame year in which the Pioneer Project re-ceived Congressional approval, NASA began todesign the Grand Tour. At the same time, theSpace Science Board of the National Academyof Sciences completed a study called "TheOuter Solar System," chaired by James VanAllen of the University of Iowa, which recom-mended that the United States undertake anexploration program:

1. To conduct exploratory investigations ofthe appearance, size, mass, magnetic pro-perties, and dynamics of each of the outerplanets and their major satellites;

2. To determine the chemical and isotopiccomposition of the atmospheres of theouter planets;

3. To determine whether biologically impor-tant organic substances exist in theseatmospheres and to characterize the loweratmospheric environments in terms ofbiologically significant parameters;

4. To describe the motions of the at-mospheres of the major planets and tocharacterize their temperature-density-composition structure;

5. To make a detailed study for each of theouter planets of the external magneticfield and respective particle population,associated radio emissions, and magneto-spheric particle-wave interactions;

6. To determine the mode of interaction ofthe solar wind with the outer planets, in-cluding the interaction of the satelliteswith the planets' magnetospheres;

7. To investigate the properties of the solarwind and the interplanetary magnetic fieldat great distances from the Sun at both

low and high solar latitudes, and to searchfor the outer boundary of the solar windflow:

8. To a,.empt to obtain the composition,energy spectra, and fluxes of cosmic raysin interstellar space, free of the modulat-ing effects of the solar wind.

The report also noted that "exceptionallyfavorable astronomical opportunities occur inthe late 1970s for multiplanet missions," andthat "professional resources for full utilizationof the outer-solar-system mission opportunitiesin the 1970s and 1980s are amply availablewithin the scientific community, and there is awidespread eagerness to participate in such mis-sions."

An additional Academy study, chaired byFrancis S. Johnson of the University of Texasat Dallas and published in 1971, was even morespecific: "An extensive study of the outer solarsystem is recognized by us to be one of themajor objectives of space science in this decade.This cndeavor is made particularly exciting bythe rare opportunity to explore several planetsand satellites in one mission using long-livedspacecraft and existing propulsion systems. Werecommend that [Mariner-class] spacecraft bedeveloped and used in Grand Tour missions forthe exploration of the outer planets in a seriesof four launches in the late 1970s."

Thus the stage was set to initiate the OuterPlanets Grand Tours. NASA's timetable calledfor dual launches to Jupiter, Saturn, and Plutoin 1976 and 1977, and dual launches to Jupiter,Uranus, and Neptune in 1979, at a total costover the decade of the 1970s of about $750million.

A necessary step was to obtain from thescientific community the best possible set ofinstruments to fly on the spacecraft. Followingits initial internal studies, NASA turned for itsdetailed scientific planning to an open competi-

The Voyager spacecraft are among the most sophisticated, automatic, and independent robots ever sent to explore theplanets. Earh craft has a mass of one ton and is dominated by the 3.7-meter-diameter white antenna used for radio commu-nication with Earth. Here Voyager undergoes final tests in a space simulator chamber. 1373-7162ACI

0 ,)a) '4

tion in which any scientist or scientific organi-zation was invit, d to propose an investigation.In October 1970 NASA issued an "Invitationfor Participation in the Mission Developmentfor Grand Tour Missions to the Outer SolarSystem," and a year later it had selected abouta dozen teams of scientists to formulate specificobjectives for these missions. At the same time,an advanced spacecraft engineering design wascarried out by the Caltech Jet Propulsion Lab-oratory (JPL), and studies were also supportedby industrial contractors. In fiscal year 1972,plans called for an appropriation by Congressof $30 million to fund these developments,leading toward a first launch in 1976.

Even as the scientific and technical prob-lems of the Grand Tour were being solved,however, political and budgetary difficulties in-tervened. The Grand Tour was an ambitious andexpensive concept, designed in the enthusiasmof the Apollo years. In the altered nationalclimate that followed the first manned lunarlandings, the United States began to pull backfrom major commitments in space. The laterApollo landings were canceled, and in fiscalyear 1972 only $10 million of the $30 millionneeded to complete Grand Tour designs was ap-

propriated. It suddenly became necessary torestructure the exploration of the outer planetsto conform to more modest space budgets.

Redesign of the Mission

The new mission concept that replaced theGrand Tour dropped the objectives of explor-ing the outer three planetsUranus, Neptune,and Pluto. In this way the lifetime of the mis-sion was greatly shortened, placing less strin-gent demands on the reliability of the millionsof components that go into a spacecraft. Limit-ing the mission to Jupiter and Saturn alsorelieved problems associated with spacecraftpower, and with communicating effectivelyover distances of more than 2 billion kilo-meters. The total cost of the new mission wasestimated at $250 million, only a third of thatpreviously planned for the Grand Tour. Be-cause it was based on the proven Mariner space-craft design, the new mission was initiallynamed Mariner Jupiter Saturn, or MJS; in 1977the name was changed to Voyager. In January1972 the President's proposed fiscal year 1973budget included $10 million specifically desig-nated for Voyager; after authorization and

The original plan for the Outer Planets Grand Tour envisaged dual launches to Jupiter, Saturn, and Pluto in th mid-1970s,and eual launches to Jupiter, Uranus, and Neptune in 1979. However, political and budgetary constraints altered the plan,and the Voyager mission to Jupiter and Saturn, with an optional encounter with Uranus, was formulated to replace it. Heiethe original Grand Tour trajectories from Earth to the outer planets are shown. IP- 10612AC1

35

Project ScientistEdward C. Stone

appropriation by Congress, the official begin-ning of Voyager was set for July 1, 1972.

With approval of the new mission appar-ently assured in the Congress, NASA issued an"Announcement of Flight Opportunity" toselect the scientific instruments to be carried onVoyager. Seventy-seven proposals were re-ceived; 31 from groups of scientists withdesigns for instruments, and 46 from in-dividuals desiring to participate in NASA-formed teams. Of these 77 proposals, 24 werefrom NASA laboratories, 48 were from scien-tists in various U.S. universities and industry,and 5 were from foreign sources. After exten-sive review, 28 proposals were accepted: 9 forinstruments and 19 for individual participation.The newly selected Principal Investigators at.dTeam Leaders met for the first time at JPI_ justbefore Christmas, 1972. To coordinate all thescience activity of the Voyager mission, NASAand JPL selected Edward Stone of Caltech, adistinguished expert on magnetospheric physics,to serve as Project Scientist.

The team assembled in 1972 by JPL and itsindustrial contractors included more than athousand highly trained engineers, scientists,and technical managers who assumed respon-sibility for the awesome task of building themost sophisticated unmanned spacecraft everdesigned and launching it across the farthestreaches of the solar system. At the head of theorganization was the Project Manager, Harris(Bud) Schurmeier. Later, Schurmeier was suc-ceeded by John Casani, Robert Parks, and RayHeacock. This team had only four years to turnthe paper concepts into hardware, ready todeliver to Kennedy Space Center for launch inthe summer of 1977.

The Objectives of Voyager

Voyager is one of the most ambitiousplanetary space missions ever undertaken.Voyager I, which encountered Jupiter onMarch 5, 1979, was to investigate Jupiter, itslarge satellites lo, Ganymede, and Callisto, and

tiny Amalthea; Saturn, its rings, and several ofits satellitesincluding Titan, the largest satel-lite in the solar system. Voyager 2, which ar-rived at Jupiter on July 9, 1979, was to examineJupiter, Europa, Ganymede, Callisto, andSaturn and several of its satellites, c cier which itwas to be hurled on toward an encounter withthe Uranian system in 1986. Both spacecraftwere also designed to study the interplanetarymedium and its interactions with the solarwind.

Scientific objectives as well as the orbitalpositions of the planets and satellites influencedthe choice of spacecraft trajectories, whichwere designed to provide close flybys of all fourof Jupiter's Galilean satellites and six ofSaturn's satellitesfeaturing a very close ap-proach to Titanand occultations of the Sunand Earth by Jupiter, Saturn, Titan, and therings of Saturn. However, all these objectivescould not be accomplished by a single space-craft. No single path through the Jupiter sys-tem, for instance, can provide close flybys of allfour Galilean satellites. Specifically, a trajec-tory for Voyager 1 that included a close en-counter with lo precluded a close encounterwith Europa and did not allow the spacecraftthe option of being targeted from Saturn toUranus without having to travel through therings of Saturn. On the other hand, a trajectorythat would send Voyager 2 on to Uranus pre-cluded not only a close encounter with 10, butalso a close encounter with Titan and withSaturn's rings. The alignment of the planetsand their satellites was such that encounter withJupiter on or before April 4, 1979 was neces-sary for optimum investigation of lo, but en-counter with Jupiter after June 15, 1979 wasmandatory for a trajectory that would maintainthe option to go on to Uranus. The latter trajec-tory also allowed Voyager 2 to maintain ahealthier distance fron' Jupiter, avoiding thefull dose of radiation tl- at would be experiencedby Voyager 1.

Although the two Voyager trajectories dohave certain fundamental differences, they alsohave a great deal in common: Both spacecraft

Protect ManagerHarris (Bud) Schurmcicr

A.

were designed to study the interplanetarymedium, and both would investigate Jupiterand Saturn and have close flybys with several oftheir major satellites. Also, if it were decidednot to send Voyager 2 on to Uranus, this space-craft could be retargeted to a Saturn trajectorysimilar to that of Voyager 1, providing a closeflyby of Titan and a closer look at the rings.The flight paths of Voyager 1 and Voyager 2complement each otherallowing the planetsand some of the satellites to be viewed from anumber of angles and over a longer period oftime that would be possible with only onespaceprobeyet the trajectories were alsodesigned to be more or less capable ofduplicating each other's scientific investiga-tions. This redundancy helps to ensure, so. faras is possible, the success of the mission.

The redundancy built into the mission isevident, not only in the design of the trajec-tories, but in the spacecraft themselves.Voyagers 1 and 2 are identical spacecraft. Inaddi'ion, many crucial elements are duplicatedon eech. For example, the computer commandsubsystem (CCS), the flight data subsystem(FDS), and the attitude and articulation controlsubsystem (AACS)which function as on-board control systemseach have multiplereprogrammable digital computers. In addi-tion, the CCS, which decodes commands fromground control and can instruct other subsys-tems from its own memory, contain. duplicatesfor all its functional units. The AACS, whichcontrols the spacecraft's stabilization and orien-tation, has duplicate star trackers and San sen-

Project ManagerJohn Casani

sors. The communications system contains tworadio receivers and four transmitters, two eachto transmit both S-band (frequency of about2295 megahertz) and X-band (frequency ofabout 8418 megahertz).

The Spacecraft

The Voyager spacecraft are more sophisti-cated, more automatic, and more independentthan were the Pioneers. This independence isimportant because the giant planets are so faraway that the correction of a malfunction byengineers on Earth would take hours to per-form. Even at "nearby" Jupiter, radio signalstake about forty minutes to travel in one direc-tion between the spacecraft and Earth. Saturnis about twice as far away as Jupiter, andUranus is twice as far as Saturn, slowing com-munication even more.

Perched atop the Centaur stage of thelaunch rocket, each Voyager spacecraft has amass of 2 tons (2066 kilograms), divided aboutequally between the spacecraft proper and thepropulsion module used for final acceleration

About the same size and weight of a subcompact car, the Voyager spacecraft carry 1[1,4 r uments for eleven science investiga-tions of the outer planets and their satellites. Power to operate the spacecraft is provided by three radioisotope thermoelectricgenerators mounted on one boom; other booms hold the science scan platform and the dual magnetometers.

Plasmadetector

Magnetometer

Extendable boom

High-gaindirectionalantenna

Cosmic raydetector Wide angle TV

Narrow angle TVTV electronics

Ultravioletspectrometer

Infrared

Planetary radioastronomy andplasma wave antenna

Propulsionfuel tank

Radioisotopethermoelectricgenerators

Photopolarimeter interferometerspectrometerand radiometer/ Low energy

/ charged particles

Thrusters

Electroniccompartments

Science instrumentcalibration paneland shunt radiator

P')Planetary radio tJ 6astronomy andplasma wave antenna

to Jupiter. The Voyager itself, with a mass of815 kilograms and typical dimensions of about3 meters, is almost the size and weight of a sub-compact carbut enormously more complex.A better analogy might be made with a largeelectronic computerbt . no terrestrial com-puter was ever asked to supply its own powersource and to operate unattended in thevacuum of space for up to a decade.

Each Voyager spacecraft carries instru-ments for eleven science investigations coveringvisual, infrared, and ultraviolet regions of thespectrum, and other remote sensing studies ofthe planets and satellites; studies of radio emis-sions, magnetic fields, cosmic rays, and lowerenergy particles; plasma (ionized gases) wavesand particles; and studies using the spacecraftradios. Each Voyager has a science boom thatholds high and low resolution TV, photopolari-meter, plasma and cosmic ray detectors, in-frared spectrometer and radiometer, ultravioletspectrometer, and low energy charged particledetector; in addition, each spacecraft has aplanetary radio astronomy and plasma waveantenna and a long boom carrying a low- and ahigh-field magnetometer.

Communication with Earth is carried outvia a high-gain antenna 3.7 meters in diameter,with a smaller low-gain antenna as backup. Thelarge white dish of the high-gain antennadominates the appearance of the spacecraft,setting it off from its predecessors, which wereable to use much smaller antennas to com-municate over the more modest distances thatseparate the planets of the inner solar system.Although the transmitter power is only 23wattsabout the power of a refrigerator lightbulbthis system is designed to transmit dataover a billion kilometers at the enormous rateof 115 200 bits per second. Data can also bestored for later transmission to Earth; an on-board digital tape recorder has a capacity ofabout 500 million (5 x 108) bits, sufficient tostore nearly 100 Voyager images.

Protect ManagerRay Heacock

During the early assembly stage, technicians at Caltech'sJet Propulsion Laboratory equip Voyager's extendableboom with low- and high-field magnetometers that measurethe intensity and direction of the outer planets' magneticfields. (373-7179 BC]

Radioisotope thermoelectric generators(RTGs), rather than solar cells, provide elec-tricity for the Voyager spacecraft. The RTGsuse radioactive plutonium oxide for this pur-pose. As the plutonium oxide decays, it givesoff heat which is converted to electricity, sup-plying a total of about 450 watts to the space-craft at launch. This power slowly declines asthe plutonium is used up, with less than 400watts expected at Saturn flyby five years afterlaunch. Hydrazine fuel is used to make mid-course corrections in trajectory and to controlthe spacecraft's orientation.

Since the Voyagers must fly through theinner magnetosphere of Jupiter, it was im-perative that the hardware systems be able towithstand the radiation from Jovian chargedparticles. The electronic microcircuits that formthe heart and brain of the spacecraft and itsscientific instruments are especially susceptibleto radiation damage. Three techniques wereused to "harden" components against radia-tion:

1. Special design using radiation-resistantmaterials;

2. Extensive testing to select those electroniccomponents which come out of themanufacturing process with highest reli-ability; and

27

VOYAGER'S GREETINGS TO THE UNIVERSE

The Voyager spacecraft will be the third and fourth human artifacts to escape entirely fromthe solar system. Pioneers 10 and 11, which preceded Voyager in outstripping the gravitationalattraction of the Sun, both carried small metal plaques identifying their time and place of originfor the benefit of any other spacefarers that might find them in the distant future. With thisexample before them, NASA placed a more ambitious message aboard Voyager 1 and 2a kindof time capsule, intended to communicate a story of our world to extraterrestrials.

The Voyager message is carried by a phonograph recorda 12-inch gold-plated copper diskcontaining sounds and images selected to portray the diversity of life and culture on Earth. Thecontents of the record were selected for NASA by a committee chaired by Carl Sagan of CornellUniversity. Dr. Sagan and his associates assembled 115 images and a variety of natural sounds,such as those made by surf, wind and thunder, birds, whales, and other animals. To this theyadded musical selections from different cultures and eras, and spoken greetings from Earth-people in sixty languages, and printed messages from President Carter and U.N. SecretaryGeneral Waldheim.

Each record is encased in a protective aluminum jacket, together with a cartridge andneedle. Instructions, in symbolic language, explain the origin of the spacecraft and indicate howthe record is to be played. The 115 images are encoded in analog form. The remainder of therecord is in audio, designed to be played at 162/3 revolutions per second. It contains the spokengreetings, beginning with Akkadian, which was spoken in Sumer about six thousand years ago,and ending with Wu, a modern Chinese dialect. Following the section on the sounds of Earth,there is an eclectic 90-minute selection of music, including both Eastern and Western classics anda variety of ethnic music.

Once the Voyager spacecraft leave the solar system (by 1990, both will be beyond the orbitof Pluto), they will find themselves in empty space. It will be forty thousand years before theycome within a light year of a star, called AC + 79 3888, and millions of years before either mightmake a close approach to any other planetary system. As Carl Sagan has noted, "The spacecraftwill be encountered and the record played only if there are advanced spacefaring civilizations ininterstellar space. But the launching of this bottle into the cosmic ocean says something veryhopeful about life on this planet."

LANGUAGES ON VOYAGER RECORD

Sumerian Cantonese Spanish Hindi Turkish Armenian Swedish KannadaAkkadian Russian Indonesian Vietnamese Welsh Polish Ukrainian TeluguHittite Thai Kechua Sinhalese Italian Netali Persian OriyaHebrew Arabic Dutch Greek Nguni Mandarin Serbian HungarianAramaic Roumanian German Latin Sotho Gujorati Luganada CzechEnglish French Bengali Japanese Wu Ha (Zambia) Amoy (Min dialect) Rajasthan'Portuguese Burmese Urdu Punjabi Korean Nyanja Marathi

SOUNDS OF EARTH ON VOYAGER RECORD

Whales Birds Laughter Riveter Tractor KissPlanets (music) Hyena Fire Morse code Truck BabyVolcanoes Elephant Tools Ships Auto gears Life signsMud pots Chimpanzee Dogs, domestic Horse and cart Jet EEG, EKGRain Wild dog Herding sheep Horse and carriage Lift-off Saturn 5 PulsarSurfCrickets, frogs

Footsteps andheartbeats

Blacksmith shopSawing

Train whistle rocket

28

VOYAGER RECORD PHOTOGRAPH INDEX

Calibration circleSolar location mapMathematical definitionsPhysical unit definitionsSolar system parametersThe SunSolar spectrumMercuryMarsJupiterEarthEgypt, Red Sea, Sinai Peninsula

and the NileChemical definitionsDNA structureDNA structure magnifiedCells and cell divisionAnatomy (eight)Human sex organsDiagram of conceptionConceptionFertilized ovumFetus diagramFetus

Diagram of male and femaleBirthNursing motherFather and daughter (Malasia)Group of childrenDiagram of family agesFamily portraitDiagram of continental driftStructure of EarthHeron Island (Great Barrier Reef

of Australia)Seashore

Snake River and Grand TetonsSand dunes

Monument ValleyForest scene with mushroomsLeafFallen leavesSequoiaSnow flakeTree with daffodilsFlying insect with flowersDiagram of vertebrate evolutionSeashell (Xancidae)DolphinsSchool of fishTree toadCrocodileEagleW aterholdJane Goodall and chimpsSketch of BushmenBushmen huntersMan from GuatamalaDancer from BaliAndean girlsThailand craftsmanElephantOld man with beard and glasses

(Turkey)Old man with dog and flowersMountain climberCathy RigbySprintersSchoolroomChildren with globeCotton harvestGrape pickerSupermarketUnderwater scene with diver

and fishFish:ng boat with nets

Cooking fishChinese dinner partyDemonstration of licking, eating

and drinkingGreat Wall of ChinaHouse construction (African)Construction scene (Amish country)House (Africa)House (New England)Modern house (Cloudcroft, New Mexico)House interior with artist and fireTaj MahalEnglish city (Oxford)BostonUN Building, dayUN Building, nightSydney Opera HouseArtisan with drillFactory interiorMuseumX-ray of handWoman with microscopeStreet scene, Asia (Pakistan)Rush hour traffic, IndiaModern highway (Ithaca)Golden Gate BridgeTrainAirplane in flightAirport (Toronto)Antarctic expeditionRadio telescope (Westerbork,

Netherlands)Radio telescope (Arecibo)Page of book (Newton,

System of the World)Astronaut in spaceTitan Centaur launchSunset with birdsString quartet (Quartetto Italiano)Violin with music score (Cavatina)

MUSIC ON VOYAGER RECORD

Bach Brandenberg Concerto Number Two, FirstMovement

"Kinds of Flowers" Javanese Court GamelanSenegalese PercussionPygmy girls initiation songAustralian Horn and Totem song"El Cascabel" Lorenzo Barcelata"Johnny B. Goode" Chuck BerryNew Guinea Men's House"Depicting the Cranes in Their Nest"Bach Partita Number Three for Violin; Gavotte et

RondeausMozart Magic Flute, Queen of the Night (Aria Num-

ber 14)ChakruloPeruvian Pan Pipes

Melancholy BluesAzerbaijan Two FlutesStravinsky, Rite of Spring, ConclusionBach Prelude and Fugue Number One in C Major trom

the Well Tempered Clavier, Book TwoBeethoven's Fifth Symphony, First MovementBulgarian Shepherdess Song "IzIel Delyo Hajdutin"Navajo Indian Night ChantThe Fairie Round from Pavans, Gar,ards, AlmainsMelanesian Pan PipesPeruvian Woman's Wedding Song"Flowing Streams"Chinese Ch'in music"Jaat Kahan Ho"--Indian Raga"Dark Was the Night"Beethoven String Quartet Number 13 "Cavatina"

29

Each Voyager carries a message in the form of a I2-inch gold-plated phonograph record. The record, together with a car-tridge and needle, is fastened to the side of the spacecraft in a gold-anodized aluminum case that also illustrates how therecord is to be played. 1P-197281

THE BRAINS OF THE VOYAGER SPACECRAFT*

The Voyager spacecraft had greater independence from Earth-based controllers and greaterversatility in carrying out complex sequences of scientific measurements than any of their prede-cessors. These capabilities resulted from three interconnected onboard computer systems: theAACS (attitude and articulation control subsystem); the FDS (flight data subsystem); and theCCS (computer command systun). Operating from "loads" of instructions transmitted earlierfrom Earth, these computers could issue commands to the spacecraft and the science instrumentsand react automatically to problems or changes in operating conditions.

The complex sequence of scientific observations and the associated engineering functionswere executed by the spacecraft under the control of an updatable program stored in the CCS byground command. At appropriate times, the CCS issued commands to the AACS for movementof the scan platform or spacecraft maneuvers; to the FDS for changes in instrument configura-tion or telemetry rate; or to numerous other subsystems within the spacecraft for specific actions.The two identical (redundant) 4096-word memories within the CCS contained both fixed routines(about 2800 words) and a variable section (about 1290 words) for changing science sequencingfunctions. A single 1290-word science sequence load could easily generate 300 000 discrete com-mands, thus providing significantly more sequencing capability than would be possible throughground commands. A 1290-word sequencing load in the CCS controlled both the science andengineering functions of the spacecraft for a period lasting for 3/4 day at closest approach andfor up to 100 days during cruise.

Each 1290-word program (or load) was built from specific science measurement units calledlinks. Some links were used repeatedly in a looping cyclic (like a computer DO loop) to performthe same obsei vation numerous times; other links that involved special measurement geometry orcritical timing occurred only once. About 175 science links were defined for the Voyage, 1Jupiter encounter. It took almost two years to convert the desired science objectives and meas-urements first into links, then into a minute-by-minute timeline for the 98-day encounter period,and finally into the specific computer instructions that could be loaded into the CCS memory forthat portion of the encounter time represented by a particular load. The total Voyager 1 Jupiterencounter period used eighteen sequence memory loads, supplemented by about 1000 groundcommands to modify the sequences because of changing conditions or calibration requirements.

For the Voyager 2 encounter, concern about the ailing spacecraft receiver limited thenumber of loads that could be transmitted, particularly while the spacecraft was deep within theJovian magnetosphere, where radiation effects caused the receiver frequency to drift unpredict-ably. However, a careful redesign of the planned sequences permitted the accomplishment ofvery nearly the original set of observations even with these constraints.

*Adapted from a paper by E. C. Stone and A. L. Lane in the Voyager 1 Thirty-Day Report.

41

Project ManagerRobert Parks

3. Spot shielding of especially sensitive areaswith radiation-absorbing materials.

All three approaches were required on theVoyager craft, especially after Pioneer 10 and11 demonstrated that the radiation at Jupiterwas even more intense than had been assumedin early design studies.

The steady streams of engineering and sci-entific data received on Earth ar transmitted

from the receiving stations of the Deep SpaceNetwork (DSN) to JPL, where the Voyagercontrol functions are centered. There, dozensof technicians check and recheck every subsys-tem to search for the slightest hint of malfunc-tion. In case of problems, there are thicknotebooks of instructions and racks of pre-coded computer tapes ready to be used to cor-rect any apparent malfunction.

Normally Voyager runs itself. Detailed in-structions are programmed into its onboardcomputers and command systems for dealingwith such potential emergencies as a stuck valvein the fuel system, loss of orientation in the startrackers, erratic gyroscope functions, failure ofradio communications, or a thousand and oneother nightmares. The instructions for operat-ing the scientific instruments are also stored onboard, with new blocks of commands sent uponce every few days to replace those for tasksalready completed. Whether in the calm ofcruise mode or the intense excitement of aplanetary encounter, the Voyager craft is alonein space, continuously sensing and reacting toits environment, tied by only a tenuous threadof radio communication to the anxiouswatchers back on Earth.

NASA PLANETARY MISSIONS

Spacecraft Launch Date Destination Encounter Date Type of EncounterMariner 2 8/26/62 Venus 12/14/62 flybyMariner 4 11/28/64 Mars 7/14/65 flybyMariner 5 6/14/67 Venus 10/19/67 flybyMariner 6 2/25/69 Mars 7/31/69 flybyMariner 7 3/27/69 Mars 8/05/69 flybyMariner 9 5/30/71 Mars 11/13/71 orbiterPioneer 10 3/03/72 Jupiter 12/04/73 flybyPioneer 11 4/06/73 Jupiter 12/03/74 flyby

Saturn 9/01/79 flybyMariner 10 11/03/73 Venus 2/05/74 flyby

Mercury 3/29/74 flybyViking I 8/20/75 Mars 6/19/76 orbiter

7/20/76 PanderViking 2 9/09/75 Mars 7/07/76 orbiter

9/03/76 landerVoyager I 8/20/77 Jupiter 3/05/79 flybyVoyager 2 9/05/77 Jupiter 7/09/79 flybyPioneer Venus 5/20/78 Venus 12/04/78 orbiter

8/8/78 Venus 12/09/78 probe

31

The Voyager scan platform contains sophisticated instruments that gather data for Voyager's remote sensing investigations.Five of the remote-sensing instrumentstwo TV cameras, the infra' ed spectrometer, the ultraviolet spectrometer, and thephotopolarimeterare mounted together on the scan platform, which can be pointed to almost any direction in space,allowing exact targeting of the observations. [373-7146BC1

32

11 3

CHAPTER 4

SCIENCE AND SCIENTISTS

Introduction

There are many reasons for sending space-craft to the planets, but in the final analysis, wesend our robot messengers across the vastnessof space for the sake of scientific exploration.Science and exploration have always gone handin hand, whether in the transcontinentaljourney of Lewis and Clark or the Pacificvoyages of Captain James Cook. In this cen-tury, as exploration has become more and moredependent on advances in technology, the scien-tific element has attained increased prominence.The greatest legacy of the NASA PlanetaryProgram is the knowledge it has provided of theother worlds that share our corner of theuniverse.

Despite the central role of science inmotivating missions to the planets, specificscientific considerations are not dominant dur-ing most of the development of a mission suchas Voyager. The problem of building a space-craft and getting it to the outer solar system istoo demanding. Of the ton of mas, in aVoyager spacecraft, the scientific instrumentsmake up only 115 kilograms (about eleven per-cent). Similarly, the cost of these instrumentsamounts to only about 10 percent of the cost ofthe spacecraft and launch vehicle. Unless thelaunch and operation of the spacecraft arenearly perfect, there can be no scientific returnin any case; ever, the most sophisticatedpackage of scientific instruments will not tellmuch about Jupiter if, following launch, it restsat the bottom of the Atlantic Ocean. But it isequally true that the ultimate purpose of themission is scientific discovery, and NASAmakes every effort to ensure that the very bestinstruments are flown and that a broad scien-tific community is given the opportunity to par-ticipate in each mission.

A decade before the 1977 launch, manyastronomers and space scientists began their in-volvement with the Voyager mission throughparticipation in study groups convened byNASA and by the National Academy ofSciences. They came primarily from univer-sities, but also in significant numbers fromNASA laboratories, from industry, and fromabroad. In 1971 the Outer Planets Grand Toursmission definition group carried out a one-yearfinal study 3f the mission that was to becomeVoyager. A competition was held in 1972 toselect the Voyager flight instruments andscience teams, and a third review stage followeda year later to confirm this selection. Out of thisprocess emerged eleven science investigations,with which more than one hundred scientistswere associated. In this chapter we look at theinstruments and the persons who designed themfor this challenging task.

Direct and Remote Measurements

The diverse measurements made byVoyager of a planet and its environment can bedivided into two broad categories, usually calleddirect or in situ measurements and remotesensing measurements. A direct measurementinvolves the analysis of the immediate environ-ment of the spacecraft; remote measurementscan be made by analyzing radiation from dis-tant objects.

The direct measurement instruments onVoyager measure cosmic ray particles, lowenergy charged particles, magnetic fields,plasma particles, and plasma waves. Theiractivity began immediately after launch, moni-toring the Earth environment and then inter-planetary space until the magnetosphere ofJupiter was reached a few days before the ac-tual flyby. Long after the flybys of Jupiter and

33

Saturn are completed, particles and fields datacan continue to be acquired by the Voyagers asthey speed outward into previously unexploredregions of space.

The remote sensing investigations areessentially astronomical in nature, measuringthe light reflected from or emitted by the planetand its satellites. On Voyager, however, theseinstruments far outstrip their terrestrial coun-terparts in capability. Primarily, they derivetheir advantage from their proximity to whatthey observeat its closest, Voyager was morethan a thousand times nearer to Jupiter thanare Earth-based telescopes; and for the stillcloser satellite encounters, Voyager was nearlyten thousand times nearer than astronomers onEarth. In addition, Voyager provided perspec-tives, such as views of the night side of Jupiter,

that are impossible from Earth. Finally, theseinstruments could exploit the full spectrum ofelectromagnetic radiation without concern forthe opacity of the terrestrial atmosphere, whichrestricts ground-based astronomers to certainspectral windows and blocks all observation atother wavelengths.

Five of the remote sensing instru-mentsthe two TV cameras, the infraredspectrometer, the ultraviolet spectrometer, andthe photopolarimeterare mounted togetheron a scan platform. This platform can bepointed to almost any direction in space, allow-ing exact targeting of the observations.

One remote sensing instrument, the plane-tary radio astronomy receiver, is not on thescan platform. It measures long-wave radioemission without requiring special pointing.

The fully deployed Voyager spacecraft is capable of a wide variety of direct and remote sensing measurements. Theinstruments and their objectives were selected many years before the first Jupiter encounter. Because of the exploratorynature of the Voyager mission, every effort was made to fly versatile instruments that could yield valuable results no matterwhat the nature of the Joy/an system. [P-18811AC]

The science instrument boom supports the plasma particle detector, the cosmic ray detector, and the low energy chargedparticle detector. These instruments began collecting data immediately after launch, monitoring the Earth environment andthen interplanetary space until the magnetosphere of Jupiterwas reached a few days before the actual flyby. [353-2992BC]

A final Voyager investigation did not fit and to study properties of the interplanetaryinto this pattern of direct versus remote sensing medium.instruments. In fact, it required no specialinstrument at all. This investigation deals with Imagingradio science, and it utilizes the regular com-munications link between the spacecraft and The eyes of Voyager are in its imaging sys-Earth to derive the masses of Jupiter and its tem. Two television cameras, each with a set ofsatellites, to probe the atmosphere of Jupiter, color filters, look at the planets and their

The Voyager Imaging Science Team

VOYAGER SCIENCE INVESTIGATIONS

Project Scientist: E. C. Stone, Caltech

InvestigationPrincipal Investigator

or Team LeaderPrimary Objectives

at Jupiter

Imaging science B. A. Smith, U. Arizona

Infrared radiation (IRIS) R. A. Hanel,NASA Goddard

Ultraviolet spectroscopy

Photopolarimetry

Planetary radio astronomy

Magnetic fields

Plasma particles

A. L. Broadfoot,Kitt Peak Observatory

C. F. Lillie/C. W. Hord,U. Colorado

High resolution reconnaissance overlarge phase angles; measurement of at-mospheric dynamics; determination ofgeologic structure of satellites; searchfor rings and new satellites.

Determination of atmospheric compo-sition, thermal structure, and dyna-mics; satellite surface composition andthermal properties.

Measurement of upper atmosphericcomposition and structure; auroralprocesses; distribution of ions andneutral atoms in the Jovian system.

Measurement of atmospheric aerosols;satellite surface texture and sodiumcloud.

J. W. Warwick, U. Colorado Determination of polarization andspectra of radio frequency emissions;lo radio modulation process; plasmadensities.

N. F. Ness, NASA Goddard Measurement of plasma electron den-sities; wave-particle interactions; low-frequency wave emissions.

H. S. Bridge, MIT Measurement of magnetospheric ionand electron distribution; solar windinteraction with Jupiter; ions fromsatellites.

Plasma waves F. L. Scarf, TRW

Low energy charged particles S. M. Krimigis,Johns Hopkins U.

Cosmic ray particles

Radio science

Measurement of plasma electron densi-ties; wave-particle interactions; low-frequency wave emissions.

Measurement of the distribution, com-position, and flow of energetic ionsand electrons; satellite energetic parti-cle interactions.

R. E. Vogt, Caltech Measurement of the distribution, com-position, and flow of high energytrapped nuclei; energetic electronspectra.

V. R. Eshleman, Stanford U. Measurement of atmospheric and ion-ospheric structure, constituents, anddynamics; satellite masses

satellites and transmit thousands of detailedpictures to Earth. The imaging system is prob-ably the most versatile and therefore the mosttruly exploratory of the Voyager instruments.No matter what is out there, the imaging system

36

will let us see it and hence, we hope, begin tounderstand its nature.

Unlike other Voyager instruments, theimaging system is not the result of a competi-tion among proposals submitted by groups of

I

1

Bradford A. Smith, imaging science Team Leader

scientists. NASA assigned the development ofthe cameras directly to JPL, to be integratedfrom the beginning with the design of theVoyager spacecraft and its subsystems. Themembers of the Voyager Imaging Science Teamwere selected, as individuals, on the basis of thescientific studies they proposed to carry out.Initially, ten members of the Imaging ScienceTeam were selected, but by the time of theJupiter encounters, the team had been ex-panded to 22 scientists.

The Team Leader is Bradford A. Smith, aprofessor in the Department of PlanetaryScience at the University of Arizona. Smith wasinvolved in imaging science on sevual previousmissions, including Mariners 6 and 7 and Vik-ing. He was also active in ground-based photo-graphy of Jupiter at both New Mexico StateUniversity and University of Arizona, and he isa member of the team developing a planetarycamera for the Space Telescope, scheduled foroperation in Earth orbit in the mid-1980s.

Originally, the Deputy Team Leader wasGeoffrey A. Briggs. a young British-bornphysicist from JPL. However, in 1977 Briggstook a position at NASA Headquarters inWashington and was replaced by geologist

Laurence A. Soderblom of the U.S. GeologicalSurvey in Flagstaff, Arizona. An energctic andarticulate scientist, Soderblom, with his interestin satellite geology, complemented Smith,whose personal scientific interests are directedmore toward the atmosphere of Jupiter.

The objectives of imaging involved multi-color photography of Jupiter and its satellites.Both wide- and narrow-angle cameras wereneeded to obtain the highest possible resolutionwhile retaining the capability to study global-scale features on Jupiter and the satellites. Innormal photographic terms, both cameras usedtelephoto lenses. For the wide-angle camera, afocal length of 200 millimeters was selected,giving a field of view of about 3 degrees. Thisfield is similar to that obtained with a400-millimeter telephoto lens on a 35-millimetercamera. The narrow-angle Voyager camerahas a focal length of 1500 millimeters and fieldof view of 0.4 degrees. The camera optics arecombinations of mirrors and lenses, designedfor extreme stability of focus and for freedomfrom distortion.

Each camera has a rotating filter wheelthat can be used to select the color of the lightthat reaches the camera. For the wide-anglecamera, these colors are clear, violet, blue,green, orange, and three special bands for selec-tive observation in sodium light (589-nano-meter wavelength), and in methane spectrallines at 541 nanometers and 618 nanometers.For the narrow-angle camera, the filters areclear, ultraviolet, violet, blue, green, andorange. To create a color picture, the camerasare commanded to take, in rapid succession,pictures of the same area in blue, green, andorange light. These three pictures can then bereconstructed on Earth into a "true" colorimage. Other combinations of colors are usedto investigate particular scientific problems andto determine the spectrum of sunlight reflectedfrom features on Jupiter and its satellites.

The detector in the cameras is not pho-tographic film but the surface of a selenium-sulfur vidicon television tube, 11 millimeterssquare. Unlike ,nost commercial TV cameras,these tubes are designed for slow-scan readout,providing 48 seconds to acquire each picture.The shutter speed can be varied from a fractionof a second (for Jupiter and lo) to manyminutes (for searches for faint features, such asaurorae on the night side of Jupiter).

Each picture consists of many numbers,each of which represents the brightness of asingle picture element, or pixel, on the image.There are 640 000 pixels in each Voyager

e: 6

image, representing a square image of 800 x800 points. This information content is morethan twice that of an ordinary television pic-ture, which has only 520 lines. For each pixel,eight binary numbers are required to specifiythe brightness; the total information in a singleimage is thus 8 x 640 000 = 5 120 000 bits.Even at a transmission of one frame per 48seconds, the "bit rate" is more than 100 000bits per second. For comparison, the bit ratefrom the first spacecraft to Mars (Mariner 4)was about 10 bits per second, requiring a weekto transmit 21 pictures, with a total informationcontent equivalent to a single Voyager picture.Altogether, Voyager took nearly 20 000 pic-tures at each Jupiter encounter, representing101Ia hundred billionbits of information.

Infrared Spectrometer

The infrared investigation on Voyager isbased on one of the most sophisticated instru-ments ever flown to another planet. In the past,most infrared instruments on planetary space-craft measured at only a few wavelengths, butVoyager carries a true spectrometer, capable ofmeasuring at nearly 2000 separate wavelengths,covering the spectrum from 4 to 50 micrometers.

Twelve scientists, led by Principal Inves-tigator Rudolph Hanel of the NASA GoddardSpace Flight Center at Greenbelt, Maryland,proposed this infrared instrument. Hanel isan acknowledged world leader in infraredspectroscopy from space. With his co-workersat Goddard, he has pioneered in adapting theextremely complex art of interferometric spec-troscopy to the rigors of space flight. His spec-trometers have made many studies of theEarth's atmosphere from meteorological satel-lites, and a Hanel interferometer flew suc-cessfully to Mars on Mariner 9.

The primary goals of the infrared spec-trometer investigation are directed towardanalysis of the composition and structure of theatmosphere of Jupiter. Among the molecules tobe searched for on Jupiter were hydrogen (H2),helium (He), methane (CH4), ammonia (NH3),phosphine (PH3), water (H2O), carbon monox-ide (CO), simple compounds of silicon andsulfur, and a variety of organic compoundsconsisting of atoms of carbon and hydrogen(e.g., C2H2, C2H4, C2H6). In addition to indi-cating the abundance of these constituents ofthe atmosphere, the infrared spectra also con-tain information on atmospheric structure, thatis, on the variation of temperature and pressurewith altitude. The presence of clouds or dust

38

--Ir."

Rudolph Hanel, infrared spectrometer Principal Inves-tigator

layers can also be inferred from the shapes ofspectral lines.

In addition to its spectroscopy of Jupiter,the Voyager instrument could be used as a heat-measuring device to map the temperatures ofboth the satellites and the atmosphere of Jupi-ter. Particularly interesting for the satellites aremeasurements of the surface cooling andheating rates, since these rates reveal thephysical compactness of the surface, easilydistinguishing between rock and sand or dust.

The infrared instrument is a Michelson in-terferometer at the focus of a gold-platedtelescope of 51-centimeter aperture. The spec-trum is not obtained directly, by moving a prismor grating, but indirectly through the interfer-ence effects of light of different wavelengths:hence its name, an interferometer. The complexinterference pattern generated by the motion ofone of the mirrors in the light path is transmit-ted to Earth, where computer analysis is re-quired to transform it into a recognizable spec-trum. The entire instrument, which has a massof 20 kilograms, is called IRIS, for infrared in-terferometer spectrometer.

The development of the infrared systemfor Voyager posed many problems. IRIS wasdesigned to cover the optimum spectral regionfor studies of the atmospheres of Jupiter andSaturn. However, when it was decided in 1974

4 9

that an extended Voyager mission might alsoallow a visit to Uranus, Hanel and his col-leagues realized that their instrument had seri-ous deficiencies for investigation of that colderand more distant planet, and they proposedthat a modified IRIS (MIRIS) be substitutedfor the original design. A crash program wasauthorized to develop MIRIS in parallel withIRIS, and in early 1977, as launch approached,it appeared that the improved instrument wouldbe ready. Problems occurred during testing,however, and for several weeks in June andJuly the decision hung in the balance. Unfor-tunately, there simply was not enough time tosolve all the problems; both Voyagers werelaunched carrying the original IRIS, and thefinal flight qualification of MIRIS came in Oc-tober, about six weeks after launch. With noother missions planned beyond Saturn, noalternate use for MIRIS has been found; it re-mains "on the shelf," one of the rare caseswhere a technological gamble by NASA did notpay off.

Ultraviolet Spectrometer

A second spectroscopic instrument on theVoyager scan platform examines short-wave, orultraviolet, radiation. The Principal Investi-gator for the ultraviolet spectrometer (UVS) isA. Lyle Broadfoot of Kitt Peak National Ob-servatory in Tucson, Arizona. Broadfoot's ex-pertise lies in instrumentation for atmospheric

Lyle Broadfoot, ultraviolet spectrometer Principal Inves-tigator

I ,iii"I'

research, and he was previously the PrincipalInvestigator for a similar ultraviolet instrumenton the Mariner 10 mission to Venus and Mer-cury. Associated with Broadfoot in the Voyagerinvestigation are fourteen other scientists fromthe United States, Canada, and France.

In the upper atmosphere of a planet, thelighter gases tend to diffuse, rising above theirheavier neighbors, and unusual chemical reac-tions take place as the action of sunlight breakssome chemical bonds and stimulates the forma-tion of others. The study of these tenuousregions of planetary atmospheres is calledaeronomy. Many of the exotic chemical pro-cesses that occur can best be studied by examin-ing radiation of very short wavelength, and it isto these problems that much of the work of theUVS team is directed.

The observations are also sensitive tospecial processes in the Jovian atmosphereresulting from the magnetosphere. At the verytop of the atmosphere, energetic charged par-ticles interacting with the atmost. heric mole-cules produce ultraviolet aurorae that indicatethe location and nature of the bombarding elec-trons and ions. Emissions from atoms in the ex-tended gas clouds around Io and possiblyaround other satellites were also expected to beobservable.

The ultraviolet instrument is a relativelystraightforward optical spectrometer, adaptedfor use in space. A diffraction grating dispersesthe light into a spectrum, and the other opticalelements are gold-coated mirrors. The field ofview is rectangular, about 0.1 degrees x 0.9degrees. An array of sensitive electronic detec-tors provides simultaneous measurements in128 wavelength channels over a wavelengthrange from 50 to 170 nanometers.

Photopolarimeter

The fourth scan-platform instrument isdesigned to measure the brightness and polari-zation of light with high precision. Unlike theimaging system, however; it can look at only asingle point (one pixel) at a time; thus it sac-rifices spatial capability in favor of precision ofmeasurement. The prime objectives of thisphotopolarimeter are related to study of theclouds of Jupiter.

During the development stage for this in-strument, the Principal Investigator wasCharles F. Lillie of the Laboratory for Atmos-pheric and Space Physics of the University ofColorado at Boulder. Later, the PrincipalInvestigator's role was assumed by Lillie's col-

50

39

la

Charles W. Hord, photopolarimeter Principal Investigator

league, Charles W. Hord, a physicist with con-siderable experience in space missions, pri-marily with ultraviolet instruments. Four otherscientists are Co-Investigators on the photo-polarimeter

The instrument itself is essentially a smallCassegrain reflecting telescope of 15-centimeteraperture. Two consecutive filter wheels selectthe color and polarization of the light, which isthen detected and measured by a photomulti-plier tube. The entire photopolarimeter weighsjust 2.5 kilograms.

In spite of their apparent simplicity, how-ever, the Voyager photopolarimeters wereplagued with troubles almost from the momentof launch. A succession of mechanical failureson Voyager 1 led to the sticking first of thepolarization wheel and then of the filter wheel.Even when repeated commands from Earthmanaged to free the wheels, their behavior waserratic. Ultimately an electronic failure also

40

51

took place, leading to the reluctant decision toshut down the Voyager 1 photopolarimeter. On

oyager 2, similar mechanical problems greatlyrestricted the ability of the instrument to carryout its obset rations. The photopolarimeter thuswas unable to contribute its share to unravelingthe mysteries of Jupiter and its satellites.

Planetary Radio Astronomy

The final Voyager remote sensing instru-ment is designed to measure radio emissionfrom Jupiter and Saturn over a wide range offrequencies. These emissions, which sound likehiss or static if played through an audioreceiver, result from interactions of chargedparticles in the magnetospheres and iono-spheres of the giant planets. The planetaryradio astronomy (PRA) Principal Investigatoris James W. Warwick, an astronomer in theDepartment of Astro-Geophysics of the Uni-versity of Colorado at Boulder. Eleven col-leagues from the United States and France par-ticipate as Co-Investigators. Warwick has beenstudying Jupiter longer than any other VoyagerPrincipal Investigator. He has monitored itsradio emissions since the 1960s, and he played acentral role in the discovery that lo influencedthese emissions.

Jupiter emits many kinds of radio radia-tion, ranging from bland thermal emission atshort (centimeter) wavelengths, to synchrotronemission from energetic electrons at interme-diate (decimeter) wavelengths, to erratic, ex-tremely intense bursts at long (meter and deca-meter) wavelengths. The origin of these latter,nonthermal emissions constitutes one of themajor unsolved problems of the Jovian system,

James W. Warwick, planetary radio astronomy PrincipalInvestigator

and, more generally, of the physics of plasmas.One of the most important advantages of theVoyager PRA for studying Jovian radio emis-sion lies in its proximity to Jupiter and hence itsability to locate the sources of differe-t kindsof radio bursts.

The PRA instrument consists of an an-tenna and a radio receiver. The antenna is madeup of two thin metal poles, each 10 meters long,extended from the spacecraft after launch at anangle of 90 degrees to each other. These areelectrically connected to two receivers of ex-tremely high sensitivity and broad frequencyresponse: from 1.2 kilohertz to 40.5 megahertz.The PRA can operate in a number of modes,depending on the measurements desired. At itslowest level of activity, it monitors intensity inall 198 bands and transmits the data at 266 bitsper second. In its highest mode, where searchesare made for variations with very short timescales, the data rate goes up to 108 000 bits persecond, essentially the same as that required byimaging. In fact, the high-rate PRA data actu-ally use an imaging frame as a display form,and occasionally throughout the mission an un-familiar looking "image" was transmitted thatwas actually a block of PRA dataa portraitof electrical events in the Jovian atnwsphereand magnetosphere that only Warwick and hiscolleagues could interpret.

Magnetometer

The first of the direct sensing instrumentsto be discussed is the magnetometer, designedto measure the magnetic fields surrounding the

Norman F. Ness, magnetometer Principal Investigator

spacecraft. Such measurements can be inter-preted to yield the intrinsic fields of Jupiter andits satellites and to characterize, in conjunctionwith data from particle and plasma instru-ments, the processes taking place in themagnetosphere of the planet. The PrincipalInvestigator for this instrument is Norman F.Ness of the NASA Goddard Space Flight Cen-ter. Ness is an intense, competitive scientist

Voyager's 13-meter-long magnetometer boom is shown fully extended. Inspace, under zero gravity conditions, the triangularepoxy glass mast spirals from its housing and provides a rigid support for two magnetometer instrumentsone at the end ofthe boom and another at about the midpoint. [260-1811

01111.111,

Igo

4

with a great deal of previous experience inspacecraft magnetometers, primarily on boardEarth satellites. Ness is also the only VoyagerPrincipal Investigator who has previous experi-ence at Jupiter; he was Principal Investigatoron one of the two magnetometer instrumentsflown on Pioneer 11. For the Voyager inves-tigation, Ness is joined by four colleagues fromGoddard and one from Germany.

The magnetometer instrument consists oftwo systems: a high-field magnetometer and alow-field magnetometer. Each system containstwo identical three-axis magnetometers thatmeasure the intensity and direction of themagnetic field. The low-field system requiresisolation from magnetic fields induced by elec-tric circuits in the spacecraft itself. To achievethis isolation, it is mounted on the largest.com-ponent of Voyagera 13-meter boom, about aslong as the width of a typical city house lot.This boom was coiled tightly in a canister dur-ing launch; later, when the package wasopened, it uncurled and extended automati-cally.

By using two magnetometers, Ness and hiscolleagues are able to correct for the residualartificial magnetic fields that reach even 13meters from the main spacecraft. The dynamicrange extends from a maximum field of 20gauss down to 2 x 10-8 gaussa factor of onebillion. The fields can be measured as fre-quently as 17 times per second. The total mass,exclusive of the 13-meter boom, is 5.6 kilo-grams.

Plasma Particles

Plasma is the term given to a "gas" ofcharged particles; the electrons and protons areseparate, yet there are equal numbers of each,producing a zero net charge. If the velocities ofthe electrons and protons are less than about0.1 percent of the speed of light, they can bemeasured by the Voyager plasma instrument; iftheir energies are higher, they are measured byone of the other two particle instrumentsthelow energy charged particle (LECP) detector orthe cosmic ray detector. The plasma instru-ment, like the magnetometer, was designed toprovide basic data on the particles and fieldsenvironment of Voyager.

The Principal Investigator for the plasmainstrument is Herbert S. Bridge of the Massa-chusetts Institute of Technology in Cambridge,Massachusetts. An experienced space physicist,Bridge has flown similar instruments on many

42

Herbert S. Bridge, plasma particle Principal Investigator

Earth satellites and planetary probes. He alsoholds the position of Director of the Labora-tory for Space Experiments at MIT. On theVoyager experiment, he is joined by one Ger-man and ten U.S. Co-Investigators.

The objectives of the plasma investigationare directed toward study of both the inter-planetary medium and the Jovian magneto-sphere. At Jupiter, Bridge expected to deter-mine the plasma populations and processes inthe inner and outer regions of the magneto-sphere and in the plasma tail that extendsbeyond Jupiter, much as a comet tail is blownoutward by the solar wind. The densities andtemperature of electrons were measured, andtheir origins determined: Some originate nearJupiter and diffuse outward through the mag-netosphere; others derive from the solar wind.

The plasma instrument, with a mass of 9.9kilograms, was designed to view in two direc-tions: one toward the Earth and Sun, primarilyto study the solar wind, and the other sideways,looking toward the direction plasma wouldflow if it were caught up in the rotating Jovianmagnetic field. If it is desired to look in otherdirections, the entire spacecraft must be tipped,a maneuver that was carried out several timesnear Jupiter. The detectors directly sense theflow of electrons, protons, and alpha particles(helium nucleii, made up of two protons andtwo neutrons each). Analysis of the energyspectra can also yield data on positive ions ofhigher mass.

M 1-_-)%J O

Plasma Waves

The plasma wave investigation on Voyagerwas a late addition to the scientific payload.It was selected to broaden the capability ofthe mission to study a wide variety of plasmaprocesses. Because of the electrically chargednature of the plasma, it responds to energyinputs in ways that ordinary gas cannot. Oneof these modes of response yields plasmawaves, which are oscillations in density andelectric field that generally cover the audiorange of frequencies. Measurement of suchwaves characterizes tne density and tempera-ture of the local plasma surrounding the space-craft, and it also allows remote sensing detec-tion of distant events from the plasma wavesthey produce.

The plasma wave Principal Investigator isphysicist Frederick L. Scarf of the TRWDefense and Space Systems Group of RedondoBeach, Californiahe is the only VoyagerPrincipal Investigator to come from industry.Scarf has been associated with many particlesand fields investigations in the terrestrial mag-netosphere, although he is better known as atheorist than as an experimenter. A member ofthe Space Science Board of the National

Frederick L. Scarf, plasma wave Principal Investigator

1.

Academy of Sciences, he is familiar in Washing-ton as an eloquent advocate of space physicsthe study of plasma-physical processes in thespace environment.

The plasma wave instrument shares withthe planetary radio astronomy investigation apair of 10-meter-long antennas. Whereas thePRA uses these as electric antennas to detectradio radiation, the plasma wave system usesthem to detect directly the oscillations in theplasma near the spacecraft. Waves are meas-ured over a broad frequency range, from 10hertz (a bit deeper than the lowest bass note wecan hear) to 56 kilohertz (about three timeshigher than the highest pitch to which thehuman ear responds). The instrument elec-tronics have a total mass of only 1.4 kilograms.

Low Energy Charged Particles

Charged particles with energies greaterthan a few thousand electron volts are not easilymeasured by a plasma instrument such as thatdesigned by Herb Bridge. Instead, these fastermoving particles, with speeds up to a few per-cent the .,peed of light, are the subject of a pairof Voyager instruments called collectively theLECP, or low energy charged particle instru-ment. Like the other particles and fields investi-gation, the LECP is designed to provide basicdata on plasma-physical processes in the Jovianmagnetosphere and the solar wind, and on theirinteractions.

The Principal Investigator for the LECPinvestigation is Stamatios Mike Krimigis, aGreek-born physicist from Johns HopkinsUniversity. Krimigis has participated in anumber of satellite studies of the terrestrialmagnetosphere, and he now serves as Head ofSpace Physics and Instrumentation at the JohnsHop: ins Applied Physics Laboratory. He isjoined in this investigation by one German andfive U.S. Co-Investigators.

The LECP instrument consists of two sub-systems. The first, called the low energy mag-netospheric particle analyzer, is optimized formeasurement of particles within the Jovianmagnetosphere, with high sensitivity over abroad dynamic range. Measurements of elec-trons, protons, and other positive ions can becarried out, determining the energy and com-position of individual particles. The totalenergy ranges covered are 10 kiloelectron volts(keV) to 11 million electron volts (MeV) forelectrons and 15 keV to 150 MeV for protonsand ions.

Oti

43

Stamatios Mike Krimigis, low . iergy 4.ltzrged particle Prin-cipal Investigator

The second LECP subsystem is a lowenergy r'iarged particle telescoi.e, designed tooperan..,:here the density of charged particles islow, such as in interplanetary space or the outermagnetosphere of Jupiter. For protons and.,-Nsitive ions, the energy range is from 50 keVto 40 MeV per nucleon. The energy and speciesresolution is again sufficient to determine thecomposition, both chemical and isotopic, ofmany ions encountered. In order to providedirectional discrimination even on a spacecraftof fixed orientation, both LECP subsystems aremounted on a moving platform that stepsthrough eight positions in a time that can becommanded to vary from 48 seconds to 48minutes. The mass of the instrument and itsplatform is 6.7 kilograms.

Cosmic Rays

The solar system is constantly bombardedby extremely energetic charged particles. Theseare called cosmic rays, although they are parti-cles, not photons"rays" are only producedwhen the particles strike something, such as themolecules of the Earth's atmosphere, and giveup their energy in a flash of x-rays and gamma-rays. One of the Voyager instruments isdesigned to study these galactic cosmic rays,particularly to look from beyond the orbit ofSaturn, where the cosmic ray particles will beless affected by the solar magnetic field andsolar wind than they are near Earth.

44

The cosmic ray Principal Investigator isRochus E. Vogt of the California Institute ofTechnology. Vogt has measured cosmic raysfrom the ground, from balloons, and fromspacecraft for many years. During 1977 and1978 he served as Chief Scientist at JPL, andthen assumed the job of directing the physics,mathematics, and astronomy programs atCaltech. Among his six Co-Investigators is EdStone, the Voyager Project Scientist.

Because the cosmic ray instrument was notdirected principally toward measuitments ofthe Jovian system, it is described only briefly.L.ke the LECP, it is designed to determine theenergy and composition of individual electronsand positive ions. For electrons, the energyrange is from 3 to 110 MeV, and for ions from 1to 500 MeV per nucleon; the correspondingvelocities are from about 10 percent to 99 per-cent of the speed of light. For the positive ions,composition can be determined for elementsfrom hydrogen to iron. At Jupiter, this systemcould be used to determine the nature of therare particles accelerated to very high energiesin the Jovian magnetosphere.

Radio Science

The final Voyager science investigation isin the field of radio science. No special instru-ment was required for this study; rather, NASAselected members of a Radio Science Team whoproposed investigations that could be carriedout using the already existing spacecraft tele-communication system.

The radio science Team. ..ader is Von R.Eshleman of tIo. Center for Radio Astronomyat Stanford University. Eshleman is a radarphysicist who has been interpreting spacecraftradio occultation data since the first such probewas carried ut when Mariner 4 passed behindMars in 1964. The Deputy Team Leader is G.Leonard Tyler, a colleague of Eshleman's atStanford. There are five other radio team mem-bers, four of them from JPL.

The radio science investigations aredivided into two groups. The first deals with theatmosphere of Jupiter. During the Voyager fly-bys, the spacecraft passed behind the planet asseen from Earth, and the i adio signal wasdimmed by the atmosphere before it was finallyextinguished. During an occultation, the propa-gation of the radio waves is slowed down bypassage through the neutral atmosphere and isspeeded up by passage through the electricallycharged ionosphere. Because of the extreme

Rochus E. Vogt, cosmic ray Principal Investigator

stability of the ground-based and spacecraftradio transmitters, it is possible to measurethese shifts in the signal with high precision.The shifts are proportional to electron densityfor the ionosphere, and to gas density for theatmosphere. From a careful study of the inter-actions of the transmitted beam with the Jovianatmosphere, Eshleman and his colleagues canreconstruct a temperature-pressure profile ofthe ionosphere and the upper atmosphere ofJupiter. The same approach can be used tosearch for tenuous atmospheres on the satellites.

The second area of study is in the field ofcelestial mechanics. The frequency stability ofthe communications system permits measure-ments of the speed of the spacecraft, relative toEarth, to a precision of one part in severalmillion. By careful tracking, gravitational per-turbations on the spacecraft can be detectedand used to measure the gravitational fields,

t

Von R Eshleman, radio science Team Leader

and hence the masses, of Jupiter and itssatellites.

These scientific instruments and theirobjectives were selected many years beforethe first Jupiter encounter in March 1977.Because Voyager was an exploratory mission,every effort was made to fly versatile instru-ments that could yield valuable results no mat-ter what the nature of the Jovian system. Inaddition, the Voyager spacecraft control systempermitted the instruments to receive commandsfrom Earth to adjust their sensitivities andobserving sequences in response to new infor-mation. By the spring of 1977, all the instru-ments were completed, ready to be installed inthe Voyager spacecraft for testing and launch.

45

CHAPTER 5

THE VOYAGE TOJUPITER-GETTING

THERE

Launch

On August 20, 1977, exactly two yearsafter the launch of the Viking spacecraft toMars, the first of the VoyagersactuallyVoyrger 2was boosted into space at 10.29a.m. EDT, less than five minutes after thelaunch window opened on the first day of thethirty-day launch period. Sixteen days later, at8:56 a.m. EDT on Labor Day, September 5,1977, Voyager 1 was hurled into space on ashorter, faster trajectory than its twin, zippingpast the orbit of the Moon only ten hours afterlaunch. Ultimately, Voyager 1 earned its title byovertaking Voyager 2 as bon spacecraft jour-neyed through the ast,roid belt, to arrive atJupiter four months ahead of Voyager 2.

The Voyagers lifted off from LaunchComplex 41, Ai Force Eastern Test Range,Kennedy Space Center, Cape Canaveral,Florida, atop the giant Titan III-E/Centaurrocket. It was the last time such a launch vehiclewas scheduled to be used, as, according to plan,the Space Shuttle would take over in the 1980s.Thus the launching of the two Voyagers signi-fied both an end and a beginning: a once-in-a-lifetime opportunity to explore, in only 81/2years' time, perhaps fifteen major bodies of theouter solar system.

But long before even the first Voyager wasto make its closest approach to Jupiter-1nfact, even before Voyager 2 was off the launchpadthere were problems to overcome.

In early August 1977, about three weeksbefore launch, failures in the attitude and ar-

ticulation control subsystem (AACS) and theflight data subsystem (FDS), two of the space-craft's three main computer subsystems, pre-vented the VGR77-2 spacecraft, originallyscheduled for launch on August 20, frombecoming Voyager 2. Instead, the "spare"spacecraft VGR77-3 was substituted, becomingVoyager 2 upon launch August 20, andVGR77-2, after proper repairs, becameVoyager I. Minor problems continued right upto launch. The low energy charged particle in-strument failed and had to be replaced, and aslate as T minus five minutes there was a halt inthe countdown to check on a stuck valve. Un-like a jinxed dress rehearsal, which is said to"assure" an opening-night success, Voyager 2'sprelaunch problems were a portent of difficul-ties to come.

The Voyager 2 launch was witnessed bythousands as the spacecraft ascended gracefullyinto the blue Florida sky, accompanied by thedeep-throated rumblings of its rocket, echoingfor miles across the beaches and scrub forestsof Cape Canaveral. The Titan-Centaur per-formance was nearly flawless, and Voyager 2quickly achieved an accurate trajectory towardJupiter. However, even while the engines werestill firing, the spacecraft began to experience a

baffling series of problems that would absorbthe attention of hundreds of persons fromPasadena to Washington, D.C., for the nextseveral weeks until they were brought undercontrol.

During the first minutes of flight, thereseemed to be two difficulties with the AACS.

The first picture to capture crescent Earth and crescent Moon in the same frame was taken by Voyager I, the second-launched spacecraft, on September 18, 1977, at a distance of 12 midion kilometers from Earth. Or the Earth eastern Asia,the western Pacific, and part of the Arctic can be seen. Since the Moon is much less reflective than the Earth, JPL imageprocessors brightened the lunar image by a factor of three to ensure that both Earth aid Moon were visible on this print.(P-19891CI

r )L.,. c )

The Titan/Centaur rocket used to launch Voyager stood astall as a I5-story building and weighed nearly 700 tons.Here the rocket waits for launch at Kennedy Space Center,with the Voyager spacecraft enclosed in the white protectiveshroud at the top. IP-19471AI

48

The first was a problem with one of the threestabilizing gyroscopes, but fortunately, thegyroscope began operating normally withoutintervention from the ground. The other prob-lem appeared to be with one of the AACS com-puters; the spacecraft switched to a backupcomputer during the Titan burn, and initialdata transmissions were incomplete. Earlyanalysis seemed to indicate that an event duringthe launch itself, rather than a faulty spacecraftcomputer system, was the cause of the dataloss. At first, on August 23, officials suspectedthat perhaps the spacecraft had been bumpedby the rocket motor one hour after liftoff andagain about seventeen hours later, whentelemetry signals indicated that the spacecrafthad been jolted. However, by the next day,flight engineers determined that electronic gyra-tions in the AACS seemed to have caused thedifficulty.

Within an hour after launch, Voyager 2'sscience scan platform boom was to have beenfully extended and locked. Instructions todeploy were given, and the boom moved out-ward; however, there was no signal to indicatethat the boom was actually locked in place. Ef-forts to command the boom to move into thelocked position were thwarted by the space-craft. The first maneuver designed to try to lockthe boom was aborted by the computer com-mand subsystem (CCS) when the AACS errone-ously indicated that it was in trouble. Threedays later another maneuver was scheduled toreprogram the faulty computer in the AACS, toalign the Sun sensors, and to try to lock thescience boom. To provide a direct check of theboom position, the scan platform was turned sothat the TV cameras could see the spacecraft.Careful measurement of these pictures verifiedthat the boom was within 1/2 degree of fulldeployment, but still there was no indicationthat it was locked into place. Ultimately, itwas decided that the sensor to signal actuationof the lock was at fault, and that the boomitself was almost certainly fully extended andoperational.

Because of the postlaunch problems ofVoyager 2, the launch of Voyager 1 was delayedtwicefrom September 1 to September 3 andthen to September 5in order to inspectVoyager l's science boom and to try to preventa repetition of Voyager 2's problems. An extraspring was attached to the science boom toassure its full extension. Finally, as if to makeup for the troubles of the first launch, Voyagerl's launch was both "flawless and accurate."

Eb

THE TITAN/CENTAUR LAUNCH VEHICLE

The two Voyager spacecraft were carried into space and accelerated toward Jupiter by theTitan 111-E Centaur rocket, the largest launch vehicle in the NASA arsenal after the retirement ofthe Saturn rockets in 1975. The Titan and Centaur vehicles were originally developed separatelyand have been used with other rocket stages for many NASA launches. They were first combinedfor the two Viking launches to Mars in 1975, and this powerful four-stage launch vehicle wasused again in 1977 for Voyager.

The Titan/Centaur stands nearly 50 meters tall, about the height of a fifteen-story building.Fully fueled, it weighs nearly 700 tons. At takeoff, the thrust of the two solid-propellant Stage-0motors is about 10.7 million newtons. These motors, which burn for 122 seconds, use powderedaluminum as fuel and ammonium perchlorate as oxidizer. Together, they have a mass of 500tons.

The first stage of the liquid propellant core of the Titan rocket ignites about 112 secondsafter takeoff. The propellant is hydrazine as fuel and nitrogen tetroxide as oxidizer. The firststage is 3 meters in diameter and 20 meters tall. Fueled, it has a mass of 130 tons. The motor pro-vides a thrust of 2.5 million newtons for a duration of 146 seconds.

About 4.3 minutes after takeoff the Titan Stage II liquid propellant motor begins to fire,and the first stage is separated and falls back into the Atlantic. The second stage is 3 meters indiameter and more than 7 meters long, with a fueled mass of 35 tons. The single liquid fuel motorburns for 210 seconds with a thrust of half a million newtons. During the second stage burn, theshroud covering the Voyager spacecraft is jettisoned.

T1._ Centaur and Titan vehicles separate 8 minutes into the flight, and the Centaur mainengine begins its burn. The Centaur is nearly 20 meters tall and 3 meters in diameter, with a massof 17 tons. The motors have a thrust of almost 200 000 newtons, operating on the most powerfulchemical fuels known: liquid oxygen and liquid hydrogen. The Centaur burns for only 1 minuteand 36 seconds as it attains Earth parking orbit; the engine then shuts down as the vehicle beginsa half-hour coasting period that carries it nearly half way around the Earth. During this time,careful tracking of the spacecraft supplies the data needed for Earth-based computers to calcu-late the proper time to leave parking orbit and start the long trip toward Jupiter.

About 50 minutes after liftoff, from a position high above the Indian Ocean, the secondburn of the Centaur main engine begins. Six minutes of additional thrust provides enough energyto break out of Earth's orbit. The Voyager then separates from the Centaur for a final boosttoward Jupiter. The solid rocket motor in the spacecraft propulsion module (acting as final stageof this five-stage launch sequence) fires for 45 seconds at a thrust of 68 000 newtons. Just anhour after liftoff, the Voyager spacecraft is on its way, coasting on an orbit toward Jupiter at aspeed of more than 10 kilometers per second.

The Voyager spacecraft is dominated by the large 3.7-meter- diameter antenna used for communication withEarth. Here the spacecraft undergoes final tests beforelaunch. The science instrument scan platform is foldedagainst the spacecraft on the right, the three cylinders onthe left are the RTG power sources. 1260-108BC]

C0

Voyager 2 was the first of the spacecraft to be launched, onAugust 20, 1977, propelled into space in a Titan/Centaurrocket. [P-19450ACJ

50

All launch and postlaunch events wentsmoothly. The launch window opened at 8:56a.m. EDT, and Voyager took off promptly at8:56:01. The booms and antennas deployed andlocked in the first hours after launch; all in-struments scheduled to be on were on andworking well.

The First Year Is the Roughest

During the autumn of 1977 Voyager 2, andto a lesser extent Voyager 1, continued toplague controllers with erratic actions. Thrust-ers fired at inappropriate times, data modesshifted, instrument filter and analyzer wheelsbecame stuck, and the various computer controlsystems occasionally overrode ground com-mands. Apparently, the spacecraft hardwarewas working properly, but the computers .onboard displayed certain traits that seemedalmost humanly perverseand perhaps a littlepsychotic. In general, these reactions were theresult of programming too much sensitivity intothe spacecraft systems, resulting in panic over-reaction by i... onboard computers to minorfluctuations in the environment. Ultimately,part of the programming had to be rewritten onEarth and then transmitted to the Voyagers, tocalm them down so that tl,'..., would ignoreminor perturbations, yet still be ready to per-form automatic sequences required to protectthe spacecraft from major threats. Meanwhile,however, more serious problems were develop-ing.

On February 23, 1978, during a series ofmovements or stews, Voyager I's scan platformslowed and stopped before completing themaneuver. This failure caused a great deal ofconcern, since the scan platform houses the op-tical instruments that are crucial to the observa-tion of the Jovian systemthe ultraviolet spec-trometer, the IRIS, the photopolarimeter, andthe two TV cameras. At JPL, tests were run ona proof-test modelan exact copy of the Voya-ger spacecraftto try to find out why Voyagerl's scan platform had become stuck. On March17, Voyager l's scan platform wls testedJPLengineers instructed the platform to moveslowly for a short distance, and Voyagerresponded as ordered. Further tests were con-ducted on March 23. This time the scan plat-form was ordered to execute a sequence of fourstews, moving away from the part of the skywhere the original failure had occurred andending with the position that it would be mostuseful to leave the platform injust in case theplatform should become stuck again. On April

4 the scan platform was commanded to per-form a sequence of 38 slews, and fifty moreslews were performed on April 5. All were suc-cessful. Yet engineers were still hesitant to forcethe platform to move through the region whereit had originally stuck, and extensive discus-sions were held to determine if the Jupiterobservations could be carried out without risk-ing a return to the danger area. It was argued,however, that full mobility of the scan platformreally was required, and on May 31 commandswere sent to maneuver the scan platformthrough the danger region. It moved normally:The scan platform was operating properlyagain. After additional slewing tests were run inmid-June, the scan platform was pronouncedfit for operation. Engineers suspected that thematerial caught in the platform gears must havebeen crushed or moved out of the way by thecontinued slewing, allowing the platform tomove once more.

An even more serious crisis soon endan-gered the Voyager 2 spacecraft. In lateNovember 1977, the S-band radio receiverbegan losing amplifier power in its high-gainmode. so the solid-state amplifier was switchedto its low-power position. No further problemswere noted until April 5, 1978, when Voyager2's primary radio receiver suddenly failed, andshocked engineers discovered that the backupreceiver was also faulty. The trouble wasdetected after Voyager's computer commandsubsystem directed the spacecraft to switchfrom the primary radio receiver to the backupreceiver. This command was issued as part of aspecial protection sequence: If the primaryradio receiver receives no commands fromEarth for seven days, the backup receiver isswitched on instead; if the secondary receiver inurn receives no instructions over a twelve-hour

period, the system reverts to the main receiver.When, on April 5, Voyager 2's radio receptionwas switched from the primary to the secondaryreceiver, flight engineers found that they wereunable to communicate with the spacecraftthe secondary receiver's tracking loop capacitorwas malfunctioning. That meant that the sec-ondary receiver could not follow a changingsignal frequency sent out from Earth. The fre-quencies of signals transmitted from Earth areaffected by the Doppler effectjust as the sirenon a fire engine seems first to rise in pitch as thetruck approaches, then falls as the truck speedsaway, so the frequency of signals transmittedfrom Earth fluctuates with the Earth's rotationas the Deep Space Network's radio antennas

move toward or away from the spacecraft. Theengineers had to wait until the primary radioreceiver was switched back on before they couldcommunicate with the spacecraft. Once the pri-mary receiver was on, Voyager 2 began receiv-ing instructions from Earth, but approximatelythirty minutes later, there was an apparentpower surge in the receiver. The fuses blew.

:11

IIIIIIi..-

?

«14'

Voyager I was launched on September 5, 1977. The launchwas delayed 5 days to make last-minute adjustments toavoid the postlaunch difficulties experienced by Voyager 2.(P- I 9480M1

C2

51

Uranus8/20/77

Voyager

Uranus1/30/86

Saturn r Saturn8/27/81 / 11/13/80

Saturn8/20/77

Jupiter7/9/79

Jupiter3/5/79

Jupiter-Saturn-Uranus

Jupiter8/20/77 /

Each Voyager spacecraft follows a billion-kilometer path toJupiter. Except for minor thruster firings to achioe smalltrajectory corrections, each Voyager coasts from Earth toJupiter, guided by the grautational pull of the Sun AtJupiter, the powerful tug of the giant planet defleLts thespacecraft and speeds them up, imparting an extra kick t9send them on their way toward Saturn

There was no recourse. The main receiver hadfailed; its loss was permanent. It remained forthe engineers to devise a way to communicatewith the slightly deaf spacecraft.

Because the switching of the radio re-ceivers was still controlled by the special protec-tion sequence discussed earlier, flight engineerswould have to wait for seven daysuntil April13before they could attempt communicationwith the spacecraft again. During that weekspecial procedures were established andrehearsed so that commands could be sent toVoyager in the short time that the backupreceiver would he on. On Thursday, April 13,1978, the seven days were up and the spacecraftshould have shifted from the dead main re-ceiver to the sick backup system. There was justa twelve-hour "window" in which to restorecommunication. At about 3:30 a.m. PST theMadrid tracking station of the Deep Space Net-work sent its first order to the spacecraft, ap-proximately 474 million kilometers away. Al-most an hour later, word arrived from Voyagerthat the command had been accepted. (One-way light time for a signal to travel the distancefrom Earth to Voyager at that time was almost27 minutes.) Elated flight controllers went

52

ahead and transmitted nine hours of commandsto the spacecraft.

Voyager 2 was successfully commandedagain on April 18 and April 26. The April 26commands included a course change maneuverthat was executed properly on May 3. On June23, Voyager 2 was programmed for a backupautomatic mission at Saturn in the event thatthe secondary radio receiver should also fail.These backup mission instructions wouldoperate all the science experiments, but only aminimum amount of data would be returned,since the scan platform would only be pro-grammed to move through three positionsrather than thousands as it would in normaloperation. Instructions for a backup minimumautomatic encounter at Jupiter were trans-mitted to Voyager 2 in two segments, thesecond of these on October 12, 1978.

With the backup instructions recorded onboard the spacecraft, Voyager personnel felttheir fears partially allayed. If Voyager 2'ssecondary radio receiver failed, the spacecraftwould still obtain some science data at Jupiterand Saturn. But that would mean that therewould be no mission beyond Saturn; our firstopportunity to explore Uranus, its satellites, itsnewly discovered ring system, and possibly evento get a look at Neptune, would not come inthis century.

Another major concern affecting bothVoyager spacecraft was the proper manage-ment of hydrazine fuel reserves. Hydrazine isused by the thrusters on the Voyagers forstabilization of the spacecraft and for tra-jectory correction maneuvers (TCM). EachVoyager was loaded with 105 kilograms ofhydrazine budgeted for use on the long flight toJupiter, Saturn, and beyond. Because of theexcellent performance of the launch rockets;both Voyagers required less hydrazine thananticipated for their final boost into propertrajectory toward Jupiter, and at first it lookedas though both spacecraft would have plenty ofpropellant to spare.

Charles E. Kolhase, Manager of MissionAnalysis and Engineering for the Voyager Proj-ect, later explained the situation: "Voyager 1should have been launched September I. Had itbeen launched on September Iand I'm glad itwasn't the maneuver to correct the trajectoryfor a Titan flyby would have required a changein velocity of 100-110 meters per secondanenormous maneuverand we would have had apropellant margin foi going on to Saturn of

(3 3

THE DEEP SPACE NETWORK

A vital component of the Voyager Mission is the communications system linking thespacecraft with controllers and scientists on Earth. The ability to communicate with spacecraftover the vast distances to the outer planets, and particularly to return the enormous amounts ofdata collected by sophisticated cameras and spectrometers, depends in large part on thetransmitters and receivers of the Deep Space Network (DSN), operated for NASA by JPL.

The original network of these receiving stations was established in 1958 to provide round-the-world tracking of the first U.S. satellite, Explorer 1. By the late 1970s, the DSN had evolvedinto a system of large antennas, low-noise receivers, and high-power transmitters at sites strategi-cally located on three continents. From these sites the data are forwarded (often using terrestrialcommunications satellites) to the mission operations center at JPL.

The three DSN stations are located in the Mohave desert at Goldstone, California; nearMadrid, Spain; and near Canberra, Australia. Each location is equipped with two 26-metersteerable antennas and a single giant steerable dish 64 meters in diameter, with approximately thecollecting area of a football field. In addition, each is equipped with transmitting, receiving, anddata handling equipment. The transmitters in Spain and Australia have 100-kilowatt power,while the 64-meter antenna at Goldstone has a 400-kilowatt transmitter. Most con.mands toVoyager are sent from Goldstone, but all three stations require the highest quality receivers topermit continuous recording of the data streams pouring in from the spacecraft.

Since the mid-1960s, the DSN's standard frequency has been S-band (2295 megahertz).Voyager introduces a new, higher frequency telemetry link at X-band (8418 megahertz). TheX-band signal can carry more information than S-band with similar power transmitters, but itrequires more exact antenna performance. In addition, the X-band signal is absorbed byterrestrial clouds and, especially, rain. Fortunately, all three DSN stations are in dry climates,but during encounters the weather forecasts on Earth become items of crucial concern if preciousdata are not to be lost by storm interference.

As a result of the development of larger antennas and improved electronics, the DSNcommand capabilities and telemetry data rates have increased dramatically over the years. Forexample, in 1965 Mariner 4 transmitted from Mars at a rate of only 81/2 bits of information persecond. In 1969, Mariners 6 and 7 transmitted picture data from Mars at 16 200 bits per second.Mariner 10, in 1973, achieved 117 200 bits per second from Mercury. Voyager operates at asimilar rate from Jupiter, about six times farther away. Many of these improvements in datatransmission result from changes in the DSN rather than in the spacecraft transmitters.

perhaps 4.5 kilograms. But, by launching onthe fifth of September we increased our marginto 23 kilograms. Fortunately, for every launchdate that went by, that velocity change maneu-ver was shrinking at a rate of 10 meters persecond per day. Now, a 1 meter per secondchange uses about a pound of hydrazine [about0.5 kilogram]. So when we launched on thefifth of September, now we suddenly had 40pounds of hydrazine excess over what we wouldhave had if we had launched on the first ofSeptember. As a result, Voyager I is in greatshape as far as hydrazine is concerned."

Problems with hydrazine managementdeveloped, however. Voyager l's first trajectorycorrection maneuver achieved only 80 percentof the required speed change. Exhaust plumesfrom the thrusters apparently struck part of the

spacecraft, causing a 20 percent loss in velocity.That being the case, Voyager might requiremore fuel than had been expected to completethe mission. The extra fuel requirements didnot threaten Voyager 1 itself, since it heldample fuel to reach Saturn; the concern was forVoyager 2, where the effective loss of fuelmight be enough to jeopardize the Uranus mis-sion.

Because of the plume impingement prob-lem on Voyager 1, Voyager 2's first trajectorycorrection maneuver was adjusted to allow forthe possibility of a 20 percent loss in thrust. TheVoyager 2 maneuver was successful, but con-trollers felt that additional action was requiredto conserve fuel. One way to save was by reduc-ing requirements on control of the spacecraftorientation. Less control fuel would be needed

53

.

or,

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11

if the already miniscule pressure exerted on thespacecraft by the solar wind could be reduced.Flight engineers at JPL calculated that thepressure would be reduced if the spacecraftwere tipped upside down; however, to accom-plish this, the spacecraft would have to besteered by a new set of guide stars. By repro-gramming the attitude control system it wasfound possible to substitute the northern star,Deneb, in the constellation of Cygnus, for theoriginal reference star, Canopus, in thesouthern constellation of Carina. With thischange, as well as readjustment of Voyager 2'strajectory near Jupiter, inflight consumption ofhydrazine was reduced significantly.

In late August 1978 both Voyagers werereprogrammed to ensure better science resultsat Jupiter encounter; for example, the repro-gramming would prevent imaging (TV) photo-graphs from blurring when the tape recorderwas operating. By early November, flight crewshad begun training exercises to rehearse for theVoyager 1 flyby of Jupiter on March 5, 1979. Anear encounter test was performed onDecember 12-14, 1978: a complete runthroughof Voyager 1's 39-hour near encounter period,which would take place March 3-5, 1979. Par-ticipants included the flight team, the DeepSpace Network tracking stations, the scientists,and the spacecraft itself. Results: Voyager andthe Voyager team were all ready for the en-counter.

Meanwhile, the spacecraft were busyreturning scientific data to Earth. Technically,the Voyagers were in the cruise phase of themissiona period that, for Voyager 2, wouldlast until April 24, 1979, and for Voyager 1, un-til January 4, 1979, when each spacecraft wouldenter its respective observatory phase.

Cruise Phase Science

In the first few days after launch, thespacecrafts' instruments were turned on andcalibrated; various tests for each instrumentwould continue to be performed throughout thecruise phase. This period presented a great op-portunity for the Voyagers to study the inter-planetary magnetic fields, solar flares, and thesolar wind. In addition, ultraviolet and infraredradiation studies of the sky were performed. Inmid-September 1977 the television cameras onVoyager 1 recorded a number of photographsof the Earth and Moon. A photograph takenSeptember 18 captured both crescent Moon andcrescent Earth. It was the first time the two

56

celestial bodies had ever been photographedtogether.

In November both Voyagers crossed theorbit of Mars, entering the asteroid belt amonth later. On December 15, at a distance ofabout 170 million kilometers from Earth, Voya-gcr 1 finally speeded past its slower twin. Thejourney through the asteroid belt was long butuneventful: Voyager 1 emerged safely in Sep-tember 1978, and Voyager 2 in October. UnlikePioneers 10 and 11, the Voyagers carried noinstruments to look at debris in the asteroidbelt.

By April both spacecraft were already half-way to Jupiter and, about two months later,Voyager 1, still approximately 265 million kilo-meters from Jupiter, began returning photo-graphs of the planet that showed considerabledetail, although less than could be obtainedwith telescopes on Earth. Both the imaging(TV) and the planetary radio astronomy in-struments began observing Jupiter, and, byOctober 2, 1978, officials announced that "thepolarization characteristics of Jupiter's radioemissions have been defined. In the high fre-quencies, there is consistent right-hand circularpolarization, while in the low frequencies, thereis a consistent left-hand circular polarization.This was an unexpected result." In addition toscientific studies of the interplanetary mediumand a first look at Jupiter, the plasma wave in-strument, which studies waves of charged par-ticles over a range of frequencies that includesaudio frequencies, was able to record the soundof the spacecraft thrusters firing, as hydrazinefuel is decomposed and ejected into space. Thesound was described as being "somewhat like a5-gallon can being hit with a leather-wrappedmallet."

On December 10, 1978, 83 million kilo-meters from Jupiter, Voyager 1 took photo-graphs that surpetssed the best photographs evertaken from ground-based telescopes, and scien-tists were anxiously awaiting the start of con-tinuous coverage of the rapidly changing cloudforms. These pictures, together with data fromseveral ground-based observatories, were care-fully scrutinized by a team of scientists at JPLto select the final targets in the atmosphere ofJupiter to be studied at high resolution duringthe flyby. The observatory phase of Voyager1's journey to Jupiter was about to begin.

The Observatory PhaseThe observatory phase, originally sched-

uled to start on December 15, 1978, eighty days

67

b

d

As Voyager 1 approached Jupiter, the resolution of the images steadily improved. In October 1978 (a), at a distance of about125 million kilometers, the image was less clear than would be obtained with an Earth-based telescope. By December 10 (b)the spacecraft had moved to a distance of 85 million kilometers, and the resolution was about 2000 kilometers, comparableto the best telescopic images. On January 9, 1979 (c), at a distance of 54 million kilometers, the image surpassed all ground-based views and approached the resolution of the Pioneer 10 and 11 photos. In (d), taken January 24 at a distance of 40million kilometers, the resolution exceeded 1000 kilometers. (P-20790; P-20829C; P-20926C; P-20945C]

57

VU

before encounter, was postponed until January4, 1979, to provide the flight team a holiday-season break. For the next two months, Voya-ger would carry out a long-term scientificstudya "time history"of Jupiter, its satel-lites, and its magnetosphere. On January 6,Voyager I began photographing Jupiter everytwo hourseach time taking a series of fourphotographs through different color filters aspart of a long-duration study of large-scale at-mospheric processes, so scientists could studythe changing cloud patterns on Jup.ter. Eventhe first of these pictures showed that the at-mosphere was dynamic "with more convectivestructure than had previously been thought."Particularly striking were the changes in theplanet, especially near the Great Red Spot, thathad taken place since the Pioneer flybys in 1973and 1974. Jupiter was wearing a new face forVoyager.

In mid-January, photos of Jupiter werealready being praised for "showing exceptionaldetails of the planet's multicolored bands ofclouds." Still 47 million kilometers from thegiant, Voyager I had by now taken more than500 photographs of Jupiter. Movements ofcloud patterns were becoming more obvious;feathery structures seemed painted across some

of the bands that encircle the planet; swirlingfeatures were huddled near the Great Red Spot.The satellites were also beginning to look morelike worlds, with a few bright spots visible onGanymede and dark red poles and a brightequatorial region clearly seen on lo when itpassed once each orbit across the turbulent faceof Jupiter.

From January 30 to February 3, Voyagersent back one photograph of Jupiter every 96seconds over a 100-hour period. Using a totalof three different color filters, the spacecraftthus produced one color picture every 43/4 min-utes, in order to make a "movie" coveringten Jovian "days." To receive these pictures,sent back over the high-rate X-band transmit-ter, the Deep Space Network's 64-meter anten-nas provided round-the-clock coverage.Voyager I was ready for its far encounterphase.

Far Encounter Phase

By early February Jupiter loomed too largein the narrow-angle camera to be photographedin one piece; 2 x 2 three-color (violet, orange,green) sets of pictures were taken for the nexttwo weeks; by February 21, Jupiter had grown

rhe Galilean satellites of Jupiter first began to show as tiny worlds, not mere points of light, as the Voyager I observatoryphase began. In this view taken January 17, 1979, at a range of 47 million kilometers, the differing sizes and surface reflet:-twines (albedos) of Ganymede (right center) and Europa (top right) are clearly visible. The view of Jupiter isunusual in thatthe Great Red Spot is not easily visible, but can just be seen at the right edge of the planet. Most pictures selected for publica-tion include the photogenic Red Spot. [P-20938C1

fi

691

By February 1, 1979, Voyager I was only 30 million kilometers from Jupiter, and the resolution of the imaging system cor-responded io about 600 kilometers at the cloud tops of the giant planet. At this time, a great deal of unexpected complexitybecame apparent around the Red Spot, and movie sequences clearly showed the cloud motions, including theapparent six-day rotation of the Red Spot. (P-20993C1 -`1.=,'''' PJ

One of the most spectacular planetary pht,tographs ever taken was obtained on February 13 as Voyager 1 continued itsapproach to Jupiter. By this time, at a range of 20 million kilometers, Jupiter loomed too large to fit within a single narrow-

angle imaging frame. Passing in front of the planet are the inner two Galilean satellites. lo, on the left, already showsbrightly colored patterns on its surface, while Europa, on the right, is a bland ice-covered world. The scale of all of these

objects is huge by terrestrial standards, lo and Europa ate each the size of our Moon, and the Red Spot is larger than the

Earth. LP- 21082C1

too large even for that, and 3 x 3 sets werescheduled to begin. When these sets of picturesare pasted together to form a single picture, theresult is called a mosaic.

While the imaging experiments were in thelimelight, the other scientific instruments hadalso begun to concentrate on the Jovian system.The ultraviolet spectrometer had been scanningthe region eight times a day; the infrared spec-trometer (IRIS) spent I1/2 hours a day analyzinginfrared emissions from various longitudes inJupiter's atmosphere; the planetary radioastronomy and plasma wave instrumentslooked for radio bursts from Jupiter and forplasma disturbances in the region; the photo-polarimeter had begun searching for the edge oflo's sodium torus; and a watch was begun for

the bow shockthe outer boundary of theJovian magnetosphere.

On February 10, Voyager 1 crossed theorbit of Sinope, Jupiter's outermost satellite.Yet the spacecraft even then had a long way togostill 23 million kilometers from Jupiter,!tut closing in on the planet at nearly a millionkilometers a day. A week later, targeted photo-graphs of Callisto began to provide coverage ofthe satellite all around its orbit; similar photosof Ganymede began on February 25.

Meanwhile excitement was building. Asearly as February 8 and 9, delight with the mis-sion and anticipation of the results of theencounter in March were already evident.Garry E. Hunt, from the Laboratory forPlanetary Atmospheres, University College,

The Voyager TV cameras do not take color pictures directly as do commercial cameras. Instead, a color image is

reconstructed on the ground from three separate monochromatic images, obtained through color filters and transmitted

separately to Earth. There are a number of possible filter combinations, but the most nearly "true" color is obtained with

originals photographed in blue (480 nanometers), green (565 nanometers), and orange (590 nanometers) he.% Before these

can be combined, the individual frames must be registered, correcting for any change in spacecraft position or pointingbetween exposures. Often, only part of a scene is contained in all three original pictures. Shown here is a reconstruction of a

plume on Jupiter, photog.aphed on March I, 1979. The colors used to print the three separate frames can be seen clearly in

the nonoverlapping areas. For other pictures in this book, the nonoverlapping partial frames are omitted. IP-211921

A

AM.

The Voyager Project was operated from the Jet PropulsionLaboratory managed for NASA by the California Instituteof Technology. Located in the hills above Pasadena, Cali-fornia, JPL is the main center for U S. exploration of thesolar system. IJBI7249BCJ

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London, a member of the Imaging Team,discussing the appearance of Jupiter's at-mosphere as seen by Voyager during theprevioue, month, said "It seems to be far morephotogenic now than it did during the Pioneerencounters; I'm ;store than delighted by itit'san incredible state of affairs. There are infinite-ly more details than ever imagined."

The pictures from Voyager are "clearlyspectacular," said Lonne Lane, Assistant Proj-ect Scientist fcr Jupiter. "We're getting evenbetter results than we had anticipated. We haveseen new phenomena in Moth optical and radioemissions. We have definitely seen things thatare differentin at least in ciic case, unantici-patedand are begging for answers we haven'tgot." There was already, still almost a monthfrom encounter, a strong feeling of accomplish-ment among the scientists and engineers; theyhad done a difficult task and it has been suc-cessful.

By the last week of February 1979, the at-tention of thousands of individuals was focusedon the activities at JPL. Scientists had arrivedfrom universities and laboratories arou.id thecountry and from abroad, many bringing grad-uate students or faculty colleagues to assistthem. Engineers and technicians from JPL con-tractors joined NASA officials as the Pasadenamotels filled up. Special badges were issued andreserved parking areas set aside for the Voyagerinflux. Twenty-four hours a day, lights burnedin the flight control rooms, the science offices,the computer areas, and t' photo processinglabs. In order to protect those with critical tasksto perform from the friendly distraction of thenew arrivals, special computer-controlled lockswere placed on doors, and security officersbegan to patrol the halls. By the end of themonth the press had begun to arrive. Amidaccelerating excitement, the Voyager I encoun-ter was about to begin.

Ied

CHAPTER 6

THE FIRST ENCOUNTER

The Chilli Is Full of Surprises

The Voyager I encounter took place at4:42 a.m. PST, March 5, 1979. About six hoursbefore, while the spacecraft continued to hurtleon toward Jupiter, overflow crowds had pouredout of Beckman Auditorium on the campus ofthe California Institute of Technology. Therehad been a symposium entitled Jupiter and theMind of Man. More than one face that eveninghad turned to look toward the Pasadena night,for there, glittering against the fabric of thesky, was the "star" of the showa planet sohuge that the Earth would be but a blemish onits Great Red Spot. One might have tried toimagine Voyager I, large by spacecraft stand-ards, rapidly closing in on Jupitera pesky,investigative "mosquito" buzzing around theJovian system.

Meanwhile, at JPL, pictures taken byVoyager i `lashing back every 48 seconds,pictures that were already revealing more andmore of Jupiter's atmosphere and would soondisclose four new worlds- the Galilean satel-lites. the encounter was almost at hand.

4:42 a.m.Voyager I had made its closestapproach to Jupiter 37 minutes earlier. Onlynow were the signals that had been sent outfrom the spacecraft at 4:05 a.m. reaching theEarth.

But the moment of closest encounter didnot have the same impact a landing would havehad. Although the champagne would flow laterfor those who had worked so long and hard onthis successful mission, there was none now. Inthe press room there were just four coffeepotsworking overtime to help keep the press alert.In the science areas, focus had shifted to thesatellite encounters, which would stretch overthe next 24 hours; meanwhile, a few personstried to catch a little sleep at their desks before

the first closeups of to came in. The instant ofencounter came . . . and went . . . with noscreams, no New Year's noisemakers. All heexcitement of the mission lay both behind . . .

and ahead.

Thursday, February 22. The encounter activitywas kicked off in Washington, D.C., by a pres-,conference at NASA He9dquarters. After in-troductory statements by NASA and JPL of-ficials, some of the scientific results from theobservatory and far encounter phases werepresented.

Both the plasma wave and the planetaryradio astronomy instruments had detected very-low-frequency radio emission that generally in-creased in intensity whenever either the north orsouth magnetic pole of Jupiter tipped towardthe spacecraft. The plasma wave experimenthad also detected radiation that seemed to comefrom a region either near or beyond the orbit ofloperhaps even as far as the outer magneto-sphere. In addition, Frederick L. Scarf, Princi-pal Investigator for the plasma wave instru-ment, released a recording of the ion soundwaves created by 5 to 10 kilovolt energy protonstraveling upstream from Jupiter.

James Warwick, Principal Investigator ofthe radio astronomy investigation, enthusiastic-ally reported the detection of a striking newlow-frequency radiation from Jupiter, at wave-lengths of tens of kilometers. Such radio wavescannot penetrate the ionospheie of the Earth,and thus had never been detected before.Because of their huge scale, these bursts proba-bly did not originate at Jupiter itself, but inmagnetospheric regions above the planetperhaps in association with the high-densityplasma torus associated with lo. Warwick com-mented that "only from the magnificent

The smaller-scale clouds on Jupiter tend to be more irregular than the large ovals and plumes. At the lower right, one of thethree large white ovals clearly shows internal structure, with the swirling cloud pattern indicating counterclockv.isc, or anti-cyclonic, flow. A smaller anticyclonic white feature near the center is surrounded by a dark, cloud-free band where one cansee to greater depths in the atmosphere. This photo was taken March 1 from a distance of 4 million kilometers 1P-21183C1

I `.,g

Launch date = 9/5/77----Jupiter arrival c ate = 315179

Sun ,/occultation/

Ganymede

Earthoccultation

lo

Cellist°

Periapsis

Amalthea

Europa\ ------'

Voyager 1 trajectorySatellite closest approach

2 hr

The Voyager I encounter with Jupiter took place during alittle more than 48 hours, from the inbound to the out-bound crossing of the orbit of Cellist°. This figure showsthe spacecraft path as it would be seen from above thenorth pole of Jupiter. Closest approach to Jupiter was350 000 kilometers. The close flybys of lo, Ganymede, andCallrsto all took place as the spacecraft was outbound.

perspective of the Voyager as it approachesJupiter have we been able to get this completepicture."

Another unexpected result was announcedby Lyle Broadfoot, Principal Investigator forthe ultraviolet spectrometer investigation. Thescientists had expected to find very weak ultra-violet emissions on the sunlit side of Jupiter,caused by sunlight being scattered from hydro-gen and helium in Jupiter's upper atmosphere."Instead we are seeing a spectacular auroraldisplay. There are two features of the emis-sionthe auroral emission which comes fromthe planet and a second type of emission whichappears to come from a radiating torus or shellarount the planet at the orbit of lo. The spec-tral content of these two radiating sources isdistinctly different. What we find is that theauroral emission from Jupiter's atmosphere isso strong that it completely dominates the emis-sion spectrum eNen on the sunlit side of theatmosphere."

"Not since Mariner 4 carried its TV camerato Mars fifteen years ago have we been lesspreparedhave we been less certain of what weare about to see over the next two weeks," saidBradford Smith, Imaging Team Leader. Hementioned the time-lapse "rotation movie," in

64

which the colorful planct spun through ten fullJ uniter days; tiny images of the satellites passedacross Jupiter's face as tl:ough being whippedalong by the rotation of the giant. A week or soearlier, when this film had been shown for thefirst time to the full Imaging Team, it providedan occasion for good-humored rivalry betweenplanet people and satellite people, with jokesabout the satellites getting in the way of theimportant studies of Jupiter. For the next fewdays, the imaging focus remained on Jupiter; itshifted to lo, Ganymede, and Callisto, as eachwas passed in turn after closest approach to theplanet.

Tuesday, February 27. (Range to Jupiter, 7.1million kilometers). At a distance of 660 millionkilometers from Earth, within 90 Jovian radii(Rj) of Jupiter, Voyager 1 was prepared tobegin the encounter with the planet's magneto-sphere. On the previous day the spacecraft hadcrossed the point, at 100 Rj, at which Pioneers10 and 11 had found the bow shock, the first in-dication of the magnetospheric boundary. Thestart of Voyager's plunge into the Jovian mag-netosphere was overdue, and scientists anx-iously watched the data from the particles andfields instruments, looking for the first indica-tion of disordered magnetic fields and alteredparticle densities that would mark the bowshock. Apparently, higher solar wind pressure,associated with increased solar activity since1974, had compressed the magnetosphere, butno one could predict how strong this compres-sion might be.

For the first time since its discovery, LyleBroadfoot and his UVS colleagues suggested aprobable identification for the unexpectedultraviolet emission near the orbit of lo. Themost likely cancidate was sulfur atoms withtwo electrons removed (S III), at an inferred

ENCOUNTER DISTANCESFOR VOYAGER I

Object

JupiterAmalthealoEuropaGanymedeCallisto

Range to Centerat ClosestApproach

(k ilome t ers)

Best ImageResolution

(km per line pair)

349 000417 000

21 000734 000115 000126 000

8

81

33

2

2

6J

temperature perhaps as high as 200 000 K, Anadditional indication of sulfur came from MikeKrimigis, who reported that the low-energycharged particles instrument had detected burstsof sulfur ions streaming away from Jupiter thathad apparently escaped from the inner mag-netosphere. No explanations were offered,however, for the presence of large amounts ofthis element.

At JPL, a press room had been opened inVon Karman Auditorium to accommodate thehundred or so reporters expected to arrive. Tokeep all the interested people informed ofVoyager progress, frequeni television reportswere beamed over closed-circuit TV throughoutJPL. From an in-lab television studio called theBlue Room, JPL scientist Al Hibbs, who hadplayed a similar role during the Viking Missionto Mars, provided hourly reports and inter-viewed members of the Voyager teams. As thepressure for constant commentary and instantanalysis increased, Garry Hunt of the ImagingTeam was also called on to host activities in theBlue Room, where his British accent added anadditional touch of class to the operation.

As the encounter progressed, the JPL tele-vision reports reached a wider audience. In theLos Angeles area, KCET Public Televisionbegan a nightly "Jupiter Watch" program,with Dr. Hibbs as host. During the encounterdays, service was extended to interested publictelevision stations throughout the nation. Inthis way, tens of thousands of persons wereable to experience the thrill of discovery, seeingthe closeup pictures of Jupiter and its satellitesat the same moment as the scientists at JPL sawthem, and listening to the excited and fre-quently awestruck commentary as the first ten-tative interpretations were attempted. Unfor-tunately, the commercial television networksdid not make use of this opportunity, and thegreatest coverage available t.) most of the coun-try was a 90-second commentary on the nightlynetwork news.

Wednesday, February 28. (Range to Jupiter,5.9 million kilometers). At 6:33 a.m., at a rangeof 86 Rj, Voyager 1 finally reached Jupiter'sbow shock. But by 12:28 p.m. the solar windhad pushed the magnetosphere back towardJupiter, and Voyager was once more outside,back in the solar wind. Not until March 2, at adistance of less than 45 Rj, would thespacecraft enter the magnetosphere for the finaltime.

At 11 a.m. the first daily briefing to thepress was given. "After nearly two months ofatmospheric imaging and perhaps a week ortwo of satellite viewing, [we're] happily bewild-ered," said Brad Smith. The Jovian atmos-phere is "where our greatest state of confusionseems to exist right at the moment, althoughover the next several days we may find thatsome of our smirking geology friends will findthemselves in a similar state. I think, for themost part, we have to say that the existing at-mospheric circulation models have all been shotto hell by Voyager. Although these models canstill explain some of the coarse zonal flow pat-terns, they fail entirely in explaining thedetailed behavior that Voyager is now reveal-ing." It was thought, from Pioneer results, thatJupiter's atmosphere showed primarily hori-zontal or zonal flow near the equatorial region,but that the zonal flow pattern broke down athigh latitudes. But Voyager found that "zonalflow exists poleward as far as we can see."

Smith also showed a time-lapse movie ofJupiter assembled from images obtained duringthe month of January. Once each rotation,approximately every ten hours, a color picturehad been taken. Viewed consecutively, theseframes displayed the complex cloud motions ona single hemisphere of Jupiter, as they would beseen from a fixed point above the equator ofthe planet. The film revealed that clouds movearound the Great Red Spot in a period of aboutsix days, at speeds of perhaps 100 meters persecond. The Great Red Spot, as well as many ofthe smaller spots that dot the planet, appearedto be rotating anticyclonically. Anticyclonicmotion is characteristic of high-pressureregions, unlike terrestrial storms. Smith notedthat "Jupiter is far more complex in its atmos-pheric motions than we had ever imagined. Weare seeing a much more complicated flow ofcyclonic and anticyclonic vorticity, circulation.We see currents which flow along and seem tointeract with an obstacle and turn around andflow back." There is a Jovian jet stream that is"moving along at well over 100 meters per sec-ond. Several of thew curious little dark featuresthat appear to be small orown spots nearJupiter's north temperate region have been seento overtake one another and gobble each otherup. And then they occasionally spit out a piecehere and there as they move along."

Thursday, March 1. (Range to Jupiter, 4.8million kilometers). At 5 a.m., at a distance of

a )65

Or

The southern hemisphere of Jupiter presents a tremendous diversity of atmospheric structure and motion. The Great RedSpot rotates counterclockwise in about six days; above and below it high-speed jet streams flow to the right and the left,while a complex, dynamic cloud pattern develops in its wake. This picture was taken on February 25, when Voyager I was 9million kilometers from the planet. [P-2l I5ICI

71 Rj, Voyager crossed the bow shock for thethird time, catching up with the contractingmagnetosphere of the planet. About noon, at66 Rj, the spacecraft finally reached the bound-ary of the magnetosphere, called the magneto-pause. Herbert Bridge, the plasma instrumentPrincipal Investigator, noted that the solar

66

wind pressure as monitored by Voyager 2, stillbetween the Sun and Jupiter, had been forseveral days from two to five times greater thanits level during the Pioneer 10 and 11 en-counters. Presumably. this high pressure wasthe cause of the compressed state of the mag-netosphere. However, in the previous few hours

77

the solar pressure had dropped, so Bridge anti-cipated that the Jovian magnetosphere mightsoon inflate and expand outward.

1red Scarf' intrigued the press with a tapeof the sounds made by high-energy protonscoming upstream from Jupiter. The plasmawave instrument had recorded the noise of theprotons, mixed with the noise of the spacecraftitself, producing sound effects that soundedsomewhat like a mixture of singing whales, aNor'easter, and the Daytona 500.

At the press briefing, interest in the imag-ing results began to shift from Jupiter towardthe satellites. Pictures of each of the four bigGalilean moons revealed bright and darkfeatures as small as about 200 kilometersacross. Unfortunately, this resolution is notenough to be diagnosticthe spots cannot beinterpreted in terms of recognizable geologicalfeatures, such as mountains or craters. Todayone could only speculate, but tomorrow or thenext day the answers would begin to come in.Deputy Imaging Team Leader Larry Soder-blom conveyed his excitement through ametaphor that would be repeated many timesduring the next week: "We're beginning a stagein this mission which represents, 1 think, one ofthe most exciting points in man's scientific ex-ploration of the solar systemin the next fewdays, we'll explore four new worlds," seeing in

a few days' time what it took us centuries tolearn about other worlds in our solar system. Interms of our experience with the exploration ofMars, "it is about 1700 AD this morning,tomorrow it will be about 1800, and it will beabout 1976 [the year of Viking] by Tuesdayevening."

Friday, March 2. (Range to Jupiter, 3.6 millionkilometers). E9rly in the morning, a twelvefoldincrease in solar wind pressure caused anothercontraction of the magnetosphere, which wasbehaving like a spring, compressing in responseto outside forces. As the magnetopause boun-dary moved rapidly inward, it crossed thespacecraft at 59 Rj from the planet. An hourlater the bow shock also flashed past, andVoyager was once more in the interplanetarymedium. By noon reinflation of themagnetosphere began again; Voyager crossedthe bow shock for the fifth and final time at 55Rj, followed by three more magnetopausecrossings, as the magnetospheric boundaryflopped in and out between 45 Rj and 50 Rj.

Some Project officials began to worryabout the contracted state of the magneto-sphere. The radiation "hardening" of Voyagerwas carried out to protect against the energeticparticle fluxes observed by Pioneers 10 and 11.

A few days before encounter, the Voyager images of the larger Galilean satellites, Callisto and Gartymede, v,!re beginning toshow distinctive surfaces with many bright spots These two pictures were taken on March 5 at a range of 8 millionkilometers; the resolution is about 100 kilometers. Although extremely tantalizing, these images were umnterpretable,because the spots could not be associated with any recognizable geological features, such as mountains or craters. Like anaked-eye view of the Moon, the e pictures seemed to reveal more than was actually meaningful. [P-21188C and P-21150C1

7

VOYAGER I BOW SHOCK (5) ANDMAGNETOPAUSE (M) CROSSINGS

Boundary Day Distance (R1)

Inbound

S 2/28 86

S 2/28 82

S 3/01 72

M 3/01 67

M 3/02 59

S 3/02 58

S 3/02 56

NI 3/03 47

Outbound

M 3/13 158

NI 3, 13 163

NI 3/13 165

S 3/16 199

S 3/18 227

S 3/18 227

S 3/19 240

S 3/20 256

S 1/2V 258

Under the new conditions, would the particlesin the inner magnetosphere be more concen-trated and perhaps increase the radiation levelsbeyond the design limits? Scientists asked howlong the compression might last and speculatedabout how much energy might be pumped intothe Jovian adiation belts, but only time couldprovide the answers.

Accurate recording of the X-band data be-ing sent from Voyager at 115 thousand bits persecond required fairly clear weather at thetracking sites. The three Deep Space Network(DSN) stations in California, Spain, andAustralia are located at norr-ally dr) sites.How ever, early in the morning, heavy rain atthe Australian site interfered with reception ofthe Voyager signal for fourteen minutes. For-tunately, the loss occurred when the DSN track-ing of the spacecraft was being switched fromGoldstone, California, to the Australian sta-tion. Mission control was able to extend Gold-stone coverage for several minutes so that onlyabout three minutes' worth of data was lostcompletely.

A more positive announcement was madeat the press conference that morning by DonaldShemansky of the Ultraviolet SpectroscopyTeam: the discovery of a high-energy torus ofdoubly ionized sulfur (S Ill) circling Jupiter inthe region of lo's orbit. "We were surprised out

68

of our chairs to see a spectacularly bright emis-sion in the 650-1100 angstrom region, immedi-ately implying that we were looking at a plasmathat had to be at a temperature of about100 000 degrees." The scientists estimated thatthe density of this torus must be at least 500ions per cubic centimeter, and that the powerneeded to keep this plasma at such a hightemperature must be in the neighborhood of500 billion watts. As Al Hibbs mentioned on"Jupiter Watch" that night, 500 billion wattsof power is the total amount of installedgenerating capacity of the United States.

Saturday, March 3. (Range ,..' Jupiter, 2.5million kilometers) Early in t e morning thefinal crossing of the magnetopause took placeat 47 Rj, and Voyager finally joined the Joviansystem. At the end of the day, the rapidly mov-ing spacecraft crossed the orbit of Callisto, butthat satellite itself was on the far side of theplanet. Not until the outbound crossing of itsorbit on March 6 would closeup views ofCallisto be obtained.

During the preceding night, a severe sum-mer thunderstorm in Australia again caused aloss of data. A line of storms over the trackingstation blocked the high-rate, X-band sciencedata for three hours and twenty minutes. At-tempts were made to save a part of the data bycommanding the spacecraft to slow its trans-mission rate, rather like speaking slowly to apartially deaf listener. But by the time the craftreceived the signal and responded, the stormshad intensified, and no signal could getthrough. Closeups of the Great Red Spot andan extended series of observations of the glow-ing sodium cloud around lo were lost.

A black-and-white movie assembled fromphotos obtained in January and February wasshown at the press conference. The movie wastaken from observatory phase pictures photo-graphed in blue light. From these pictures, theimaging team "put together this so-called 'bluemovie'," said Brad Smith, introducing thefilm. The film showed changes taking place inJupiter's atmosphere over a period of aboutseventy Jovian days. Near the equator therewere "bright plumes floating byat high speedthe plumes seem to wave around, somethinglike a flag waving in the breeze." Farther north,along the edge of the north temperate zone. onecould see one of the dark ovals"a ratherfuzzy one would move up, catch up with the onejust ahead of it, get stuck to the outside and rollaround on it for a while, then get ejected a little

At a resolution of about 100 kilometers a planetary surface just begins to reveal its personality lo was photcgraphed againstthe disk of Jupiter on February 26, from a distance of about 8 million kilometers Such early pictures whetted the appetitesof the Voyager scientists at JPL as they anxiously speculated about what they would I Ind in closers iews of the surlat.c of thisremarkable satellite. [P-21185 WWI

later." Dr. Smith also showed new closeupviews of the region around the Red Spot, show-ing not only the anticyclonic features but fila-mentary "spaghettilike" material which wasrotating in a cyclonic direction, indicating alow-pressure region. "The filamentary materialstill seems to be rather a mysteryvery difficultto see the details of the motion." But the photo-graphs were already showing what seemed to bestream lines in the white ovals. "In appearancethe white ovals seem to resemble the Red Spot.The stream lines are at least suggestive of diver-gent flow, that is, material in each of these

spots which is upwelling in these areas and thenmoving out tends to go around in a counter-clockwise anticyclonic motion but may, at thesame time, be slowly diserging outward."

The large white ovals are about forty yearsold. Dr. Smith explained how they formed: Be-tween 1939 and 1940, "where those three whitesnots exist right now, was a rather bright bandsimilar to the north temperate zone we see onJupiter right now. In that time period of a yearor so, three darkish spots formed. A dark cloudspread out at each one of those three locationsand just kept spreading longitudinally until the

69

white material condensed between them." Inthe end, everything was dark except for thethree ovals, which have persisted ever since.

Torrence Johnson of the Imaging Teamcommented on a photo of to in which featuresresembling circular crater-type structuresseemed to be visible. "Whatever they are,[those circular features] are approaching thesize of the things that we would call basins ifthey were impact structures on other planets.We don't really know whether they're impactstructures. They have some characteristics thatlook reminiscent of impact structures. Theycould be endogenicvolcanic in originor in-ternally generated in some other way." Johnsonalso showed another photograph of lo, this one

taken against black sky, showing a "strikinglydifferent face" looking, perhaps, likesomeone's nightmare, glaring back at the in-truder from Earth. One huge featurea "bulls-eye" or "hoof print" on loappeared to beapproximately 1000 kilometers long. No onehad ever seen such a feature, and the imagingscientists could only speculate about its sig-nificance.

A few days earlier, someone had posted inthe Imaging Team area a quote from a 1975review paper on the Jovian satellites by DavidMorrison and Joseph Burns. The section on lobegan, "lo is one of the most intriguing objectsin the solar system." This statement seemedmore and more appropriate as Voyager images

Small-scale structures in the jet streams of Jupiter's north tropical zone reveal details of atmospheric circulation. The smalldark oval near the right edge of the zone may offer a glimpse deep into Jupitei s atmosphere. Between the regularly spaceddark ovals near the bottom of the frame are more small-scale features that are being studied for their roles in Jovian atmos-pheric activity. The blue-gray regions along the shear line between the equatorial zone and the north equatorial belt also ap-pear to be windows into the deeper regions of the atmosphere. This photo was taken Febrt.ary 19 by Voyager 1 from adistance of 14 million kilometers. IP-21160C]

-ty

improved. Johnson referred to this day asequivalent to the "late 1960s" in our study ofthe Jovian satellites. "We can see much moreclearly than ever before, but still not clearlyenough to provide understanding of what weare seeing."

Sunda), March 4. (Range to Jupiter, 1.2million kilometers). At 4:37 a.m., the near en-counter phase began: Voyager I was almostthere! In the press room, someone taped a for-tune cookie message to the Voyager TVmonitor: "There is a prospect of a thrilling timeahead for you."

Pulled by the powerful gravity o' Jupiter,the spacecraft was now on a curved paththrough the inner Jovian system. At 2 p.m.

PST, it crossed the orbit of Ganymede, andlater in the afternoon it passed within less than2 million kilometers of Europa, providingVoyager l's closest look at this satellite. Duringthe afternoon and evening, a number of viewswere obtained of Amalthea, the small innersatellite, at a range of less than 500 000kilometers. At 8 p.m. the orbit of Europa wascrossed, and increasing attention was drawn tothe coming encounter with lo. At about 7 p.m.a full-frame color sequence of lo was receivedwith a resolution of 16 kilometers. During thenight, as Imaging Team members scratchedtheir heads trying to prepare a press release cap-tion to interpret the peculiar structures seen, theJPL Image Processing Lab rushed to prepare acolor version for release the next day.

The Great Red Spot became more and more spectacular as Voyager I approached, with each day revealing new and intricatedetail in the clouds. This view was obtained on March 1 at a distance of 5 million kilometers; the smallest featuresthat can bemade out are about 100 kilometers across. To the west of the Great Red Spot is a region of great turbulence, and to the southis one of the three white ovals. [P-21182C]

re,

Large brown ovals in the northern hemisphere of Jupiter are apparently regions in which an opening in the upper, ammoniaclouds reveals darlf-r regions below. This oval, about the same length as the diameter of the Earth, was at latitude 15°N.Features of this sort are not rare on Jupiter and have an average lifetime of one to two years. Abode the feature is the paleorange nort.1 temperate belt, bounded on the south by the high-speed north temperate current, with winds of 120 meters persecond. The range to Jupiter at the time this photograph was obtained on March 2 was 4 million kilometers, with the smallestresolvable featu.zs being 75 kilometers across. IP-21194C]

72

Once again, tracking station problemsinterrupted the smooth flow of data from thespacecraft. Failure at Madrid to reestablish thecorrect receiver frequency after a spacecraftmaneuver resulted in the loss of 53 minutes ofirreplaceable data; an additional eleven minuteswere lost an hour later. As the first problemwas being corrected, the tracking antennabecame misaligned, resulting in a noisy datareturn. Meanwhile, only a few of the , atchersat JPL mourned the lost data, so exciting werethe other new results that kept pouring in.

"For the highlights of this morning Dr.Soderblom will come up and show some beauti-ful satellite pictures," Brad Smith announcedat the daily press briefing. Larry Soderblom'sexcitement was hard to contain. "Today isprobably going to t -n out to be one of themost memorable days in our exploration pro-gram. For the planetary geol"..ists it's trulyChristmas Ee. We see tonight tine beginning ofthe exploration of four new worlds. We're rac-ing through time and space at an incredibleratethe rate at which we are learning things isawe-inspiring in itself." Ca Ilisto was still too

tar assay to see well "but the things we're seeingon the closer three satellites have really got uscharged in anticipation." Ganymede"Whatcan I say? Loops and swirls and incrediblepatches that are difficult now to haiard a guessabout." !oan eerie-looking red, orange, andyellow world"this one we've got all figuredout. [Laughter from the press and applause.] Itis covered with thin candy shells of anythingfrom sulfates and sulfur and salts to all kinds ofstrange things."

The low energy charged particle instru-ment had discovered high-speed sulfur in theouter Jovian magnetosphere, ten times as farfrom the planet as the sulfur torus around to.The high-speed sulfur, which whizzed byVoyager I at 8000 kilometers per second, wasfirst detected as the spacecraft crossed themagnetopause and entered the magnetosphere.Apparently this sulfur had picked up speed as itmoved outward from lo's torus, but themechanism foi this acceleration was notknown. Roby Vogt reported that the cosmic rayinstrument was detecting two distinct groups oratoms closer in to the planet. One group was

The best Voyager I photos of Europa were obtained on March 4 from a distance of about 2 million kilometers. This % to% ofthe hemisphere centered at about 300° longitude has a resolution of about 40 kilometers. Most of the surface is bland andhighly refleeto c, being composed almost cntirel of water -tee No craters resulting from meteoric impacts can he seen Themost striking N isua features, which set Europa oft from the other satellites, are the dark streaks, as much as seseral thou-sand kilometers long, that cross the surface. Just barely risible to Voyager I, these streaks were dramatically apparent toVoyager 2 in its closer flyby in July. [P-21208C]

A

4

apparently of solar composition (derived fromthe solar wind), and the other showed enhancedoxygen as well as sulfur. The plasma instrumenthad also begun to measure sulfur in the santeform (S III) as that detected by the U VS in thelo torus. Where, the scientists asked, could theoxygen and sulfur be coming trout? Could lo bethe source of these atoms?

At one point, as questions from the pressbecame too detailed, asking for "instant analy-ses" of the data, Dr. Smith was prompted tomake the following statement: "I want to saysomething about what's going on right now inthe imaging area because we get a lot of pointedquestions. We have a remarkably good systemthat is getting extremely good photographs.Not only are the cameras working well, butJupiter and the satellites are cooperating byshowing a tot of truly remarkable detail. And itmay sound unprofessional, but a lot of the peo-ple up in the Imaging Team area are just stand-ing around with their mouths hanging openwatching the pictures come in, and you don'tlike to tear yourself away to go and start look-ing at numbers on a printout. We will do that,but in the meantime we're just caught up in theexcitement of what's going on."

While the press briefing was underway, he-twoen 11:38 and 11:50 a.m., a special experi-ment was being tried on board the Voyager. Asthe smixecraft passed through the plane of theequator of Jupiter, it aimed its narrow-anglecamera to a point in space halfway between thecloud tops and the orbit of Amalthea and tooka single, eleven-minute exposure. The purpose:to search for a possible faint ring aroundJupiter, which, if present, could best be seen bysighting along the plane of the equator. Theimage appeared briefly on the TV monitors,clearly showing somethinga strange band oflightstreaking across the renter of the frame.Up in the imaging area, analyses began at onceto determine if the strange band were really thesought-for ring, but it would not be until March7 that the identification would be confirmedand the discovery announced.

Monday, March 5. (Encounter Day. Minimumrange to Jupiter, 780 000 kilometers; speed ofspacecraft, almost 100 000 kilometers pernour). Many celebrities, including the Govei-nor of California, spent the night at WI, towitness the historic occasion. In Washington,

1 he "bullsee ot lo was t Irst photographed at a range 01 2 8 million kilometers on March 3. and this solor s efS1011 n as

released on Ntart.h 4 At the time Imaging Team sLientists 11 role that "The large heart-shaped feature 11ith a dark spot nearits Leiner could he lo's equi. alent ot an impao basin such as Nlare Orientate on the Nloon Its outer dimensions are about800 by 1000 kilometers. Subsequent high-resolu In coserage should reseal whether the small dark spots are impact Lraters,or perhaps something more e \ one such as soleanoes The reddish Lolor ot lo has been attributed to sulfur in the salts ss hit.hare behesed by some to make up the surface ot lo." It would he another week before this :cature, later named "Pete" lor theHawaiian soleano goddess. would be recognized as an acme. erupting ,olcano. IP-21187(1

D.C., a special TV monitor was set up in theWhite House for the President and ht' family.

As late Sunday night eased into the earlymorning of encounter, closeup images ofJupiter, looking more like abstract art than likeplanetary science, flashed across the TVscreens, and verbal images far less wi'd then thevisuals from Jupiter were heard from commen-tators and from members of the press. "Areyou sure Van Gogh didn't paint that?" "That'snot Jupiter; it looks like a closeup of a salad.""They're not showing us Jupiter, that's somemedical school anatomy slide."

Shortly before closest approach to Jupiter,Voyager began its intensive observations of lo.Much of this information, taken while theAustralian station was tracking the spacecraft,was recorded on Voyager's onboard tape-recorder for playback later that day. But evenbefore the results of that imaging were known,Larry Soderblom was calling lo "one of themost spectacular bodies in the solar system."As more and more vivid photos of to appearedon the monitors, members of the Imaging Teamin the Blue Room buzzed with excitement."This is incredible." "The element of surpriseis coming up in every one of these frames." "Iknew it would be wild from what wz saw on ap-proach but to anticipate anything like thiswould have required some sort of heavenlyperspective. I think this is incredible."

At 7:35 a.m. Voyager was scheduled topass through the flux tube of lo, the region inwhich tremendous electric currents were cr''.:u-lated to be flowing back and forth betw' hesatellite and Jupiter. Norm Ness suggested,after examining magnetometer data, thatVoyager skirted the edge of the flux tube, andthat the current in the tube was about one mil-lion amps. As the flux tube results were re-ceived, champagne bottles began to pop in theparticles and fields science offices, in celebra-tion of the successful passage through the innermagietosphere. Meanwhile, at 7:47 a.m., clos-est approach to to occurred, at a range of only22 000 kilometers. Voyager was 25 000 timescloser to this satellite than were the watchers onEarth.

At 8:14 a.m., while still within 30 000 kilo-meters of lo, the spacecraft passed out of sightbehind the edge of Jupiter. All scientific datafor the next two hours and six minutes werestored on the onboard tape recorder for latertransmission to Eartn. Meanwhile, the radiocommunicatio.n signal was used to probe the at-mosphere of Jupiter, yielding a profile of elec-

tron density in the icnosphere and of the gaspressure and temperature in the upper atmos-phere. While out of sight from Earth, at 9:07a.m., Voyager plunged into the shadow ofJupiter. As the Sun set on the spacecraft, theultralolet instrument used the absorption ofsunlight to determine the composition and tem-perature cf the upper atmosphere. In the dark-ness, the infrared IRIS measured the night-sidetemperatures of the planet, and long-exposureimages were taken to search forQurora, light-ning, and fireballs in the Jovian atmosphere. At10:20 a.m., Voyager reappeared from behindJupiter and radio contact was restored; at 11:24a.m., it emerged from shadow into sunlight,speeding on toward encounter with Ganymede.

At 8 a.m. a special press conference washeld to mark the successful Jupiter flyby. NoelHinners, Associate Administrator for SpaceScience and the highest ranking NASA officialpresent, congratulated all those who had madethe Voyager Mission a success. The encounterwas the "culmination of a fantastic amount ofdedication and effort. The result is a spec-tacular feat of technology and a beginning of anew era of science for the solar system. Justwatching the data come in has been fantastic. Ihad a fear that things on the satellites were go-ing to look like the lunar highlands. Naturewins again. If we're going to see exploration ofthis nature occurring in the 1980s and 1990s wemust continue to expound the results of whatwe're finding here, the role of exploration inthe history of our country, what it means to usas a vizorous national society."

As time passed, it became apparent thatVoyager 1 had been affected by Jupiter's radia-tion environment. The basic timingthe mainclock on the spacecrafthad slowed down.First it slowed by 6.3 seconds, but by March 6 itwas found to have slowed a total of eight sec-onds. In addition, the two central computersapparently got out of synchronization bothwith themselves and with the flight data subsys-,,:m. On March 6 it was renorted that the space-craft cameras were shuttering one frametime(48 seconds) early: this was partly offset by theeight-second spacecraft "masterclock" slow-down resulting in images (according to ourclocks) being photographed forty secondsearly. This timing error resulted in the camerataking some pictures while the scan platformwas moving, causing some blurred images. Anumber of the highest resolution images of loand Ganymede were seriously degraded by thismalfunction.

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At the regular I I a.m. press briefing, BradSmith glowed. "We're all recovering from whatI would call the most exciting, the most fasci-nating, what may ultimately prove to be themost scientifically rewarding mission in the un-manned space program. The lo pictures thismorning were truly spectacular and the atmos-phere up in the imaging area was punctuated bywhoops of joy or amazement or both." Thenew color photo of lo taken the night beforesas released, showing strange surface featuresin tones of yellow, orange, and white. Theimage defied description; the Imaging Teamused terms like "grotesque," "diseased,""gross," "bizarre." Smith introduced the pic-ture with the comment, "lo looks better than alot of pizzas I've seen." Larry Soderblomadded, "Well, you may recall [that we] told youyesterday that when we flew by we'd figure allthis out. I hope you didn't believe it."

One thing was certain: There were no im-pact craters on lo. Unless the satellites of

Jupiter had somehow been shielded from themeteoric impacts that cratered objects such asthe Moon, Mars, and Mercury, the absence ofcraters must indicate the presence of erosion orof internal processes that destroy or cover upcraters. lo did not look like a dead planet.Imaging Team member Hal Masursk y, lookingat the "pizza" picture, estimated that the sur-face of lo must be no more than 100 millionyears oldthat is, some agent must have erasedimpact craters during the last 100 million years.This interpretation depended on how oftencatering impacts occur on lo. No one could besure that there had been any interplanetarydebris in the Jovian system to impact the sur-faces of the satellites. 13;rhaps none of themwould be cratered. The forthcoming flybys ofGanymede and Callisto would soon providethis information.

The close flyby of Ganymede took place at6:53 p.m., at a range of 115 000 kilometers.During the preceding four hours, photos re-

When the first color close-up of lo was released, Imaging Team Leader Brad Smith said that he had seer, "better lookingpizzas"; hence this view, taken March 4 at a range of about 860 000 kilometers, became known as "the pizza picture." Thecircular feature in the center (the piece of pepperoni) was later revealed to be the active volcano Prometheus, but at the timeof its release this lovely but bizarre picture baffled scientists and press alike. [P-21457C1

The giant volcanic feature Pe le, about 1000 kilometers across, mystified Voyager scient;:hs"hen this picture of lo was takenon March 5 from a distance of about 400 000 kilometers. The brilliant colors, the stra,ige shapes of the surface deposits, andthe absence of impact craters all testified that lo was unlike any world previously encot.ntered in the exploration of the solarsystem. 1P21226C]

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As Voyager I approached lo, the images of the surface became more and more spectacular. On the morning of March 5, at arange of 130 000 kilometers, this picture was taken centered near longitude 320° and latitude 10°S. The width of the picture

about 1000 kilometers (the distance from the Mexican border to the northern edge of California) There are no impact,raters, signifying a geologic .11y young surface, and the dark center with radiating red flows indicates recent volcanic activityof some sort. IP-21277C]

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vealed a surface cosered with impact craters.Watching these photos and supplying commen-tary for the television listeners, David Morrisonremarked, "While I'm delighted to se: craters,it's just the opposite of what I would ha% e ex-pected. I was telling everyone a few days agothat I thought lo would have plenty of cratersand that Ganymede, because of the ice surface,simply would not be able to hold large cratersover geological time. So this is fascinating andthis is confusingboth what has happened onlo to erase the craters and why Ganymede's sur-face is strong enough to preserve them."

Just before closest approach, at 6:35 p.m.,the ultraviolet instrument watched as a brightstar passed behind Ganymede and reemergedten minutes later. No dimming that could be at-tributed to an atmosphere was seen; when thedata were analyzed later, scientists set an upperlimit for any gas on this satellite at one-billionth

of the atmospheric pi essuie at the surface ofthe Lartn.

As encounter day drew to a close, celebra-tions took place all oser JP1.. 1 or many, how -

eser,er, the excitement was tempered by exhaus-tion. After 48 hours of intense activity, sleepwas imperative for some. But the close ap-proach to Callisto was still to come, as was anexamination of the data already received.

Tuesday, March 6. Voyager was now recedingfrom Jupiter, accelerated on a new trajectoryone that would speed it on toward its November1980 encounter with Saturn. But first came theencounter with Callisto, with closest approachat 9:50 a.m., at a range of 126 000 kilometers.The satellite was littered with craters and thereappeared one huge "bullseye" pattern thatmight have been the result of an impact. As thephotos of Callisto came in, Garry Hunt de-

4.

After its close flyby of lo, Voyager I headed for Ganymede, the largest of the Galilean satellites, This global view ofGanymede, taken on March 4 at a range of 2.6 million kilometers, shows features as small as 50 kilometers across. At thetime, Voyager scientists speculated that the numerous white spots were impact craters, surroundedin some cases by icy ejectablankets splashed onto the surrounding surface. However, many narrow white streaks, especially those in the lower leftquadrant, promised new and exciting geological features on this smelt 1,P-21207CI

As Voyager I approached Ganymede on March 3, manystrange new surface features became visible. In this frame,taken from a distance of 250 000 kilometers and showingfeatures as small as 5 kilometers across, three distinct typesof terrain are seen: polygons of old dark surface, extensiveareas of lighter, younger material, and brilliant white ejectapatterns (probably water-ice) around fresh craters. Thebright rays in the upper part of the picture are 300-500kilometers long; at the bottom are several craters with onlyfaint, muted ejecta patterns. IP-2[262C]

Voyager 1 found that the surface of Ganymede was geologi-cally very complex. This frame, taken March 5 from arange of 250 000 kilometers, shows a region about 1000kilometers across centered near longitude 0° and latitude20°S. The surface displays many craters, some (probablythe younger ones) with bright ray systems Bright groovedbands traverse the surface in various directions. One ofthese bands, running in a north-south direction in the lowerleft of the picture, is offset along a white line that may rep-resent a fault. Ganymede is the only Galilean satellite toshow indications of such lateral offsets in the crust.IP-21266 ]

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scribed the scene in the imaging area: "Theactivity around the monitors now is quite in-credible with people caught breathless by thepictures coming in. Every satellite we've seenhas been a different world." Asked how he fe;tabout the images of Jupiter's atmosphere, hereplied, "I'm absolutely delighted with whatI've seen, and I'm delighted Voyager 2 is notfar behind since I'm convinced that we'll see yetanother face of Jupiter by then. The weatherwill have changed by July."

At the morning press briefing, the wealthof new data began to be revealed. "If Jupiterhad ever posed for Monet, it would probablyhave turned out like this," said Brad Smith ashe introduced some enhanced (exaggerated)color images that had been specially processedto show more detail in the Great Red Spotregion. Indeed, these photographs did look likepaintings with Jupiter displaying some of itsbest abstract art.

The first picture of Amalthea was shown,revealing an elongated, dark, reddish objectabout 265 kilometers long. Smith reported:"There is actually some structure that one canseea cratera couple of bright features for

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which we have no interpretation and someother evidence of cratering. It doesn't look likemuch, but after all, Amalthea has never beenseen as anything more than a point of lightfrom the Earth, and, in fact, there are very fewpeople that have even seen it as a point of light.I doubt that more than one astronomer out of ahundred has actually seen Amalthea."

Larry Soderblom introduced new photo-graphs of the satellites. lo still showed nocraters, even at high resolution. 1 he craters that"should" have been on lo were on Ganymedeand Callisto. Ganymede not only had craters, ithad fault lines as well. "Then is transversemotion along these faults. Thin is get offset,

apparently, for hundreds of kilometers. So it'sthe first time we've seen major kinds of trans-verse motion on the surface of another planet."Ganymede, in effect, had shown evidence ofhaving its own icy version of the San AndreasFault.

Although Callisto was heavily coveredwith craters up to 200 kilometers in diameter,Dr. Soderblom commented on the absence oflarger impact basins. Perhaps there was one inthe huge bullseye feature; however, it was not a"standard" looking basin like those on theMoon. Callisto was "extremely smooth andfree of any relief. The structure [the bullseye],if impact caused, shows no relief; the limb does

Amalthea was thought to be the Innermost satellite of Jupiter until Voyager 2 discovered tiny Adrastea (1979JI). TheVoyager I camera revealed Amalthea as an irregular dark reddish object with dimensions of 270 x 160 kilometers. Thesethree images have resolutions of (top right, b) 25 kilometers; (bottom left, c) 13 kilomeers; (bottom right, d) 8 kilometers.[260-5031

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not show any relief; maybe it's possible thatCallisto cannot support relief." By the nextday, the geologists on the Imaging Team werebecoming more convinced that the Callisto"bullseye" was the frozen remnant of an enor-mous impact into Callisto's surface. SinceCallisto is composed in large part of water andhas an icy crust, the team speculated that anyraised features created by the impact wouldeventually "slide" back into the surface, "andthe ripple marks from the shock wave caused bythe impact were frozen" into the surface.

Lyle Broadfoot announced that the ultra-violet spectrometer had detected very strongauroral emission in Jupiter's north and southpolar regions. The aurora seemed to be causedprimarily by the excitation of molecular hydro-gen, although some atomic hydrogen was alsodetected. Auroral emissions from helium atomswere not Getected.

The IRIS infrared measurements requiredmore computer processing than other Voyagerdata, and therefore they were not available un-til a day or two after the observations were

allisto was the last of the Galilean satellites to be studied by oyager 1 In this photo, taken March 5 from a distance of 1.2million kilometers, with a resolution of about 25 kilometers, the extensise cratering of the surface began to be apparent.Near the upper left edge is the large impact basin Valhalla; the numerous light spots are craters 100 kilometers or more indiameter. This is the same side of Callisto that was photographed at higher resolution during the Voyager I flyby of thesatellite a day later. [P-21284C]

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The largest ancient impact basin on Callisto is called Valhalla. The central light area is about 600 kilometers across.Surrounding it is a set of concentric low ridges, looking like frozen ripple marks, extending about 1500 kilometers from thecenter. This picture was taken by Voyager I on March 6 at a range of 350 030 kilometers. IP- 21287C1

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made. However, Rudy Hanel already had twonew results to report. First, the Great Red Spotwas about 3° C cooler than its surroundings,and this cooling extended many kilometersabove the clouds, into the thin upper atmos-phere. Second, the thermal emission from towas peculiar, with an unexpected shape to thespectrum. Tentatively, Hanel suggested theremight be same hot spots on the surface of thesatellite.

Jupiter has been full of surprises, but theexcitement was far from over. Major dis-coveries were yet to be made.

Wednesday, March 7. At the press briefing,Brad Smith made a spectacular announcement:"This morning I would like to add yet anotherimportant discovery to be claimed by this out-standing missionthat of a thin flat ring ofparticles surrounding Jupiter. Thus Jupiternow joins Saturn and Uranus to become thethird planet of our solar system known topossess a planetary ring system and leaves Nep-tune as the only member of the group of giantplanets without a known ring. The discovery ofthe ring was unexpected in that the currenttheory which treats long-term stability of plane-tary rings would not predict the existence ofsuch a ring around Jupiter. The single Voyager

Voyager I discovered the rings of Jupiter on March 4 in a single eleven-minute exposure with the narrow-angle cameraSpacecraft motion during the time exposure streaked the picture, as can be seen from the hairpin-like images of the stars(The star field was unusually rich, since it happened to include the Beehive star cluster in Cancer.) The ring image itself is amultiple exposure, with six separate images side by side. The ring does not extend out of the right side of the picture.indicating that this image captured the outer edge of the ring, about halfway between the cloud tops and the orbit ofAmaithea. 1P-212581

Wide angle field of view

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Since the Voyager I ring photograph was taken exactly edge-on, it was not possible to determine the width of the ring. In thisartist's conception, the ring is drawn as if it were ribbon-like, with very little width, qui,e unlike the broad flat rings ofSaturn. Voyager 2 later showed this to have been a lucky guess. IP-212591

camera image which recorded the ring wasplanned by the Voyager Imaging Team severalyears ago, not really with any great expectationof a positive result, but more for the purpose ofproviding a degree of completeness toVoyager's survey of the entire Jupiter system.The observation, as planned, involved lookingoff to the right of the limb of Jupiter in theplanet's equatorial plane at the exact momentthat the spacecraft would be crossing the equa-torial plane. The image actually was taken at 16hours and 52 minutes before encounter from adistance of about 1.2 million kilometers. Ex-posure time was 11.2 minutes. We weren't cer-tain of the exact moment that we would crossthe equatorial plane, so we planned to open ourshutter and leave it open as we went through."

The image showed six exposures of thering, together with streaked trails of back-ground stars. Smith reported that the thicknessof the ring was less than 30 kilometers, and thatit extended to a point 128 000 kilometers fromthe center of Jupiter, or 57 000 kilometersabove the clouds. In response to this discovery,scientists were eagerly planning to alter theVoyager 2 encounter sequence to try to obtainadditional information on the ling.

The newspapers carried stories of the ringdiscovery on Thursday, March 8. One story wasseen by two University of Hawaii astronomers,Eli. Becklin and Gareth Wynn-Williams, whowere observing at Mauna Kea Observatory, atan altitude of 4200 meters on the Big Island ofHawaii. Within two days they had succeeded in

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detecting the rings by their reflected sunlight, atan infrared wavelength of 2.2 micrometers,providing a rapid confirmation of the Voyagerdiscovery.

As the plasma measurements from the loflyby were analyzed, an additional clue to theorigin of sulfur and oxygen was revealed. HerbBridge reported the detection of sulfur dioxide(SO2), the simplest molecule composed of thesetwo atoms.

The high-resolution tape recorded picturesof to baffled imaging scientists. A number offeatures looked like volcanic flows; togetherwith the absence of impact craters, these fea-tures indicated a geologically active planet. Acentral point of discussion was the recent theo-retical work on to by Stanton Peale of theUniversity of California and two NASA scien-tists, Pat Cassen and Ray Reynolds. Theseauthors had just published a paper in theMarch 2 issue of Science showing how tidalheating from Jupiter's gravity could melt theinterior of lo. They wrote that "widespreadand recurrent surface volcanism might occur."It began to look as if the prediction had beencorrect.

Thursday, March 8. The last Voyager 1 pressbriefing was held. Each speaker was allottedonly a few minutes, prompting Larry Soder-blom to preface his remarks by trying to explainhow difficult it was to describe four newplanetsthe Galilean satellitesin the timeallowed. "Torrence [Johnson] was sitting withme last night, puzzling. He said, 'You know,Larry, it's sort of like imagining we'd flowninto the solar system the day before yesterday,and said, "There's a thing we'll call Mercury,and there's the Moon, and there's Earth, andthere's Mars. Now let's explain them in tenminutes ".' " There was Callisto, with thehighest density of craters of any Galilean satel-litethe oldest of the Galilean surfacesfeaturing a huge "bullseye" that is "the largestsingle contiguous feature seen so far in the solarsystem." There was Ganymede, cratered, butalso overrun with fault lines that looked,according to one person, like "tire tracks in thedesert," showing a surface that "had laterallyslidfaulted and sheared and sheared againtwisted and torn apart." There was lo, the mostbizzarethe one that scientists had thoughtwould be most lunarlikeshowing a surfacethat had apparently been "cooked and steamedand fumed out leaving deposits all over the sur-

86

face much like you might see around afumarole at Yellowstone National Park. Thefact that these things exhibit such youth makesit likely that the planet is still volcanically ac-tive." There was Europa, with huge linearfeatures unlike those of the other three Gali-leansEuropa the mystery satellite, waiting forthe probing eyes of Voyager 2 to survey it inearly July.

Ed Stone summed it all up: "I think wehave had almost a decade's worth of discoveryin this two-week period, and I think that all ofthe people who have been talking to you feel thesame saturation of new information which hasoccurred. And in fact, we will probably bestudying it in great detail for at least fiveyears."

Over the next day or so the press packed upand went home. The TV monitors showed thespacecraft's parting glance at a crescentJupiter, only hinting at the vastness of spaceVoyager 1 would travel until its encounter withSaturn in the autumn of 1980. Maybe now therewould be a relative calm that would allow thescientists to begin analyzing that "decade'sworth of data." But things were not to be calmjust yet.

Fire and Brimstone

At about 5 a.m. on March 8, Voyager hadtaken a historic picture. Looking back at a cres-cent to from a distance of 4.5 million kilo-meters, the camera had been used to obtain along-exposure view for the spacecraft naviga-tion teamone that shcwed the satellite againstthe field of background stars. During the day,Linda Morabito, an optical navigation engi-neer, began to work with this picture on hercomputer-controlled image display. She notedwhat appeared to be a crescent cloud, extendingbeyond the edge of lo. But to has no atmos-phere, so a cloud rising hundreds of kilometersabove the surface did not seem to make sense.

The next day, working with her colleagues,Morabito eliminated all possibilities for the newfeature on to except the obviousa cloud. If itwere a cloud, it must be the result of an ongoingvolcanic eruption of incredible violence. Thepicture was shown to members of the ImagingTeam, who agreed with the identification. Butit was Friday and Brad Smith and Larry Soder-blom, along with most of the other team mem-bers, had left for the weekend to try to get somerest. The picture would have to wait two moredays.

Co .

HIGHLIGHTS OF THE VOYAGER 1 SCIENTIFIC FINDINGS*

Atmosphere

Uniform wind speeds for cloud features withwidely different size scales, suggesting thatmass motion and not wave motion is beingobserved.

Rapid formation and spreading of brightcloud material, perhaps the result of disturb-ances that trigger convective activity.

A pattern of east-west winds in the polarregions, previously thought to have beendominated by convective upwelling anddownwelling

Anticyclonic motion of material associatedwith the Great Red Spot, with a rotationalperiod of about six days.

Interactions of smaller spots with the GreatRed Spot and with each other.

Auroral emissions in the polar regions, bothin the ultraviolet (which were not presentduring the 1973 Pioneer encounter) and inthe visible.

Cloud-top lightning bolts, similar to terres-trial superbolts, and meteoritic fireballs.

A temperature inversion layer in the strato-sphere and a temperature of 160 K at thelevel at which the atmospheric pressure is0.01 bar.

Very strong ultraviolet emission from thedisk, indicating a thermospheric temperatureof more than 1000 K.

A hot (1100 K) upper ionosphere on the day-side that was not observed by Pioneer 10,suggesting there may be large temporal orspatial changes.

An atmospheric composition with volumefraction of helium of 0.11 ± 0.03.

A substantially colder atmosphere above theGreat Red Spot than in the surroundingregions.

Satellites and Rings

At least eight currently active volcanoes onto, probably the result of tidal heating of theinterior of the satellite, with plumes extend-ing up to 250 kilometers above the surface.

A large hot spot on lo near the volcano Lokithat is about 150 K warmer than the sur-rounding surface.

Numerous intersecting, linear features onEuropa, possibly due to crustal cracking.

Two distinct types of terrain, cratered andgrooved, on Ganymede, suggesting that theentire ice-rich crust was once under tension.

An ancient, heavily cratered crust onCallisto, with vestigial rings of enormousimpact basins since erased by flow of the ice-laden crust.

The elliptical shape of Amalthea (270 x 160kilometers).

A faint ring of material about Jupiter, withan outer edge of 128 000 kilometers from thecenter of the planet.

Magnetosphere

An electrical current of more than a millionamperes flowing in the magnetic flux tubelinking Jupiter and to.

Very strong ultraviolet emissions fromionized sulfur and oxygen in the to plasmatorus, indicating a hot (hundred thousanddegree) plasma that evidently was not pres-ent at the time of Pioneer 10 encounter.

Plasma electron densities exceeding 4500 percubic centimeter in some regions of the loplasma torus.

A cold, corotating plasma inside 6 Ri withions of sulfur, oxygen, and sulfur dioxide.

High-energy trapped particles inside 6 RIwith significantly enhanced abundances ofoxygen, sodium, and sulfur.

Hot plasma near the magnetopause predom-inantly composed of protons, oxygen, andsulfur.

Jovian radio emission at kilometer wave-lengths, which may be generated by plasmaoscillations in the lo plasma torus.

Corotating plasma flows unexpectedly farfrom Jupiter in the dayside outer magneto-sphere.

Evidence suggesting a transition from closedmagnetic field lines to a Jovian magnetotailat about 25 Ri from Jupiter.

Whistler emission interpreted as lightningwhistlers from the Jovian atmosphere.

*Adapted from a summary prepared by E. C. Stone and A. L. Lane for the Voyager 1 Thirty-Day Report.

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The dramatic discovery of active volcanoes on to was made by Linda Morabito and her colleagues from this navigationpicture, taken March 8 at a range from to of 4.5 million kilometers. On the bright edge, the immense plume ofvolcanic ashfrom Pele (P1) rises nearly 300 kilometers above the surface. At the terminator, the border between day and night on lo, asecond smaller cloud from the volcano Loki (P2) catches the sunlight. These two eruptionscaptured on this singlediscovery photographare much larger than the largest tt ,.restrial volcanic eruption known. [P-21306 B/W]

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aI IOnce the existence of giant volcanic et uptions on lo was recognized, a reexamination of the Voyager I encounter picturesrevealed many more plumes. These two views of Prometheus (P1) were found by Joseph Vuerka and Robert Strom onMarch 12 when they reproduced earlier pictures. (Bottom lett) The plume is silhouetted against the black space, although it isalso possible to see dark "feet" where the falling material reaches the surface (Above) The complex jets of material areclearly seen as dad streaks against the light background of the surface of lo 1 he plume itself rose more than 100 kilometersabove lo's surface. [P-21295 and P-2124

Meanwhile, new information about Jupiterwas released to the public. A long-exposure(three minutes and twelve seconds) image of thedark side of the planet, taken with the wide-angle camera while in the shadow of the planet,caught Jupiter showing off some Jovian "fire-works," A long, broad, white streak across thepicture was a visible aurora, the largest auroraever seenalmost 29 000 kilometers long. Inaddition, nineteen smaller bright splotches,looking insignificant by comparison, were in

reality "superbolts" of lightning. Since hugeelectrical discharges such as lit ling can,under the right circumstances, per chemicalreactions that form complex organic molecules,the discovery of lightning on Jupiter could haveprofound implications. Was "lightning-inspired" organic synthesis going on inJupiter's atmosphere? No one knew.

Returning to JPL on Sunday night, BradSmith got his first look at the Morabito pictureof the volcanic cloud. Early Monday morning,

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other Imaging Team scientists saw it. As soonas the JPL computers were operating, JosephVeverka and Robert Strom began working withthe two interactive TV terminals to look for evi-dence in other pictures of ongoing eruptions.Faint clouds or plumes would not show up innormally processed pictures, but could bebrought out easily with the computer-controlleddisplays. By midmorning, several additionalvolcanic plumes had been found.

Meanwhile, on March 11, John Pearl ofthe IRIS team had independently drawn theconclusion that volcanic activity must be takingplace on lo. He and Rudy Hanel found evi-dence of strongly enhanced thermal emissionfrom parts of the satellite. The most prominentwas a source nearly 200° C hotter than its sur-roundings. On March 12, Pearl brought his newresults to the Imaging Team, and sure enough,the hot spot was located near one of thevolcanic plumes! A month later, continuing

Linda Morabito shows the discovery photo of the volcaniceruptions on lo. LP-217181

The dark side of Jupiter revealed many surprising phenomena to Voyager 1. A Hide -angle view, taken on March 5, led to thediscovery of a double auroral arc at north-polar latitudes and numerous flashes of lightning illuminating the clouds duringthis 3-minute, I2-second exposure, taken at a range of half a million kilometers. 1260-4601

analysis of IRIS spectra yielded identificationof sulfur dioxide (SO,) gas over this sameerupting volcano. At last a source had beenlocated for the enigmatic sulfur and oxygenions in the magnetosphere.

The volcanoes provided a thread withwhich to weave together the disparate data onlo. A few months earlier there had been areport of a sudden brightening of lo in the in-frared; now it seemed plausible that thermalemission from an eruption was the source. TheVoyager ultraviolet experimenters had beenworrying over the source of the intense sulfuremissions they had seen and had been disturbedby the changes in the gas clouds around lo sincethe Pioneer 10 and II flybys; now a variablesource for these gas clouds was identified. Inaddition, the craterfree surface and bizarrefeatures seen in the Voyager images could berecognized as the product of violent explosiveeruptions on lo. It appeared that Peale, Cassen,and Reynolds had found, in their theoreticalcalculations, the key to the most geologicallyactive body ever encountered in the solarsystem,

News of the discovery was released to thepress on Monday, March 12. During the nextfew days, a total of eight gigantic eruptionswere located in the Voyager pictures of lo.Within a few weeks, scientists all over the worldwere thinking with renewed energy about thisincredible satellite,

With four new planet-sized satellites nowphotographed, there was a sudden requirementfor maps and for names to be assigned to thenewly discovered features. The maps were pro-duced from Voyager images at the Astrogeol-ogy Branch of the U.S. Geological Survey atFlagstaff, Arizona. The names, proposed by agroup of scientists headed by Voyager ImagingTeam members Tobias Owen and Hal Masur-sky, were given official approval by the Inter-national Astronomical Union in August. For atime, a dual nomenclature persisted for theerupting volcanoes on lo. The eruption plumeswere given numbers, P1, P2, etc., while thevolcanic features were given names taken fromthe mythology of fire and volcano legends.Thus the "hoof print" of lo was called Pe le,for the Hawaiian volcano goddess, and the 280-

kilometer -high plume associated with it wascalled P1. By the time of the Voyager 2 en-counter, scientists had prepared maps on whichto plot then new discoveries.

While the Voyager scientists fanned outacross the world to share their findings withcolleagues, attention at JPL turned to Voyager2. In response to the discoveries of the first en-counter, changes were required in the sequenc-ing of scientific observations for July. Voyager2, still troubled by a faulty receiver, might re-quire more coddling from the spacecraft teamthan had its sister spacecraft, now safely on theway to Saturn.

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CHAPTER 7

THE SECOND ENCOUNTER:MORE SURPRISES FROM

THE "LAND" OF THE GIANT

Approaching Jupiter

At the beginning of July, the dry summerheat had returned to Pasadena, and so had thepress. The scientists had come days or weeksearlier to look at data being transmitted fromthe second Voyager as it approached Jupiterand its satellites. The mood at JPL seemedquieter than it had been in March for Voyager1, although the press room would once again bedeluged with observers on the day of encounter.This would be our second good, close look atthe Jovian system, but it was to be no summerrerun. Voyager 2 would have a different view ofeach world, and, in addition, both lo and Jupi-ter had undergone changes, as though to ensurethat no one would become bored and fall asleepin front of a TV monitor. In a sense, this en-counter was to be another first look at Jupiterand its satellites, with a view of each object thatwas quite different from what had been seenbefore.

Changes in Jupiter's cloud formationsbecame noticeable long before July. After a gapof six weeks following the first flyby, Voyager2's observatory phase began on April 24, 1979,seventy-six days before its July 9 encounterwith Jupiter. During this time, the spacecraft'sultraviolet and fields and particles instrumentsstudied the Jovian system and its interactionwith the solar wind. Between April 24 andMay 27, Voyager 2's imaging system concen-trated on the motions in Jupiter's atmosphere,creating another approach time-lapse "movie."From May 27 to 29 photographs were taken in amore rapid sequence, showing the planet duringfive 10-hour rotations. From these studies it

was apparent that Garry Hunt's prediction hadbeen rightthe weather had changed by July.A month before the encounter JPL's VoyagerBulletinMission Status Report announcedthat "Jupiter is sporting quite a different facethan it did just four months ago. The bright`tongue' extending upward from the Red Spotis interacting with a thin, bright cloud above itthat has traveled twice around Jupiter in fourmonths." The turbulent region west of theGreat Red Spot had begun to break up andseparate from the Red Spot. The white ovalssouth of the Red Spot had drifted to the east(about 0.35 degrees a day), while the Red Spotitself had drifted west (about 0.26 degrees aday). The white zone seen just south of the RedSpot by Voyager 1 had become very nar-rowlike a thin white line just barely outliningthe bottom of the spot. The Red Spot had alsochanged: It had become a more uniformorange-red, perhaps reverting to the color seenby Pioneers 10 and 11. The brown spots thathad been seen in the north temperate region atthe same longitude as the Red Spot were nowon the other side of the planet. A dark brownspot not present during the Voyager I flyby haddeveloped along the northern edge of the brownequatorial region on the Red Spot side of theplanet. Some of the white markings that seemedto have protruded into the equatorial region atthe time of the first flyby were missing in theVoyager 2 photographs.

As Voyager 2 entered the far encounterperiod on May 29, all instruments on the space-craft (except for the photopolarimeter) seemedto be in good shape for encounter. As was thecase with Voyager 1, the polarization wheel on

The particles of the rings of Jupiter are stronger reflectors of red light than of blue, as can be seen in this view, assembledfrom two images taken in orange and violet light. Since the images were registered on the rings, not the planet, the brightbands of colors along the edge of Jupiter are an artifact of misalignment. IP217791

t 1 0

In early June, a Voyager 2 carried out its observatory phase, additional changes in Jupiter's face began to be apparent.

These two images, taken from a distance of 24 million kilometers, have a resolution of about 500 kilometers. (Top) TheGreat Red Spot and the white oval south of it are seen to be followed on the west by regions of chaotic and turbulent clouds.

This is not the same white oval that was near the Red Spot in March; the differential rotation of the planet carried a differentoval close to the Red Spot during the intervening three months. (Right) to is visible to the right of the planet, and the shadow

of Ganymede falls on the colored clouds of Jupiter's equatorial belt. IP-21713C and P-21714C1

94

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Launch date = 8120/77Jupiter arrival date = 719179

View ,formal toJupiter equator

1=+i Voyager 2 trajectorySatellite closest approachI= MD MD

Callisto

The Voyager 2 trajectory was compitmentart to that of k oyager I 1 his time, the satellites sere encountered before Jupiter,resealing their other hemispheres As shown in this dram, mg, the spacecraft Hew by first Callisto, then Ganymede, thenEuropa The ten-hour to solcano watch took place immediately atter closest approach to Jupiter 1260-533A)

These two (aces of Jupiter were photographed by Voyager 2 on May 9 at a distance of 46 million kilometers from the planet.Voyager scientists began to detect signtt leant changes in the cloud pattern, since the Voyager I encounter two months earlier.1260-5071

107

weather is changing over one of the northern hemisphere brown ovals in this picture taken July 6 The brown ovals areregions in which breaks in the upper layer of ammonia clouds reveal darker clouds below. A high, white cloud is seen mos ingover the darker cloud, providing an indication of the structure of the cloud layers. Thin w hite clouds are also seen w it hin thedark cloud. At right, blue areas, free of high clouds, are seen. [1321753C1

Voyager 2's2's photopolarimeter was stuck, so theinstrument was able to obtain only color pho-tometry measurements.

Although Voyager 2's radio receiver stillcould not track a Doppler-shifted radio signalfrom Earth the problem is that it "hears amonotone," explained Deputy Project Man-ager Esker K. Davis), the Deep Space Networkengineers had learned to work with the space-craft, determining what frequency the space-craft would listen to at any particular time. Theyhad discovered that some of the "housekeep-ing" telemetry signals from the receiver weresensitive to the match between the incomingfrequency and the receiver frequency. By moni-toring these signals, they could detect a fre-

quency drift in time to correct the transmission,thus keeping the system in tune in spite of slowchanges in the receiver. The system was slowand demanding but effective; all the necessarycommand sequences were successfully loadedinto the computer, and communications duringthe encounter were entirely successful.

The timing offset experienced by VoyagerI as a result of Jupiter's intense radiation envi-ronment was not expected to be a problem onVoyager 2 for two reasons: Even at closestapproach, Voyager 2 would still be more thantwice as far from Jupiter as Voyager I hadbeen, and the Voyager 2 computer was pro-grammed to resynchronize the spacecraft's tim-ing systems automatically every hour. In this

The

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Complex activity in the southern hemisphere of Jupiter continued during the Voyager 2 encounter, although changes hadoccurred in the region of the Great Red Spot. A white oval, different from the one observed in a similar position at the timeof the Voyager I encounter, was situated south of the Red Spot. The region of white clouds extended from east of the RedSpot and around its northern boundary, preventing small cloud vortices from circling the feature. The disturbed region westof the Red Spot had also changed since the equivalent Voyager I image. The picture was taken on July 3 from a distance of 6million kilometers. (13-21742C1

way, even if the radiation environment provedto be much higher than anticipated, the imagesmear that might occur from a timing offsetwould be prevented.

As a result of the discoveries made byVoyager 1, the project scientists decided tomodify some of Voyager 2's preplanned se-quences. As early as April 1, the painstakingjob of constructing new computer commandsbegan. A ten-hour lo Volcano Watch wasadded to the spacecraft's program, takingadvantage of the fact that shortly after closestapproach to Jupiter, the spacecraft would re-

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main within about 1 million kilometers of tofor a long period, keeping nearly the same facein view. Provisions were also made to tak.: ex-tensive ultraviolet measurements of the emis-sion from the glowing torus surroundingJupiter near the orbit of lo. Further studieswould be made of the dark side of Jupiter tosearch for lightning and auroral activity, andthere was also the hope that the plasma wave in-strument would De able to detect lightningwhistlers (radio signets created by lightningbolts) as the Voyag.. I instrument had done. Ahigh priority was given to observations of the

to appeared in front of Jupiter as seen by Voyager on June 25, at a range of 12 million kilometers. At a resolution of about200 kilometers, the bright and dark spots on the satellite are just beginning to be resolved, but it was not possible todetermine if any eruptions were still in progress. (P-71719C1

newly discovered ring, which had not been inthe original Voyager 2 sequence at all. Thespacecraft would cross the ring plane twice,photographing the ring during both the in-bound and the outbound passages.

As was originally planned, Voyager 2would make its closest approaches to Callisto,Ganymede, Europa, and Amalthea before en-counter with Jupiter. Because of the differencein the trajectories of the two spacecraft,Voyager 2 would see the faces of Callisto andGanymede not seen by Voyager I. The most im-portant difference, however, was that t' ec-ond Voyager would fly much closer to E spathan Voyager 1 did, giving scientists their firstgood look at the mysterious streaks scratched

on the surface of that bright golden world.Voyager 2 would also have a closer flyby ofGanymede, giving us a second chance to exam-ine its strange "snowmobile tracks." Themajor loss, of course, was lo, which would beseen from Voyager 2 only at distances of amillion kilometers or :mare.

The Encounter

Wednesday, Jul) 4 (Range to Jupiter, 5.3 mil-lion kilometers; range to Earth, 921 millionkilometers). While most of the nation cele-brated Independence Day with picnics, sportsevents, and fireworks, the scientists and engi-neers at JPL were working around the clock.

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1_s 0

ENCOUNTER DISTANCESFOR VOYAGER 2

Object

Range to Centerat ClosestApproach

(kilometers)

Best ImageResolution

(km per line pair)

Jupiter 722 000 15

Amalthea 558 000 10

to 1 130 000 20

Europa 206 000 4

Ganymede 62 000 1

Callisto 215 000 4

Voyager 2 had already entered Jupiter's ter-ritory, crossing the bow shock for the first timeon July 2 at a distance of 99 Ili from Jupiter,indicating that the magnetosphere had ex-panded in the interval between the two en-counters. At about noon on July 3, the space-craft encountered the magnetopause, but onJuly 4, the data from the particles and fields in-struments were ambiguous. Apparently themagnetosphere was pulsating in response tochanging pressures, and the spacecraft wasplaying iag with the rapidly shifting boundariesof the bow shock and the magnetopause.

As the low energy charged particle instru-ment began to measure particles coming frominside the Jovian magnetosphere, it became ap-parent that some important changes had takenplace since Voyager I's encounter. From thecomposition of the particles, it appeared thatthey were largely of solar origin, unlike theheavy concentrations of ions of sulfur and oxy-gen seen by Voyager I. Scientists began tospeculate that the to volcanoes, which presum-ably eject sulfur and oxygen into the magneto-sphere, might have declined in activity. In theevening, the first images of to at a resolutionhigh enough to allow the volcanic plumes to beseen would be beamed back to Earth.

Thursday, July 5 (Range to Jupiter, 4.4 Millionkilometers). The press room at Von KarmanAuditorium opened and the members of thepress, most of them veterans of the first en-counter, arrived at JPL. Meanwhile, the space-craft continued to measure fluctuations in themagnetospheric boundary. By noon, JPL hadreported at least eleven crossings of the bowshock as the solar wind flirted with Jupiter'smagnetosphere. Apparently the solar wind wasmuch more variable in July than it had been inMarch. At times the bow shock seemed to be

Voyager \ dentists an lously awaited the first views of to that would show whether the volcanic eruptions seen in March werestill active. This picture was taken on July 4, at a range of 4.7 million kilometers, about the same as that of the volcano

discovery picture on March 8. One large plume is clearly visible, rising nearly 200 kilometers above the surface. At the time

of release of this picture on July 6, the scientists wrote, "The volcano apparently has been erupting since it was observed by

Voyager I in March. This suggest:, that the volcanoes on lo probably are in continuous eruption." [13-21738B/WI

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thicker than that experienced by Voyager 1; oneVoyager 2 crossing took ten minutes, whereasthe longest Voyager 1 crossing was only oneminute long. Even though the processes affect-ing the magnetosphere seemed more complex,the magnetosphere was less compressed; whenVoyager 2 actually entered the magnetosphereat a distance of 62 Rj, it was much farthel fromJupiter than Voyager 1 had been at its finalcrossing (47 Rj).

Photos obtained the day before from over4 million kilometers showed that at least one oflo's volcanoes was still active. A total of eightongoing eruptions had been seen by Voyager 1,and scientists were anxious to see how many ofthese were still erupting four months later.

While attention at JPL focused on the un-folding drama of the Jupiter encounter, manymembers of the world's press seemed more in-terested in the fate of Skylab, which was near-ing its death plunge into the Earth's atmos-phere. Launched in 1973, Skylab had been oneof NASA's more successful projects. Threecrews of astronauts had visited it, carrying outintensive studies of the Sun and breaking onerecord after another for the duration ofmanned space flight. Since the departure of thefinal group of three astronauts in 1974, Skylab

VOYAGER 2 BOW SHOCK (S) ANDMAGNFTOPAUSE (M) CROSSINGS

Boundary

S

S

S

MMS

S

M

Day Distance (R j)

Inbound

7/02 99 (multiple)7/02 977/03 877/04 72 (multiple)7/05 717/05 697/05 677/05 62

Outbound

7/23 1697/23 1737/24 1747/24 1757/24 1767/24 1777/25 1847/25 1857/27 2137/31 2538/01 2588/01 262 (multiple)8/03 279 (multiple)8/03 283 (multiple)

Although Voyager 2 did not come as close to lo as had Voyager 1, some changes in the surface during the four monthsbetween encounters were so large that they could still be easily seen. These two pictures of the region of the volcano Pete weretaken in early March and early July, respectively. The most dramatic change was the filliug in of the indentation in the ejectaring, turning the hoofprtnt into a symmetric oval The oval is about 1000 by 700 kilometers in outermost dimension, and thearea that changed amounts to more than 10 000 square kilometers. 1260- 687AC1

had been sinking gradually lower as a result offriction with the extreme upper atmosphere ofEarth. During the past year, higher tempera-tures in the atmosphere had increased this drag,and now the end was near. With a strange fas-cination, the world watched the end of this oldspacecraft, altrost seeming to forget the spec-tacular new Jesuits being transmitted fromJupiter . To the frustration of the Voyager teamand the press "camped out" in Von KarmanAuditorium for the second encounter, theexaggerated stoi ies of a possible Skylab disastertook precedence over Voyager news. Ulti-mately, Skylab fell over the Indian Ocean andAustralia on Wednesday, July 11, just as themajor findings of Voyager 2 were being re-leased.

Friday, July 6 (Range to Jupiter, 3.5 millionkilometers). With the first satellite encounterstill two days away, Voyager 2 continued tomake a variety of measurements of Jupiter andall the Galilean satellites. As the distance to lodecreased, it was possible to see detailed surfacefeatures as well as to look for the volcanic,.... .mes at the edge of the disk, silhouettedagainst black space. By the end of the day, theGreat Red Spot loomed so large that six imag-ing frames (a 2 x 3 mosaic) were required toencompass it and its immediate surroundings.

At the first formal press conference of theVoyager 2 encounter, Project Scientist EdStone reviewed the progress of the mission.Because the ailing spacecraft receiver was work-ing so well, Ray Heacock, Voyager ProjectManager, announced that the major trajectorycorrection maneuver at Jupiter had been re-scheduled to take place only two hours afterclosest approach. Since the geometry was espe-cially favorable at this time, the 76-minuterocket burn could put the spacecraft on itsplanned route to Saturn with a minimumexpenditure of fuel, thereby preserving theoption of sending the spacecraft on to Uranus.

The lo torus was uncle' observation, bothdirectly by the ultraviolet spectrometer, and in-directly by the charged particle instruments.The LECP instrument had begun to pick upsulfur ions, but at lower energies and lowerconcentrations than those recorded during thefirst encounter. In the ultraviolet, glows couldbe seen both from the torus and from auroraein the polar regions of Jupiter.

Photographs of lo showed that the heart-shaped feature surrounding Pe le (P1), lo'slargest volcano, had changed shape. The inden-

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tation of the heart had disappeared, making theheart into an oval. Apparently a new deposit ofvolcanic ejecta had blanketed the surface, alter-ing its color. Perhaps an earlier obstruction inthe volcanic vent, or the shape of the vent itself,had caused the area surrounding Pe le to lookheart-shaped. In any case, whatever had causedthe indentation was now gone. At the sametime, new photos failed to show a plume abovePe le, and there was speculation that changes inthis eruption might be related to the alteredpopulation of charged particles in the magneto-sphere.

Saturday, July 7 (Range to Jupiter, 2.6 millionkilometers). As the spacecraft rapidly closedon Callisto, better and better photographs weretaken of the previously unseen hemisphere. Aswith the Voyager 1 observations, however, themain impression was one of heavy cratering,unrelieved by other geologic structures. Mean-while, the coverage of lo had improved as thesatellite rotated to the point at which a census

The Voyager 2 pictures of Callisto looked remarkably simi-lar to those obtained of the other side of the satellite by

Voyager I. Seen from a distance of 2.3 million kilometers,the large craters (100 kilometers or more across) appear aslight spots. No new major impact features such as Valhalla,discovered by Voyager 1, are visibk on the hemisphere seenby Voyager 2. [P-21740CI

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of the volcanic eruptions seen in the first en-counter began to emerge.

At the 11:00 a.m. press conference, LarrySoderblom announced that four of the vol-canoes discovered by Voyager 1 had beenlooked at again by Voyager 2, and three ofthemPrometheus (P2), Loki (P3), andMarduk (P7) were still active. However, therewas no trace of volcanic activity coming fromPe le, the source of the largest plume seen byVoyager 1. P1 was either greatly subdued orhad turned off completely.

Dr. Soderblom also announced that Voy-ager 2 images had detected another giant ringstructure on Callisto, bringing the total tothree, and there were probably more. This par-ticular ring feature was estimated to be about1500 kilometers across. It was also noted thatalthough Callisto generally seemed to be satu-rated with shoulder-to-shoulder craters, thecrater density near the ring structures seemed tobe lower.

Saturday was a fairly quiet time for the sci-entists but not for the spacecraft or the space-craft team. "We blocked out about 71/2 hours,"

explained Michael Devirian, Ground DataSystems Development, Integration, and TestDirector, "in which we could send it a set ofcommands and re-send it if necessary to makesure all close-encounter commands were re-ceived by Voyager 2 until all the commands gotthrough. The whole thing went perfectly thefirst time." So everything was "go" for closeencounter. The near encounter phase began at6:36 p.m. PDT.

Sunda), Jul) 8 (Range to Jupiter, 1.5 millionkilometers). At 2:30 a.m. the first long-exposure sequence of ring pictures was taken,and at 3:00 a.m. the intensive period of the Cal-listo encounter began. Eighty high-resolutionimages were obtained of the satellite, centeredaround closest approach (215 000 kilometers)at 6:13 a.m. Incoming photos showed some fea-tures that looked like double-walled craters, butno more giant ring structures were seen. It ap-peared that there was an asymmetry in thedistribution of large impact features over Cal-listo's surface. "Callisto may turn out to be the

A new face of Ganymede was revealed to Voyager 2. This image was taken July 7 from a distance of 1.2 million kilometersand clearly shows the large dark area Regio Galileo, as well as much of the lighter grooved terrain discovered by Voyager 1.The bright spots are impact craters. This image also shows what appear to be polar caps, extending down to about latitude45° in both the northern and southern hemispheres. 1260-6701

J.-

'........1-`

most heavily cratered body in the solarsystem," Torrence Johnson remarked. GarryHunt was to add later on, "There's just notroom for another crater on that bodyit'stotally full."

At the press conference, Brad Smith con-firmed the earlier finding that lo's volcano Pe lewas quite deadat least for now. Although P4had not yet been looked at, all other volcanoesdiscovered by Voyager I were still active, butno new plumes had been found. However, newultraviolet images of P2 (Loki) suggested thatthe eruption had increased in size. (In a laterreport, the imaging team announced that P2had increased in height to 175 kilometers andhad changed to a two-column plume.) The newphotographs of Jupiter's ring showed it to bequite narrow and ribbonlike, Dr. Smith an-nounced. The artist's drawing (released duringthe Voyager I encounter), intended to show theouter edge of the ring, turned out to be a goodrepresentation of the actual ring, Dr. Smith said.

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There seemed to be less high-speed sulfurand oxygen inside Jupiter's magnetospherethan there had been during the Voyager I

encounter, George Gloeckler announced.Voyager 2's low energy charged particle instru-ment was finding substantial amounts of car-bon, silicon, magnesium, and other elements ofsolar °rig', but the lo-associated elementswere almost depleted. The ultraviolet instru-ment had found as much glowing sulfur in to'storus as before, but less of it seemed to beraised to energies high enough to leave the torusand be detected elsewhere in the magneto-sphere.

There were other indications of Jupiter'schanging weather. In a Voyager report Sundayevening, Garry Hunt remarked, "One very ex-citing observation came the other day whichcaused major excitement down in the imagingarea. We actually saw a white cloud starting tointrude across a dark barge [large brownishoval-shaped feature in Jupiter's northern

The first close-up views of Europa were both exciting and perplexing to Voyager scientists. The best Voyager I resolutionhad been only about 30 kilometers, but the Voyager 2 trajectory permitted a much closer flyby. These picture, taken on July9 at a range of 240 000 kilometers., have a resolution of about 5 kilometers. The bright icy crust of Europa is covered with aspectacular series of dark streaks, giving the satellite a cracked appearance. In a few cases, narrower light lines run down thecenters of the dark streaks, which are typically a ten tens of kilometers in width. Very few, if any, impact craters are visibleon Europa. 1P-21760C and P-2176401

hemisphere]. Atmospheric scientists get veryexcited by that because this is showing us howthe colors layer themselves upthat whitecloud is clearly above the dark brown. We'redesperately trying to understand the relation-ship of colors on Jupiter."

Monday, July 9 (Range to Jupiter at encounter,722 000 kilometers). Encounter day! And notjust one encounter, but a whole sequence:

Ganymede, Europa, Amalthea, Jupiter, andto. By early Sunday evening, a wealth of newdata on Ganymede was pouring in. Not onlywas this a side of the satellite not seen before,but Voyager 2 would pass closer to Ganymedethan had Voyager 1. Encounter took place at1:06 a.m., at a range of 62 000 kilometers. Be-tween 9:00 p.m. Sunday night and 1:30 a.m.Monday morning a total of 217 photos, plus in-frared and ultraviolet spectra, were scheduled.

11 6105

Sixty-nine photos were sent back in real time;others were recorded for playback later.

As the Ganymede pictures appeared on theTV screens, they revealed a world of tremen-dous variety. Some regions were heavilycratered: "Ganymede looks like Mercury or thehighlands of the moon," one Voyager scientistremarked. Other parts of the surface, however,showed very different features: long, parallelmountain ridges that looked like grooves madewith a giant's rake; narrow, segmented lines;white ejecta blankets from impacts that look edlike a dazzling, snow-covered landscape. Someof the pictures suggested cracking and slippingof Ganymede's crust, while others showed whatappeared to be remnants of ancient terrainunaffected by subsequent intense geologicactivity. Many of the highest resolution frameswere not seen at this time; they were recordedon the spacecraft for playback later.

Starting at about 8 a.m., Earth began re-ceiving the first closeup views of Europa.Europa "could be the most exciting satellite inthe whole Jovian system," said Larry Soder-blom, "because it's sort of the transition bodybetween the solid silicate body, lo, and the iceballs, Ganymede and Callisto." The icy crustlooked as though it "had been ruptured alloveras though it was in piecesjust asthough it had been broken in place and leftthere." At 11:43 a.m., closest approach tookplace at a range of 206 000 kilometers. By thistime the scientists were dazzled by what theyhad seen; some were calling Europa the mostbizzare of all the Galilean satellites. In theImaging Team viewing area, David Morrisoncompared Europa's surface to "a crackedegg," and Gene Shoemaker said, "It looks likesea ice to me." When someone commented thatthe canal-like streaks were reminiscent of Mars,Torrence Johnson replied, "It looks like somepictures of Mars I've seen, but only on the wallsof Lowell Observatory." Another quipped,"Where is Percival Lowell, now that we needhim!"

There were to be two press conferences:one to present spacecraft and scientific resultsand one to celebrate the second successful flybyand to talk of new goalsSaturn, and perhapsUranus.

At the first conference, Ed Stone began bydiscussing the radiation Voyager was experi-encing. One of Jupiter's surprises was that theradiation environment was greater than hadbeen anticipated, and this caused problems with

106

the radio receive!. The receiver frequency wasshifting "more rapidly than we hadanticipated," said Ray Heacock, "and we havenot been able to keep an uplink continuouslywith the spacecraft." The solution was to keepsending up commands at different frequenciesuntil a frequency the spacecraft would acceptwas found. Just how bad were the radiationlevels? Ed Stone commented, "The penetratingradiation at a given distance is more intensenow at this distance than it was when Voyagerflew by." From a preliminary analysis itseemed that, overall, Voyager 2 would still besubjected to lower radiation levels thanVoyager 1 had been, but to higher levels thanhad been expected. In addition, Voyager 2'sradio receiver was much more sensitive. Thehigher than expected radiation intensity also ledVoyager scientists to have the ultraviolet spec-trometer shut off, since that instrument wasalso quite sensitive to the radiation.

The fourth member of the Galilean satel-lites had finally been seen, and Larry Soder-blom happily introduced Europa. "Well, somefew months ago, before the Voyager 1 en-counter, we thought we had some idea of whatplanets were likeat least the planets in theinner solar system: Mars, Mercury, the Moon,the Earth. And we've discovered many timesover in the last couple of months how narrowour vision really was. Included in the Joviancollection of satellites are the oldest (Callisto),the youngest (1o), the darkest (Amalthea), thebrightest (Europa), the reddest (Amalthea and10), the whitest (Europa), the most active (1o),and the least active (Callisto). Today we foundthe flattest (Europa)."

In spite of the appearance of a cracked orbroken surface, Europa showed no topographyat all. Toward the sunset line, where the lowangle of illumination should reveal even lowrelief, "the surface disappearsas if it were thesurface of a billiard ball." It seemed clear thatEuropa has much less relief than the other twoicy satellites, Ganymede and Callisto. But whycan't Europa's surface support relief? PerhapsEuropa has a thick ice mantleon the order of100 kilometers. If Europa is affected by tidalheating, then such an ice mantle might be "sortof soft and slushy" rather than rigid as are thecrusts of Ganymede and Callisto. "The factthat the surface of Europa cannot support reliefof any substantial amount suggests that the sur-face must be soft." But, Dr. Soderblom added,there does not appear to be much lateral motion

Regio Galileo is the largest remnant of the ancient, heavily cratered crust of Ganymede. ThisVoyager 2 color reconstructionwas made from pictures taken at a range of 310 000 kilometers; the scene is about 1300 kilometers across. Numerous craters,many with central peaks, ';re visible. The large bright circular features have little relief and are probably theremnants of oldlarge craters that have been annealed by the flow of icy near-surface material. The closely spaced, arcuate linear features areanalogous to features on Ca Ilisto, such as the "ripple" marks surrounding the ancient impact feature Valhalla. IP-21761(1

107

At high resolution, the grooved terrain on Ganymede showsa wonderful complexity. Surface features as small as 1

kilometer across can be seen in this mosaic of Voyager 2images taken July 9. The grooves are basically long, parallelmountain ridges, 10 to 15 kilometers from crest to crestabout the same scale as the Appalachian mountains in theEastern United States. The numerous impact craters super-posed on the mountain ridges indicate that they are oldprobably formed se% eral billion years ago. [260-6371

or rotation causing the surface markingstheydon't seem to be offsetrather, "it's as ifEuropa had been cracked, broken, by someprocess which crushed it like an eggshell andjust left the pieces sitting there. Expansion andcontraction of ice and water are a good way tocrunch up the surface."

The other side of Ganymede presentedquite a different face from the one Voyager 1had seen. Here were the dark ancient crateredterrains, the shoulder-to-shoulder cratersreminiscent of Callisto, and there Has a hugecircular feature on Ganymede looking like theremnant of a Callisto-style ringed basin,preserved in the ancient, dark terrain. The very

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large dark feature revealed by Voyager in thenorthern hemisphere which bears these impactscars was later named "Regio Galileo," for thediscoverer of the Galilean satellites. It was seenin the low-resolution Pioneer 10 picture ofGanymede taken in 1973, but its nature was notunderstood. It is so large it has even beenglimpsed on occasions of exceptionally stable"seeing" with ground-based telescopes.

3:29 p.m. PDTJupiter Encounter! In thepress room half a dozen cameras clicked inunison as the universal clock declared theVoyager 2 had made its closest approach toJupiter-650 000 kilometers from the cloudtops, zipping by at about 73 000 kilometers perhourneither as close nor as fast as Voyager 1.By the time of the special press conference at4:30 p.m., everyone at JPL was in a partymood. Thomas A. Mutch, who had replacedNoel Hinners as NASA Associate Administra-tor for Space Science, Robert Parks, andRodney Mills were the speakers.

The Jovian system is a place of "incrediblebeauty and mystery. Jupiter has been a niceplace to go by, but we wouldn't want to stoptherewe're going on to Saturn," Rod Millsexplained, and Bob Parks agreed.

Tim Mutch had a different perspective."Although we have just heard Jupiter some-what downgraded in favor of Saturn, nonethe-less what we have been witnessing, first inMarch and now, in July, is a truly revolutionaryjourney of exploration. We have gone beyondthe familiar part of the solar system to objectsthat are so exotic that their very existence, atleast as far as I'm concerned, was something I'daccepted intellectually, but didn't really acceptin an immediate sense. We're starting out in ourown space program on a new stage of space ex-plorationon our own long journeys beyondthe solar system to distant lands. We never liketo think, or rather, it's statistically unlikely,that we're at a turning point in history. But ifyou look back at history books, such events areclearly read into the record. And I submit toyou that when the history books are written ahundred years from now, two hundred yearsfrom now, the historians are going to cite thisparticular period of exploration as a turningpoint in our cultural, our scientific, our intellec-tual development."

Although everyone was already celebratinganother successful mission, the encounter wasfar from over. Data continued to come in; therewas still the ten-hour lo Volcano Watch, whichhad begun at 4:31 p.m.; there were more obser-

J

vations of Jupiter, including scheduled ringobservations and dark side searches for auroraeand lightning bolts. There was a lot of workand excitement yet to come. Jupiter hadanother surprise in store for Voyager 2.

Tuesday, July 10 (Range to Jupiter, 1.4 millionkilometers). The lo watch continued through

the night. As time passed, the satellite rotatedin the same direction as the motion of the space-craft, keeping nearly the same side in view.Because of this, a few volcanoes could beclosely watched, but most would be missed en-tirely. During the sequence, the illuminatedcrescent steadily shrank, until at the end,volcanic plumes could be seen on both edges,

During the 10-hour to volcano watch on July 9, the spacecraft kept nearly the same face of lo in view. Most of the surfacewas turned away from the Sun, however, and only a thin crescent could be seen, shrinking as the observations continuedThese four frames were all photographed with identical exposures from a range of about 1 million kilometers. Theseimagesshow Amiran: (135) and Maui (P6) on the west edge, brightening as the Sun Illuminates them more nearly from behind.Masubi (Ps) is faintly visible in the crescent (top right and left); (bottom right) Loki (P2) rises 250 kilometers above the sur-face, catching the morning sunlight on the east edge of lo. (260-6771

109

one illuminated by the setting Sun, the othershining in the dawn light.

At the 11 a.m. press conference, Eska Dav isannounced that engineers had lost contact withthe spacecraft radio receiver Monday evening(probably due to Jupiter's radiation) and hadto "chase it around most of the night," sendingcommands at various frequencies until theylocked on to the frequency the spacecraft wouldaccept. The major trajectory correction maneu-ver, begun at about the same time contact withthe receiver was lost, was successful. The76-minute thruster firing, done at periapsis in-stead of two weeks after encounter, enabled thespacecraft to get a bigger "boost" from Jupiterthan was originally planned, amounting to afuel saving of about 10 kilograms of hydrazine,enough to preserve the option of going on fromSaturn to Uranus.

Andrew Ingersoll discussed some results ofthe analysis of the Jovian atmosphere. "Atfirst, Voyager seemed to do nothing but empha-size the chaos, not the order." But, with thehelp of ground-based observations, Reta Beebefound that there is a "regular alternation ofeastward and westward jets" underlying theseemingly chaotic visible features. "The turbu-lence we see in the visible clouds seems to be aminor side show, or a process without muchenergy or mass compared to the very greatenergy and mass that might be moving aroundin the deep atmosphere."

"We're continuing to operate in our panicmode to try to get pictures to the press," BradSmith said as he introduced new photographsof the satellites. In earlier photos, Ganymedehad seemed to have two different kinds ofterrainan ancient, cratered, Callisto-like sur-face, and the stranger, grooved terrainterrains that might be representative of twovery different types of major episodes in Gany-mede's history. The most recent images showeda much more confused picture, with several ad-ditional types of surface geology.

At the daily project science briefing,another interpretation was being discussed.Lyle Broadfoot reported that new measure-ments of the position of the ultraviolet aurorademonstrated that :t was caused by chargedparticles from the lo torus, not from the outerparts of the magnetosphere. Apparently theseplasma particles arise in the volcanic eruptions,

110

are trapped for a time in the torus, and then fallinto the polar regions of Jupiter, where they ex-cite auroral emissions. A terrestrial aurora, incontrast, is caused by particles that originate inthe soiar wind. Jim Sullivan of the plasma in-vestigation estimated that about two tons ofmaterial each second are fed from lo into theplasma torus. This plasma, driven by the rota-tion of tne Jovian magnetic field, appears to beable to supply the million-million watts ofpower radiated in the ultraviolet.

By 5:00 p.m. the excitement had dieddown; many of the scientists had parties to at-tend that evening, and some members of thepress vvere planning parties of their own. Theschedule of spacecraft activities also seemed tohave slowed. There were dark-side observationsplanned to search for lightning and aurorae.There would be a few more ring picturesnottoo much to see on the monitors that night . . .

or so many people thought. But a few peoplewere waiting around, perhaps to catch a

glimpse of lightning or auroral activity, or towait for another look at Jupiter's faint ring.

Between 5:52 and 6:16 p.m., six long-exposure, wide-angle photographs of the darkside of Jupiter had been scheduled to search foraurorae and lightning. The spacecraft was1 450 000 kilometers ft-3m Jupiter and abouttwo degrees below the equatorial plane.

Shortly after 6 p.m., the first of these ringphotographs appeared on the TV monitors withunexpected brilliance. Taken in orange andviolet light, the images showed the outline ofJupiter and, protruding from it, two narrowlinesone reaching all the way to Jupiter'slimb, the other broken off, apparently hiddenby the shadow of the giant planet. Seen from:he new perspective of the shadow of Jupiter,the tenuous rings were remarkably clear. A sud-den renewal of excitement surged through thedevotees remaining in the press room. About6:15, Brad Smith came down to join the pressto watch the remainder of this series of picturescome in. "Hey Brad, are you going to burn outthe camera with the ring?" someone joked."Well, the rings do forward scatter nicely,don't they?" Dr. Smith replied. As the wide-angle pictures were followed by narrow-angleviews, more and more detail became apparent.For the first time, a definite width for the ringcould be seen, and there was even a hint of ad-ditional material inside the main ring. All in all,

1 °i.., ,

From a vantage point 2.5 degrees above the ring plane,Voyager 2 was able for the I irst time to determine the widthof Jupiter's ring. This picture shows that the ring is ribbon-like and only a few thousand kilometers wide, quite unlikethe broad rings of Saturn. Ii-21757E37M

Voyager had provided one more splendid seriesof pictures before it took off for Saturn.

Wednesda), Jul) 11. JPL Public InformationOfficer Frank Bristow opened the 10 a.m. pressconference with an announcement: "We'll havethe report from the Imaging Team including thetremendous pictures that we received here lastnight of the Jupiter ring that excited the entireteam."

Brad Smith showed the ring pictures. "Asmany of you v4ho were here last night know, wegot some rather nice pictures of the ring ofJupiter. It's as though Voyager 2 was fearfulthat we might be becoming just a little bitapathetic after this series of marvelous discov-eries and felt that it had to dazzle us one moretime before it left for Saturn. The rings appearvery much brighter than we had expected themto be." The outer ring is about 6500 kilometerswide. There is material inside the ring. There isa rather sharp outer boundary and a somewhatdiffuse inner region. "And it is now our beliefthat the material in the ring goes all the waydown to the surface of Jupiter." There is a verynarrow relatively bright outer ring and an ex-tremely faint inner ring that goes all the waydown to Jupiter's cloud tops.

Larry Soderblom summarized the satellitedata: With respect to the Galilean satellites,"We're in a relatively high state of ignorance."

The lo Volcano Watch images seemed toindicate that plume P2 was now the highestvolcano on lo, since P1 seemed to have becomequiet. to may be the easiest Galilean satellite totry to understand, because we can actually seethe geological processes that are shaping theplanet. lo's "twin," Europa, seems to be where"our highest state of ib. Jrance" lies. "Thefaint bright streaks which show some relief areevidently different from the diffuse dark bandswhich don't seem to show topography, but thesimilarity of these forms [that both the lightand dark markings are of planetary scale] sug-gests that they must be related."

Ed Stone speculated about the other twoGalilean satellites. Ganymede and Callisto areessentially identical in size, mass, and probablycomposition. By examining them, we canperhaps learn what happens when bodies withvery similar chemistry have different "lifehistories" and different surface properties(there are indications that Ganymede's crustmay not have been as rigid as Callisto's). Going

1°111

4'

One of the most spectacular of the Voyager 2 images was obtained from inside the shadow of Jupiter. Looking back towardthe planet and the rings with its wide-angle camera, the spacecraft took these photos on July 10 from a distance of 1.5 millionkilometers. The ribbon-like nature o. the rings is clearly shown. The planet is outlined by sunlight scattered from a haze layerhigh in the atmosphere On each side, the arms of the ring curving back toward the spacecraft are cut off by the planet'sshadow as they approach the brightly outlined disk. [P-21774B/W)

The rings of Jupiter proved to be unexpectedly bright when seen with the Sun nearly behind them. Strong forward catteringof sunlight is characteristic of small particles. These two views were obtained by Voyage' 2 on July 10 from a perspciiveInside the shadow of Jupiter. The distance of the spacecraft from the rings was about 1.5 million kilometers. Although theresolutim has been degraded by camera motion during the time exposures, these images reveal that the rings have someradial structure. [260-610B/W and 260-6741

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HIGHLIGHTS OF THE VOYAGER 2 SCIENTIFIC FINDINGS

Atmosphere

The main atmospheric jet streals were pres-ent during both Voyager encounters, withsome changes in velocity.

The Great Red Spot, the white ovals, and thesmaller white spots at 41°S, appear to bemeteorologically similar.

The formation of a structure east of theGreat Red Spot created a barrier to the flowof small spots which earlier were circulatingabout the Great Red Spot.

The ethane to acetylene abundance ratio inthe upper atmosphere appears to be larger inthe polar regions than at lower latitudes andappears to be 1.7 times higher on Voyager 2than on Voyager 1.

An ultraviolet map of Jupiter shows thedistribution of absorbing haze. The polarregions are surprisingly dark, suggesting thatthe absorbing material must be at highaltitudes.

Equatorial ultraviolet emissions indicateplanet-wide precipitation of charged parti-cles into the atmosphere from the magneto-sphei e.

The high-latitude ultraviolet auroral activityis due to charged particles that originate inthe lo torus.

Satellites and Ring System

The ring consists of a bright, narrow seg-ment surrounded by a broader, dimmer seg-ment, with a total width of about 5800kilometers.

The interior of the ring is filled with muchfainter material that may extend down to thetop of the atmosphere.

Images of Amalthea in silhouette againstJupiter indicate that the satellite may befaceted or diamond shaped.

Volcanic activity on lo changed somewhat,with six of the plumes observed by Voyager 1still erupting.

The largest Voyager 1 plume (Pele) hadceased, while the dimensions of anotherplume (Loki) had increased by 50 percent.

Several large-scale changes in lo's appear-ance had occurred, consistent with surfacedeposition rates calculated for the largeeruptions.

Europa is remarkably smooth with very fewcraters. The surface consists primarily ofuniformly bright terrain crossed by linearmarkings and very low ridges.

There are four basic terrain type- onGanymede, including younger, smooth ter-rain and a rugged impact basin first observedby Voyager 2.

Callisto's entire surface is densely crateredand is likely to be !everal billion years old.

Equatorial surface temperatures on theGalilean satellites range from 80 K (night) to155 K (the subsolar point on Callisto).

Magnetosphere

The outer region of the magnetosphere con-tains a hot plasma consisting primarily ofhydrogen, oxygen, and sulfur ions.

1 he hot plasma generally flows in the coro-tation direction out to the boundary of themagnetosphere.

Beyond about 160 Rj, the hot plasmastreams nearly antisunward.

Outbound the spacecraft experienced mul-tiple magnetopause crossings between 204 R jand 215 Rj.

The abundance of oxygen and sulfur relativeto helium at high energy increases withdecreasing distance from Jupiter.

Measurements of high energy oxygen suggestthat these nuclei are diffusing inward towardJupiter.

The ultraviolet emission from the lo plasmatorus was twice as bright as four monthsearlier and the temperature had decreased by30 percent to 60 000 K.

The low-frequency (kilometric) radio emis-sions from Jupiter have a strong latitudedependence and often contain narrowbandemissions that drift to lower or higher fre-quencies with time.

A complex magnetospheric interaction withGanymede was observed in the magneticfield, plasma, and energetic particles up toabout 200 000 kilometers from the satellite.

*Adapted from a summary prepared by E. C. Stone and A. L. Lane for the Voyager 2 Thirty-Day Report.

A new inner satellite of Jupiter, provisionally designated 197911, was discovered by David Lewitt and Ed Danielson ofCal.ech in these Voyager 2 ring photographs. (Top) In a 15- second exposure with the wide-angle camera, the edge-on ringshows as a faint line, and the satellite is the dot indicated by the arrow. (Bottom) In a narrow angle 96 second exposure, themotion of the satellite can be seen. Agai the faint band is the ring, blurred by camera motion, and the arrow indicates thestreak due to the satellite. A star streak i!, ,ocated above and to the left of the satellite; note that the length and angle of thetwo trails are different, owing to satellite motion [260-807 and P-221721

further, he added that Callisto and Mercury,the least dense and the most dense, respectively,of the terrestrial-style planets, although totallydifferent in composition and density, seem tohave similar surfaces and similar histories.What would have happened to Mercury if ithad been made of ice, water, and rock asCallisto is? Would it have evolved as Callistodid?

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A New Satellite

One of the most fascinating discoveries ofVoyager 2 was not recognized at first. Graduatestudent David Jew' of the California Instituteof Technology, working with Imaging Teammember Ed Danielson, began a detailedanalysis of all the ring photos in late summer.In early October he determined that a short

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A fifteenth satellite of Jupiter was discovered in the spring of 1980 by Steven Synnott of JPL. It was first seen on thisVoyager I image taken March 5, 1979, in which the 75-kilometer-diameter satellite shows as a dark oval against the planet.Also visible is the shadow of the satellite, designated 1979J2. This satellite orbits between lo and Amalthea with a period of16 hours and 11 minutes. [P-22580B/W1

streak on a photo taken Juiy 8, previouslypresumed to be an image of a star trailed by thetime exposure, did not correspond to any knownstar position. Perhaps this was a new satellite!Additional sleuthing turned up a second imageof the same part of the ring that also showedthe anomalous oh,,ect, together with trails dueto known stars. The differing angles andlengths of the trails of the object and the starsconfirmed that this was indeed a 14th satelliteof Jupiter. Following the guidelines of the In-ternational Astronomical Union, it wasdesignated 1979) I, pending later assignment ofa mythological name. The proposed name isAdrastea, a nymph who nursed the infant Zeusin Greek legend.

The newly discovered satellite orbitsJupiter at a distance of 58 000 kilometers abovethe equatorial cloud tops, placing it just at theouter edge of the ring and much closer to theplanet than is Amalthea, previously thought tobe the innermost satellite. It travels at 30kilometers per second (nearly 70 000 miles perhour), circling Jupiter in just seven hours andeight minutes. From its brightness, scientistsguessed that A might be 30-40 kilometers indiameter.

The proximity of Adrastea to the ring sug-gests a relationship between the two. When thediscovery was announced to the press in mid-October, it was speculated that the ringmaterial might originate on the satellite,

perhaps eroded away by the energetic chargedparticles in the inner Jovian magnetosphere.Once again, Voyager had added to our perspec-tive on planetary processes, suggesting that un-discovered but similar small satellites mightalso be a ,sociated with the rings of Saturn andUranus.

Voyager 2 had certainly added a few years'of data of its own to Voyager 1's "ten years'worth of data." It had given a different view ofthe Jovian system, helping to solve some of themystery surrounding Jupiter and its satellites,and creating new mysteries. As Voyager 2 spedout away from Jupiter, riding along the giantplanet's huge magnetotail, attention turned toSaturn: What would Pioneer 11, thePathfinder, discover in September 1979? Whatwould the Voyagers learn in November 1980and August 1981? Would all go well? WouldVoyager 2 fly on to Uranus?

There was also a yearning to examine moreclosely, with the Galileo Project, what had beenunknown for so long, yet had become sofamiliar in only a few months' timethe littledark, red "potato" Amalthea, the volcano-covered world lo, the mysterious "crackedbilliard ball" Europa, cratered and groovyGanymede, ancient Callisto, and the king of theplanets itself, a colorful, banded world ofstable climate and ever-changing weather pat-terns.

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CHAPTER 8

JUPITER KING OFTHE PLANETS

A Star That Failed

More massive than all the other planetscombined, Jupiter dominates the planetarysystem. The giant revealed by Voyager is a gasplanet of great complexity; its atmosphere is inconstant motion, driven by heat escaping froma glowing interior as well as by sunlight ab-sorbed from above. Energetic atomic particlesstream around it, caught in a magnetic fieldthat reaches out nearly 10 million kilometersinto the surrounding space, embracing the seveninner satellites. From its deep interior throughits seething clouds out to its pulsating magneto-sphere, Jupiter is a place where forces of in-credible energy contend.

At its birth, Jupiter shone like a star. Theenergy released by infalling material from thesolar nebula heated its interior, and the larger itgrew the hotter it became. Theorists calculatethat when the nebular material was finally ex-hausted, Jupiter had a diameter more than tentimes its present one, a central temperature ofabout 50 000 K, and a luminosity about onepercent as great as that of the Sun today.

At this early stage, Jupiter rivaled the Sun.Had it been perhaps 70 times more massivethan it was, it would have continued to contractand increase in temperature, until self-sustaining nuclear reactions could ignite in itsinterior. If this had happened, the Sun wouldhave been a double star, and the Earth and theother planets might not have formed. However,Jupiter did not make it as a star; after a briefflash of glory, it began to cool.

At first Jupiter continued to collapse.Within the first ten million years of its life, the

planet was reduced to nearly its present size,with only a few percent additional shrinkageduring the past 4.5 billion years. The luminosityalso dropped as internal heat was carried to thesurface by convection and radiated away tospace. After a million years Jupiter emittedonly one-hundred thousandth as much radia-tion as the Sun, and today its luminosity is onlyone-ten billionth of the Sun's.

Jupiter's internal energy, although smallby stellar standards, has important effects onthe planet. About 1017 watts of power, com-parable to that received by Jupiter from theSun, reach the surface from the still-luminousinterior. The central temperature is still thoughtto be about 30 000 K, sufficient to maintain theinterior in a molten state. Scientists generallyagree that Jupiter is an entirely fluid planet,with no solid core whatever.

Composition and AtmosphericStructure

Because of its great mass, Jupiter has beenundiscriminating in its composition. All gasesand solids available in the early solar nebulawere attracted and held by its powerful gravity.Thus it is expected that Jupiter has the samebasic composition as the Sun, with both bodiespreserving a sample of the original cosmicmaterial from which the solar system formed.

The primary constituents of Jupiter havelong been suspected to be hydrogen and helium,the two sfraplest and lightest atoms. However,it has proved impossible to derive accuratemeasurements of the abundance of these twoelements from astronomical observations. On

The J.Ipiter seen by the Voyager camera:, is a cloud belted world of rapid jet streams and complex cloud forms. Prominent inthis oyager 1 image, taken February 5 at a range of 28.4 million kilometers, is the alternating structure of light zones anddark I elts, and the Great Red Spot and numerous smaller spots. Also easily visible are the two inner Galilean satellites, loand Euiopa. The resolution in this picture is 500 kilometers, about five times better than can be obtained from Earth -hatedtelescopes. Callisto can be faintly seen at the lower left. [P-21083C1

1° (,)

Jupiter is a gas giant, composed of the same elements as the Sun and starsprimarily hydrogen and helium. Its internalstructure is dominated by the properties of hydrogen, its most abundant constituent and by the high temperatures in the deepinterior that remain from its luminous youth. Most of the interior is liquid: metallic hydrogen at great depths and highpressures, and normal hydrogen nearer the surface. In the upper few thousand kilometers, the hydrogen is a gas. Theprimary known or suspected cloud layers are, from the top down, thin hydrocarbon "smog"; ammonia; ammonium hydro-sulfide; water-ice, and liquid water. [260-8281

118

J1"

the basis of a rather simple infrared measure-ment, Pioneer investigators found He /H, =0.14 ± 0.08. On Voyager, IRIS was able to ob-tain much improved infrared spectra, yieldingan initial value of He/ H2 = 0.11 ± 0.3. Voy-ager scientists expect that further analysis willreduce the uncertainty to about ± 0.01. Theratio of 0.11 is in excellent agreement with thesolar value of about 0.12, supporting the ideathat Jupiter and the Sun have similar elementalcompositions.

Astronomers have known for a long timethat, in addition to hydrogen and helium, thecompounds methane (CH4) and ammonia(NH3) are present in the visible atmosphere ofJupiter. In the 1970s, additional spectra in theinfrared resulted in the discovery of water(H2 3), ethane (C2H6), germane (GeH4),acetylene (C2H2), phosphine (PH3), carbonmonoxide (CO), hydrogen cyanide (HCN), andcarbon dioxide (CO2). All these are traceconstituents, with two of them, ethane andacetylene, apparently formed at high altitudesby the action of sunlight on methane.

A total of approximately 100 000 infraredspectra, many of small regions on the disk,were obtained by IRIS. These spectra generallyshow hydrogen, helium, methane, ammonia,phosphine, ethane, and acetylene. In addition,excellent spectra were obtained in "hot spots,"regions in which breaks in the upper clouds per-mit radiation from deeper layers to escape.(The hot spots generally correspond tc darkbrown regions on photographs of the planet.)IRIS measured temperatures in the hot spots upto 13° C but no higher; apparently this tem-perature corresponds to the top of a deepercloud deck. Spectral features indicative of thepresence of water vapor and germane wereclearly seen in the hot spots.

Further analysis of the IRIS spectra will berequired to derive the abundances of the gasesdetected. However, even the preliminary datashowed how variable Jupiter can be, especiallyin its upper atmosphere. The two hydrocar-bons, ethane and acetylene, vary in relativeabundance with latitude; there is less acetylenenear the poles. In addition to this planetwidetrend, smaller variations were seen from placeto place and between the observations in Marchand July. All the variations will eventually pro-vide information on the processes of forma-tion, transportation, and destruction of hydro-carbons in the upper atmosphere.

Voyager did not make any direct measure-ments of the chemical composition of the

ELEMENTS DETECTED IN THEJOVIAN MAGNETOSPHERE

ElementAtomicNumber Instruments

Hydrogen (H) 1 UVS, Plasma,LECP, CRS

Helium (He) 2 Plasma, LECPCarbon (C) 6 LECPNitrogen (N) 7 CRSOxygen (0) 8 UVS, Plasma,

LECP, CRSNeon (Ne) 10 CRSSodium (Na) 11 LECP, CRSMagnesium (Mg) 12 CRSSilicon (Si) 14 CRSSulfur (S) 16 UVS, Plasma,

LECP, CRSIron (Fe) 26 CRS

clouds, but theorists generally agree that theuppermost clouds are ammonia cirrus, and thatlayers of ammonium hydrosulfide (NH4SH)and water exist at deeper levels. All these cloudsare formed in the troposphere, the layer of theatmosphere in which convection takes place.The top of the ammonia cloud deck is thoughtto have a pressure of about 1 atmosphere and atemperature of about 113° C.

Ammonia cirrus is white, yet Jupiter'sclouds display a spectacular range of colors.Voyager did not determine the nature of thecoloring agents; they may be minor constit-uentstrace impurities in a sea of white clouds.Perhaps organic polymers, formed from atmos-pheric chemicals such as methane and ammoniathat have reacted with lightniag, are responsiblefor the oranges and yellows. The color of theRed Spot could be caused by red phosphorus(P4). According to this theory, phosphine(PH3) from deep in Jupiter's atmosphere isbrought to high altitudes by the upwelling ofthe Great Red Spot. Ultraviolet light, penetrat-ing the upper reaches of the Red Spot, splits thephosphine molecules, and, through a series ofchemical reactions, converts the phosphine intopure phosphorus. However, this theory fails toexplain the existence of the smaller -ed spots onJupiter; these spots are not at such highaltitudes as the Great Red Spot (which is thehighest and coldest of Jupiter's visible clouds),so it is unlikely that ultraviolet light could reactwith any phosphine in these areas to producered phosphorus.

Various forms of elemental sulfur might beresponsible for the riot of color we see on

.1.:,- 0119

North pole

Although the Voyager spacecraft never flew over the poles of Jupiter, it is possible to reconstruct from several images theview that would be seen from directly above or below the planet. The north pole is shown above and the south pole at right.Note the absence of a strong banded structure near both poles. The regular spacing of cloud features is obvious. In thesouthern hemisphere, the three white ovals are 90 degrees apart in longitude, b a fourth oval at the other quadrant ismissing. The irregular black areas at each pole are places for which no Voyager data exist. The resolution of the originalpictures from which these polar projections were made was about 600 kilometers. [P- 21638C and P-21639C1

120 1I)o ..,.

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South pole

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Jupiter. Sulfur forms polymers (S3, S4, S5, S8,)that are yellow, red, and brown, but no sulfurin any form has been detected on Jupiter. "Wenever promised you we were going to identifythe colors on Jupiter with this mission," one ofthe atmospheric scientists remarked, "but wewill have a probe that is going into the atmos-phere in the mid-1980sGalileo." Perhaps themystery of the Jovian clouds will have to waittill then.

Temperature maps of Jupiter were ob-tained by IRIS in radiation arising at differentlevels above the clouds. Maps show terrpera-tures at pressures of 0.8 atmosphere near theclouds, and 0.2 atmosphere near the top of thetroposphere. In addition to the low tempera-tures over the bright zones and the highertemperatures over dark belts, there is a greatdeal of smaller scale structure. It is interestingthat a cold area corresponding to the Great RedSpot is clearly visible even near the top of thetroposphere, indicating that this featuredisturbs the atmosphere to very high altitudes.

The structure of the atmosphere of Jupiterabove the troposphere was investigated throughthe radio occultation experiment as well as by

IRIS. The level in which the minimum tempera-ture of about 173° C occurs has a pressure of0.1 atmosphere. Above this point lies the strato-sphere, in which temperatures increase withaltitude as a result of sunlight absorbed by thegas or by aerosol particles resembling smog. At70 kilometers above the ammonia clouds, thetemperature is about 113° C. Above thislevel, the temperature sta s approximately con-stant, although at extreme altitudes the tem-perature again rises in the ionosphere.

Weather on Jupiter

The Voyager pictures reveal a planet ofcomplex atmospheric motions. Spots chaseafter each other, meet, whirl around, mingle,and then split up again; filamentary structurescurl into spirals that open outward; featherycloud systems reach out toward neighboringregions; cumulus clouds that look like ostrichplumes may brighten suddenly as they floattoward the east; spots stream around the RedSpot or get caught up in its vortical motionallin an incredible interplay of color, texture, andeastward and westward flows. Such changes canbe noticed in the space of only a few Jovian days.

If one could "unwrap" Jupiter like a map, views such as these would be obtained. The planet as It appeared about March Itsshown at the top and as it was in early July at the bottom The comparison between the pictures shows the relative motions offeatures in Jupiter's atmosphere. It can be seen, for example, that the Great Red Spot moved westward and the white ovalseastward during the time between the acc 'isition of these pictures. Regular plume patterns are equidistant around thenorthern edge of the equator, while a train of small spots moved eastward at approximately latitude 80° S. In addition tothese relative motions, significant changes are evident in the recirculating flow east of the Great Red Spot, in the disturbedregion west of the Great Red Spot, and as seen in the brightening of material spreading Into the equatorial region from themore southerly latitudes. [P-2177IC]

-

On a broader time scale, greater changeson the face of Jupiter can be seen. Featuresdrift around the planet; even the large whiteovals and the Great Red Spot slide along intheir respective latitudes. Belts or zones intrudeupon each other, resulting in one of the bandedstructures splitting up or seeming to squeezetogether and eventually disappear. Small struc-tures form, then die. The largest spots mayslowly shrink in size, and the Red Spot itselfchanges its size and color.

The Jupiter of Pioneers 10 and 11 wasquite unlike the planet seen by Voyager 1. At

the time of the Pioneer exploration, the GreatRed Spot, embedded in a huge white zone, wasmore uniformly colored, and pale brown bandscircled the northern hemisphere. In the inter-vening years, the south temperate latitudes havechanged completely, developing the complexturbulent clouds seen around the Red Spot byVoyager 1. Yet, even between the two Voy-agers, Jupiter appeared to be undergoing adynamic "facelift." At a quick glance, Voyager2 photographs showed the visage that had beenfamiliar since early in 1979, but a closer lookshowed that it is not quite the same. The white

Differing characteristics of Jupiter's meteorology are apparent in high-resolution images, such as this one taken byVoyager 1 on March 2 at a range of 4 million kilometers. The well-defined pale orange line running from southwest tonortheast (north is at the top) marks the high-speed north temperate current with wind speeds of about 120 meters persecond. Toward the top of the picture, a weaker jet of approximately 30 meters per second is characterized by wave patternsand cloud features which have been observed to rotate in a clockwise manner at these latitudes of about 35°N. These cloudshave hezn observed to hay'. lifetimes of one to two years. [P-21193C)

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Jupiter's cloud patterns changed significantly in the few months between the two Voyager flybys. Most of the changes are theresult of differential rotation, in which the prevailing winds at different latitudes shift long-lived features with respect tothose north or south. Thus, for example, the three large white ovals shifted nearly 90 degrees in longitude, relative to theGreat Red Spot, between March and July. [P-215991

band below the Great Red Spot, fairly broadduring the first flyby, had become a thin whiteribbon where it rims the southern edge of theSpot. The turbulence to the west of the RedSpot had stretched out and become "blander"than it was before. Small rotating cloudsseemed to be forming out of the waves in thisregion. The cloud structure that had been eastof the Red Spot during the Voyager 1 flybyspread out, covering the northern boundaryand preventing small clouds from circling thehuge red oval. The Red Spot itself alsochanged. Its northern boundary seemedatleast visuallyto be more set off from theclouds that surround it, and the feature ap-peared to be more uniform in color, perhapsreverting back to the personality it had inPioneer days.

The most obvious features in the atmos-phere of Jupiter, after the banded belts andzones, are the Great Red Spot and the threewhite ovals. These have often been described as"storms" in Jupiter's atmosphere. The ovalsare about the size of the Moon, and the Red

Spot is larger than the Earth. Voyager hasrevealed that in many respects the white ovals,which fc, med in 1939, resemble their ancientred relative. All four spots are southern hemi-spheric anticyclonic features that exhibitcounterclockwise motion; hence they are mete-orologically similar. Other smaller bright ellip-tical and circular spots also exhibit anticyclonicmotion, rotating clockwise in the northernhemisphere and counterclockwise in the south-ern hemisphere. In general, these features arecircled by filamentary rings that are darker thanthe spots they surround. Hints of interior spiralstructure can be seen in some of these spots. Allthe elliptical features in the southern hemi-sphere lie to the south of the strong westward-blowing jet streams. The spots tend to becomerounder the closer they are to the poles.

Along the northern edge of the equator area number of cloud plumes, which appear to beregularly spaced all around the planet. Some ofthe plumes have been observed to brightenrapidly, which may be an indication of convec-tive activity; indeed, some of the plume struc-

The Great Red Spot of Jupiter is a magnificent sight, whether viewed in normal or exaggerated color These pictures weretaken by Voyager I at a range of about I million kilometers; the area shown is about 25 000 kilometers, with features visible

on the originals that are as small as 30 kilometers across. The Red Spot is partly obscured on the north by a thin layer ofoverlying ammonia cirrus cloud. South of the Red Spot is one of the three white ovals, which are also anticyclonic vortices inthe atmosphere. The frame at the top right is in natural color, while the red and blue have been greatly exaggerated in theframe at the right to bring ou.. fine detail in the cloud structure. [P-21430C and P-21431C1

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125

tures seem to resemble the convective stormsthat form in the Earth's tropics. The plumestravel eastward at speeds ranging from about100 to 150 meters per second, but they do notmove as a unit.

The most visible cloud interactions takeplace in the region of the Great Red Spot.Material within the Red Spot rotates aboutonce every six days. Infrared measurementsshow that the Red Spot is a region of atmos-pheric upwelling, which extends to very highaltitudes; however, the divergent flow suggestedby this upwelling seems to be very smallonebright feature was observed to circle the RedSpot for sixty days without appreciably chang-ing its distance from the spot's center. Duringthe Voyagcr I flyby, spots were seen to movetoward the Red Spot from the east, flow alongits northern border, then either flow on to thewest past the Red Spot or into the outer regions

of its vortex. A spot caught on the outer edge ofthe Red Spot flow might break in two as itreached the eastern edge of the spot, with onepiece remaining in the vortex and the othermoving off to the east. Alternatively, a spotfloating toward the Red Spot from the eastmight be pushed northward to join the east-ward current flowing north of the giant redoval.

By the time of the second Voyager en-counter, a ribbon of white clouds curled aroundthe northern border of the Red Spot, blockingthe motion of small spots that might otherwisehave been caught up in the vortex. Spots ap-proaching the Red Spot from the east justturned around and headed back in the directionfrom which they had come.

In the northern hemisphere, small brownanticyclonic features speed around the planet,often colliding with one another. On collision,

Voyager 2 captured the Red Spot region four months atter Voyager 1, when some changes had taken place in the cloudcirculation pastern around it. This is a mosaic of Voyager 2 frames, taken on July 6. The white oval to the south is not thesame one that was present in a similar location during the Voyager 1 flyby, because of differential rotation at the twolatitudes. 1260-6061

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the spots may combine and roll around togetherfor a while. Ultimately, part of the mass of thecombined spots is ejected as a streamer, and theremaining material continues on its eastwardpath.

Order out of Chaos

Despite all the turbulence in Jupiter's at-mospherethis ever-changing chaotic mixtureof cyclonic and anticyclonic flows, of ovals andfilaments, of reds, browns, and whitesa pat-tern may be emerging: There is an underlyingorder to the seemingly random mixing of pat-terns we see in the Jovian atmosphere.

First, the changing weather patterns are insome sense cyclic. The fact that Jupiter may bereverting-to the appearance it presented at thetime of Pioneers 10 and 11 is not surprising.From Earth-based studies, astronomers havefound that the face of Jupiter often goesthrough a major change every few years. Thetransition is very rapid, but the planet main-tains its new "look" for some timeuntil thenext major transition. At the time of the Pio-neer flybys, "The Red Spot was prominent, butit was surrounded by intense cloud. There wasno visible structure at all in the south tropicalzoneit was totally bland. You couldn't see theturbulent area to the west," explained GarryMint. "And, I believe that the buildup of cloudthat we're seeing to the east of the Red Spot isthe beginning of the transition that will producethe Pioneer look."

There is even more order underlyingJupiter's changing atmosphere. This order isrevealed in part in the alternating belts andzones. It is believed that the cloud-coveredzones are regions of rising air, and the belts areregions of descending air. The internal energyof Jupiter provides the power to maintain thispattern of slow vertical circulation. In addition,there are horizontal or zonal flows that arcmuch more regular than the changing cloudpatterns.

The three large white orals are the longest-10 ed feature~ inJupner's atmosphere, alter the Great Red Spot Like theRed Spot, they arc anticytionk, or high-pressure, regions.During Voyager 2 encounter, one 01 the oral, v. as justsouth of the Red Spot This penile shuns the other bnoorals as they looked in catty July 1 he clouds slum %crysimilar internal structures 10 the east of each of them,recirculating currents are clearly seen In the loser I rame, asimilar structure h %col to the v. est of the cloud II'-21754C)

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A sequence of pictures of Jupiter, taken once per rotation (about ten hours), can be used to construct a time-lapse movie ofthe circulation of the Jovian atmosphere. These frames are from the Voyager I "Blue Movie" of the Great Red Spot region.Every odd Jovian rotation is shown, so the 24 frames correspond to 48 Jupiter days, or about 20 Earth days. White spots canbe seen entering the Red Spot from the upper right and being carried around by its six-day rotation until they are ejectedtoward the lower right. Above the Red Spot, the flow is toward the right; below it, toward the left. The rotation of the Spot iscounterclockwise, or anticyclonic. (260 -449]

"At first, Voyager seemed to do nothingbut emphasize the chaos, not the order inJupiter's atmosphere," Andy Ingersoll stated."There are turbulent regions in which indi-vidual little spots seem to change every Jovianrotation. And the whole texture in certain tur-bulent regions is unrecognizable in one earthday." With the Voyager spacecraft, more detailcould be seen than ever before; in addition,changes could be observed on a small timescale,as they happened. "It became much more of amystery how large-scale order could exist in the

60

40

0

20

40

60100 0

Zonal velocity (m/s)

1--,I Earth-basedmeasurements(1974-1979)

face of all this small-scale chaos. But I think weare beginning to see the order underneath.What we are looking at when we observeJupiter are minute cloud particles representingonly a small fraction of the mass of the atmos-phere." The large-scale order the scientists hadfound was a regular alternation of eastwardand westward jets. "If we take all the measure-ments from Earth-based observations over thelast 75 years, we find that every current that hasever been seen from the Earth over 75 years isvisible in one ten-hour rotation. They're alltherethey were just invisible." The ever-changing appearance was dancing above aregular, almost stationary pattern of alternat-ing flows, which may come from deep withinJupiter's atmosphere. Why this alternating pat-tern persists remains a mystery. Even if theunderlying pattern can be thought of as a sortof Jovian climate, it still does not explain themechanics of "the minor sideshow"thecharging weather patterns. Analysis of theVoyager pictures is sure to keep planetary mete-orologists busy for many years to come.

lights in the Night Sky

Toward the end of the first encounterperiod, Voyager 1 flew behind Jupiter, and thespacecraft's wide-angle camera scanned thenorthern hemisphere on the nightside of theplanet, searching for aurorae and lightningbolts. The most impressive darkside featurefound was a tremendous aurora in the northpolar region. But this was not the first timeJovian aurorae had been detected. Very-high-energy auroral emissions resulting fromultraviolet glows of atomic and molecularhydrogen had been detected prior to encounteron the bright side of Jupiter by the ultravioletspectrometer. The ultraviolet observations indi-cate that atmospheric temperatures in theauroral regions are at least 1000 K. In both thevisible and the ultraviolet spectra, the auroraeare confined to the polar regions and resultfrom charged magnetospheric particles strikingthe upper atmosphere. The ultraviolet auroraeare created when high-energy particles from thelo plasma torus spiral in toward Jupiter onmagnetic field lines.

Several meteor trails were also evident inthe darkside pictures of Jupiter's atmosphere.Traveling at roughly 60 kilometers per secondas they entered, these fireballs brightened

At different latitudes, the strong presailing zonal winds produce different apparent rotation rates. Plotted here are thehorizontal velocities measured from a pair of Voyager images taken one rotation (about ten hours) apart. Also shown forcomparison are older ground-based measurements obtained from careful timings of the apparent rotation rate at differentlatitudes. The excellent agreement of the two plots indicates he stability of the zonal winds. Also, the wind pattern showsmuch greater symmetry between northern and southern hemispheres than do the more superficial cloud lx sterns.

. - 10 0

The night side of Jupiter is not dull. A large aurora (northern light) arcs across the northern horizon, while farther southabout twenty large bolts of lightning illuminate electrical storms in the clouds. Similar pictures also resealed fireballs, orlarge meteors, burning up in the atmosphere 01 Jupiter 113-21283B/W)

quickly and seemed to survive for about 1000kilometers before they died.

Clusters of lightning boltsindicative ofelectrical stormswere also discovered onJupiter's nightside. This particular phenom-enon does not seem to depend on latitude. TheVoyager I photograph that captured the hugeJovian aurora also caught the electricaldischarges of 19 superboits of lightning, andVoyager 2 photographs located eight additionalflashes. Radio emission (whistlers) from light-ning discharges were also detected by theVoyager radio astronomy receivers and theplasma wave instrument.

Magnetic Field

Deep in the interior of Jupiter, the pres-sures are so great that hydrogen becomes an

electrical conductor, like a metal. Currentsdriven by the rapid rotation of the planet arethought to flow in this metallic core. The resultis a magnetic field that penetrates the spacearound Jupiter.

Direct measurements of the Jovian mag-netic field were first made by the Pioneers, andVoyager results generally confirm the initialfindings. The strength of the Jovian field isabout 4000 times greater than that of the Earth.The dipolar axis is not at the center of Jupiter,but offset by about 10 000 kilometers andtipped by 11 degrees from the axis of rotation.Each time the planet spins, the field wobbles upand down, carrying with it the trapped plasmaof the radiation belts. The Voyager particlesand fields instruments concentrated not on theplanetary magnetic field but on the processestaking place in the magnetosphere.

0.03

0.1

03

10

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60

80

100

100 110 120 130 140 150 160

Temperature MI

A probe of the Josian atmosphere is obtained each time a spaLecratt passes behind the planet as seen from the EarthPassage through the ionosphere and atmosphere alters the phase of the radio telernetr), signal, and subsequent computeranalysis allows members of the Radio Science Team to reconstruct the profile of the atmosphere Shown here is theatmospheric temperature as a function of pressure as derived from the Voyager I X-band occultation data, corresponding toa point at latitude 12°S, longitude 63° The two curse, represent estreme interpretations of the same data; the best fit liessomewhere between. Accuracy is high at greater depths but poor at lesels abos e a pressure of about 0.03 bar. Clearly show nis the temperature minimum near 0.1 bar and the steady increase of temperature with depth as the radio beam probed towardthe cloud level near 1.0 bar

At a wavelength near 5 micrometers, the primary gases in the atmosphere of Jupiter are particularly transparent, and theinfrared radiation from the planet comes from relatisely great depths At these depths, it is possible to see es idence of gasessuch as water sapor that condense at higher altitudes where the temperaiures are lower This IRIS spectrum in the5-micrometer spectral region shows features identified with water (1-120), germane (CieEld, and deuterated methane ICH,D),as well as the more easily detected ammonia asIH,).

280

240

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160

Wavelength (pH56 5 45

1800 2000

Wave number Icm

1 4,

2200

280 2

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160

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90 110 130

Temperature (K)

150 170

The structure of the )o% Ian atmosphere can be derned from infrared spectra as well as from the radio occultation data. Thisprofile of temperature as a function of pressure corers the same range of altitudes as does the preceding figure and is in goodagreement Both profiles locate a temperature minimum of about 110 K near a pressure of 0.1 bar (1000 mb = 1 bar = I

atm). The IRIS data also show variation of structure with position, including a cooler minimum temperature (about 100 K)over the Great Red Spot

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The Magnetosphere

Giant Jupiter has an enormous realmfrom the size of its satellite system to its tremen-dous aurorae and superbolts of lightning, to thehuge planet-sized cloud features that surroundits atmosphere. The most gargantuan Jovianfeature is its magnetosphere, which envelopesthe satellites and constantly changes in size,pumping in and out at the whim of the solarwind. The Pioneer and Voyager spacecraft pro-vided four cuts through this dynamic region,showing that its borders in the upwind solardirection lie between 50 Rj and 100 Rj fromJupiter. Downwind, away from the Sun, themagnetosphere extends much farther; some sci-entists postulate that a magnetotail may reachas far as the orbit of Saturn.

Charged particles in the magnetosphere aresubject to powerful forces. Tightly embeddedin Jupiter, the magnetic field spins with a ten-hour period as the planet rotates. The particlesare caught in the spinning field and acceleratedto high speeds. The result is a co-rotatingplasma in the magnetic equator of Jupiter, ex-tending outward to at least 20 R. Beyond thisdistance, the flow breaks up and the magneto-sphere is more unstable. Within the co-rotationregion, the spinning plasma sets up a powerfulelectric current girdling the planet.

Charged particles can be accelerated in themagnetosphere to high energies, correspondingto speeds tens of thousands of kilometers persecond. Some of these particle streams escapefrom the inner parts of the magnetosphere andcan penetrate the magnetopause and be ejectedfrom the Jovian system. On Voyager 1, the lowener harged particle instrument begandetecting these streams of "hot" plasma on 22January, when Voyager 1 was still 600 Rj(almost 50 million kilometers) from the planet.Voyager 2 first detected Jovian particles at aneven greater distance, 800 R J. Hydrogen andhelium ions (protons and alpha particles)dominate the magnetosphere at great distancesfrom Jupiter, but increasing amounts of sulfurand oxygen appeared as the spacecraft crossedthe magnetopause. The heavier ions presum-ably originate from lo.

In the inner magnetosphere, the Galileansatellites have a powerful influence on thepopulations of fast-moving particles. Duringthe Voyager 1 encounter, the primary effectwas observed at lo, where the satellite ap-parently sweeps up energetic electrons. In themillion-volt energy range, these particles aredepleted near lo, with peaks observed both in-side and outside the satellite's orbit. Voyager 2passed close to Ganymede, and here also majoreffects were seen, with the satellite apparently

The structure of the atmosphere can be inferred from IRIS spectra at many locations user the disk of Jupiter. Scientists arebeginning to assemble this vast amount of information intomaps that show the temperatures at a gix., n pressure. Shown hereare the observed temperatures (bottom left) at a depth near the cloud tops (0.8 bar) and (below) at an altitude about 30kilometers higher (0.15 bar). The temperature contours are labeled in degrees Kelvin. The banded structure, with highertemperatures near the dark equatorial belt, is most clearly evident at the lower altitude Surprisingly, the cool regionassociated with the Great Red Spot (latitude 23°S) is more apparent at high altitude.

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The magnetosphere of Jupiter can be "seen" from Earth by its emissions at radio wavelengths. The recent development ofimaging radio telescopes in Great Britain, The Nether la nth, and the United States allows frequent mapping of the large-scalefeatures of the innermost magnetosphere, inside the orbit of lo. These six image% showing one rotation of Jupiter %%ereobtained near the time of the Voyager I flyby by Imke de Pater at the Westerbork Radio Observatory near Leiden. Thebrightest emission is %hov.n b,, dark red or black. The %lie 01 the planet is indicated by a dotted %line circle The tilt of themagnetosphere relato,e to the rotation axis of the planet can he seen by the Vi,obble of the magnetosphere as Jupiter rotates

134

absorbing electrons. In the wake created by themotion of the co-rotating plasma pastGanymede, the particle populations showedlarge and complex variations.

Just inside the Jovian magnetosphere is the"hot spot" of the solar system: a 300-400million degree plasma detected by Voyager Iwhile it was still about 5 million kilometersfrom Jupiter. T. P. Armstrong commented,"Even the interior of the Sun is estimated to beless than 20 million degrees." S. M. Krimigisadded that the temperature of this plasma is"the highest yet measured anywhere in the solarsystem." Fortunately for Voyager, this regionof incredibly hot plasma is also one of the solarsystem's best vacuums. The spacecraft was inlittle danger because the bow shock protectsthis region from the solar wind, and most of theparticles in Jupiter's magnetosphere are held inmuch closer to the planet.

The very hot plasma in the outer magneto-sphere discovered by Voyager is thought to playan important role in establishing the size of theJovian magnetosphere. Although the density islow, only about one charged particle per hun-dred cubic centimeters, this plasma actuallycarries a great deal of energy because of thehigh speed of the particles. It is this plasmapressure, rather than the magnetic fieldpressure, that appears to hold off the pressureof the solar wind. However, the balance be-tween hot plasma inside the magnetopause andthe solar wind outside is not very stable. TheVoyager experimenters suggest that a smallchange in solar wind pressure can cause the

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100 i 200plasmaspherey ,' Plasma sheet 1/.

Hot plasma region( 20 to 40 keV) ?\

Bow shock

DuskMagnetosphericwind

.

Each voyager passed through the boundaries of themagnetospherethe bow shock (BS) and the magnetopauseIMP)on both the inbound and the outbound legs of itspassage through the Josian system. In this diagram, theIlea% y solid line represents the spacecraft trajectory, as seenlooking down from the north Also shown are the positionsof the bow shock in March and of the magnetosphere inboth March and luly

b

R MMagnetosphei icwind

Sun Magnetosphericwind

eyeral regions of plasma (charged partiLles) make up theImian magnetosphere These skett.fies, based on Voyagerdata, show the magnetosphere as stewed from above (left)and as seen from the Jovian equatorial plane (above) Mostof the plasma co-rotates with the planet and is confined nearthe magnetic equator, where it forms a broad plasma sheetabout 1(X) R, across.

RG135

The rings of Jupiter are best seen when looking nearly in the direction of the Sun, since the small particles that comprise themare good forward scatterers of sunlight. This mosaic is of Voyager 2 images (two wide angle and four narrow angle) obtainedfrom a perspective behind the planet and inside the shadow of Jupiter. The spacecraft was 2 degrees below the equator ofJupiter and 1.5 million kilometers from the rings. The ,hadow of the planet can be seen to obscure the near segment of thering near the edge of the planet. The brightest region of the ring is about 1.8 R1 from the center of Jupiter [260-678B]

boundary to become suddenly unstable. A largequantity of the hot plasma can then be lost,producing the bursts seen at large distances andpermitting a sudden collapse of the outer mag-netosphere. Continued injection of hot plasmafrom within would then reinflate the magneto-sphere, which would expand like a balloon untilanother instability developed. Processes of thissort may be the cause of the rapidly varyingmagnetospheric boundaries observed by bothVoyager spacecraft.

Rings of Jupiter

One of the spectacular discoveries ofVoyager was the existence of a ring system of

136

small particles circling Jupiter. Saturn andUranus were known to have rings, but none hadbeen seen before at Jupiter.

As revealed by the Voyager cameras, therings extend outward from the upper atmo-sphere to a distance of 53 000 kilometers abovethe cloud tops, 1.8 R j from the center of theplanet. The main rings, however, are much nar-rower, spanning from 47 000 to 53 000 kilo-meters above Jupiter. There are two main rings,a 5000-kilometer-wide segment, and a brighter,outer 800-kilometer segment. The thickness ofthe rings is unknown, except that it is certainlyless than 30 kilometers, and probably under 1kilometer.

Structure within the ring can be seen in the best Voyager 2 images, taken about 27 hours after closest approach to Jupiter.This enlarged portion of a wide-angle picture taken with a clear filter shows a bright core about 800 kilometers across with adimmer region a few thousand kilometers across on the inside, and a narrow dim region on the outside. 1260-6741

The rings of Jupiter are quite tenuous,which explains why they are invisible fromEarth [although they have been detected fromEarth since Voyager]. Seen face on, thebrightest part of the ring blocks less than onepart in ten thousand of the light passingthrough, making it essentially transparent. Infact, the ring does not even offer much resist-ance to a spacecraft; Pioneer II traversed thering in 1974 with no obvious ill consequences.Apparently the individual particles that makeup the ring are widely dispersed. They can beseen only when the rings are viewed nearlyedge-on, or toward the Sun, where they showup well in forward scattered light. It is this extrabrilliance when backlit that created the excellentphotos taken by Voyager 2 from inside theshadow of Jupiter.

The individual ring particles are probablydark, rocky fragments that are very smallessentially dust grains. They move aroundJupiter in individual orbits, circling the pl..netin 5 7 hours. Scientists postulate that such or-bits are not stable and that the particles fallslowly in toward Jupiter. Apparently the ringsare constantly renewed from some source,which may be the satellite Adrastea (J14),discovered by Voyager 2. There has also beenspeculation that Adrastea may influence thering structure by sweeping particles out of thering. At present the rings of Jupiter remainmysterious. They are clearly very differentfrom the rings of Saturn and Uranus, andreaching an understanding of their origins anddynamics presents many challenges to planetaryscientists.

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137

The four Galilean satellites of Jupiter are planet-like worlds, revealed by Voyager to be as diverse and fascinating as theterrestrial planets Mercury, Venus, Earth, and Mars. In this Voyager I composite, all four are shown in their correct relativesize, as they would appear from a distance of about I million kilometers. Relative color and reflectivity are also approxi-mately preserved, although it is not possible to show on a single print the full range of brightness from the dark rocky surfaceof Ca llisto to the brilliant white of Europa or orange of lo. Clockwise from the upper left, the satellites are lo (longitude1401, Europa (longitude 300°), Callisto (longitude 350°), and Ganymede (longitude 320°). 1260-499C1

1 ../ft

CHAPTER 9

FOUR NEW WORLDS

Jupiter's Satellite System

In a sense, the Voyager Mission revealed anew planetary system. Astronomers had longbeen fascinated by the large Galilean satellitesof Jupiter, but they had only looked from afar,watching the dancing points of light in theirtelescopes, and, occasionally, as the atmos-phere steadied, seeing these points resolvethemselves into tiny disks before dissolvingagain in the turbulence of the terrestrial atmos-phere. Much had been learned from telescopicstudies, but not until the Voyager flights had wetruly seen the Galilean satellites. The historichours as Voyager 1 cruised past each sateihicMarch 5 and 6, 1979, fundamentally altered ourperspective. Four new worlds were revealed, asdiverse and fascinating as the more familiar ter-restrial planets. Although not yet householdwords, the names Io, Europa, Ganymede andCallisto have now been added to Mercury,Venus, Moon, and Mars in the lexicon of im-portal), 'T'arth-sized" bodies in the solarsystem.

Jupiter has fifteen known satellites,counting the two new satellites discovered byVoyager. These moons vary greatly in size,composition, and orbit. The four outermostsatellites, Sinope, Pasiphae, Carme, andA nanke, circle the planet in retrograde orbits ofhigh inclination; their distances from Jupitervary between 20 and 24 million kilometers (290R j to 333 Rj). These small bodies, none morethan 50 kilometers in diameter, require nearlytwo years for each orbit of Jui,iter. It is possiblethat they are captured asteroids, but so little isknown about them that astronomers cannot tellif their surface properties resemble those ofasteroids, or if these four satellites are evensimilar to each other.

The next group of Jovian satellites consistsof four small difficult-to-observe objects.

These are Lysithea, Elara, Himalia, and Leda,the latter discovered by Charles Kowal of HaleObservatories in 1974. They have similar orbits,varying in distance from Jupiter between 11and 12 million kilometers (about 160 R j). Likethe outer group, these satellites have orbits ofhigh inclination; unlike the outer group, theymove in the proper, prograde direction aroundJupiter. The largest, Hinialia (170 kilometers indiameter) and Elara (80 kilometers diameter),are known to be very dark, rocky objects, and itseems probable that the others are similar. It isunlikely that the census of the outer groups ofirregular satellites is complete, and new satel-lites less than 10 kilometers in diameter willprobably be discovered.

The J ovian system is dominated, ofcourse, by the large Galilean satellites, whichvary in size from just smaller than the Moon(Europa) to nearly as large as Mars(Ganymede). These satellites are in regular,nearly circular orbits in the same plane as theequator of Jupiter, and all four lie within theinner magnetosphere of Jupiter, where they in-teract strongly with energetic charged particlesend plasma. Most of this chapter will bedc9ted to a discussion of these fascinatingworlds.

We now know of three additional small sat-ellites inside the orbit of Io, orbiting close toJupiter. The first, Amalthea, was discovered in1892; it orbits Jupiter in just twelve hours at adistance of 181 000 kilometers (2.55 Rj). Asmaller object, Adrastea (officially i979J1 forthe first new satellite of Jupiter discoveredin 1979), is much closer, at 134 000 kilometers(1.76 R j) As described in Chapter 7, it skirtsthe outer edge of the ring, circling Jupiter injust over seven hours. The inner satellite movesfaster than Jupiter's rotation; seen from theplanet, it would rise in the west and set in theeast. Both Amalthea and Adraste, Ire buried

15o139

Callisto was revealed by the Voyager cameras to be a heavily cratered and hence geologically inactive world. This mosaic ofVoyager I images, obtained on March 6 from a distance of about 400 000 kilometers. shows surface detail as small as 10 kilo-meters across. 1 he prominent old Impact feature Valhalla has a central bright spot about 600 kilometers across, probablyrepresenting the original impact basil The concentric bright rings extend outward about 1500 kilometers from the impactcenter. 1260-4501

1404...)

...L.i

SATELLITES OF JUPITER

Name

Distance From JupiterPeriod(days)

Year ofDiscovery163 kilometers Jupiter Radii

Adrastea J14 134 I.76 0.30 1979Amalthea J5 181 2.55 0.49 18921 979J2 J15 222 3.11 0.67 1980lo J 1 422 5.95 1.77 1610Europa J2 671 9.47 3.55 1610Ganymede J3 1070 15.10 7.15 1610Callisto J4 1880 26.60 16.70 1610Lyda J13 II 110 156 240 1974Himalia J6 11 470 161 251 1904Lysithea J10 1 1 710 164 260 1938Elara J7 II 740 165 260 1904Ananke J12 20 700 291 617 1951Carme J11 22 350 314 692 1938Pasiphae J8 23 300 327 735 1908Sinope J9 23 700 333 758 1914

The state of the interior of the Galilean satellites can be judged from their sizes and densities. These cross-sectional viewsrepresent the best guess following the Voyager flybys as to the composition and structure of the objects. lo, with a densityequal to that of the Moon and a long history of volcanic activity, is a dry, rocky object. Europa is less dense, and it probablyhas a global ocean of ice as much as 100 kilometers thick over a rocky interior Ganymede and Callisto both have densitiesnear 2 grams per cubic centimeter, suggesting a composition about half water and half rock. There is probably a rocky coresurrounded by an icy riantle.

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deep within the inner magnetosphere wherethey are continually bombarded by energeticelectrons, protons, and ions. Depletion of theJovian radiation belt particles was observed atthe orbits of both satellites by Pioneer II,which went much closer to Jupiter than theVoyagers, testifying to the intensity of the in-teraction between these objects and theirsurroundings.

Long after the flybys of Jupiter, continuedanalysis of Voyager images resealed anothernew satellite, Jupiter's fifteenth. Intiallydesignated 1979J2, the unexpected new satelliteorbits the planet at 3.17 Rj, between lo andAmalthea. Stephen Synnott of the JPL OpticalNavigation Team discovered the satellite onpictures taken during the Voyager I events onMarch 5, 1979, while searching for additionalimages of satellite 1979J1. It is about 75 kilo-meters in diameter, but nothing else is knownabout its physical properties.

Together, the 15 satellites cifcling giantJupiter form a mini-solar system. Perhaps theouter, irregular satellites were captured or,:sulted from the catastrophic collisions of one

or more larger satellites with passing asteroids.The inner seven satellites constitute a coherentsystem, almost certainly formed together withJupiter and sharing a common 4.5-billion-yearhistory. They ate fascinating as individualworlds, and also as brothers and sisters, and thestudy of their interrelationships undoubtedlywill provide insights into the general problemsof planetary formation and evolution.

S17I-S AND DINSITIES 01- THEGALILEAN SATELLITES

NameDiameter

(kilometers)

Densii(grants per cubic

centimeter)

to 3640 3.5

Europa 3130 3.0Ganymede 5270 1.9

Ca llisto 4840 1.8

Ca Nista

Callisto is the least active geologically ofthe Galilean satellites. Basically a dead world, itbears the scars of innumerable meteoric im-pacts, with virtually no sign of major internalactivity. Callisto is a world of craters, and tounderstand it we must explore the role thatcratering plays in molding planetary surfaces.

The space between the planets is filled withdebris, ranging from the larger asteroids, hun-dreds of kilometers in size, down tomicroscopic grains of dust. Inevitably, eachplanet collides with some of these fragments.The smallei particles do little damage; in thecase of a planet with an atmosphere, like Earth,they burn up as meteors before reaching thesurface, whereas on an airless planet, theyerode the surface by sandblasting the exposedrock. The larger impacts are another matter,and the craters they produce can be the domi-nant features on the surface of a planet.

Voyagers I and 2 photographed most of the surf ace of Callisto at resolutions of a ley, kilometers or better ShoAn here is apreliminary shaded relief map. Additional measurements s' ill improve the accuracy of the coordinate system. 1260-6721

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Callisto is a world of craters, as is well shown in this Voyager 2 photomosaic taken from a distance of 400 000 kilometers.Craters about 100 kilometers in diameter cover the surface uniformly. Many have bright rims, perhaps composed of exposedwater-ice. There are very few craters larger than 150 kilometers in diameter, however, indicating that the scars of very largeimpacts do not survive on the surface of Ca lbw. (P- 21746B/WI

154

-A,

We who live on Earth tend not to realizethe importance of cratering, for the simple rea-son that our planet has very few craters, andthese are frequently of volcanic rather thanmeteoric origin. Why are we so favored? Isthere an invisible shield to protect us from thecosmic shooting gallery? Clearly not; the Earthhas experienced just as many cratering impactsas has the Moon or other planets. The dif-ference is not that craters are formed less often,but that the great geological activity of Eartherosion, volcanism, mountain building, conti-nental drift, etc.erases craters as fast as theyare formed. On the average, a 10-kilometer-wide crater is formed on Earth about onceevery million years, but all those older than afew million years have been eroded away, filledin, or crushed beyond recognition by crustalmotion.

If a planet lacks peat internal geologicforces, large craters can survive almost indefi-nitely. Such is the case for the Moon. Most ofthe volcanism and other activity on the Moonce- ed 3'/ billion years ago, as the dating of

samples obtained by the Apollo astro-nauts showed. Since that time, the lunar surfacehas been passively accumulating impact scars.

The longer any particular surface area has beenexposed, the more densely packed are the cra-ters. Thus crater density is the first thing aplanetary geologist looks for in photos of a newworld. Craters are the touchstone of this field,revealing the degree of internal activity andallowing the determination of the relative agesof different surface units.

On Callisto the density of craters is veryhigh. In some places they are packed as closelyas one can imagine, particularly for cratersseveral tens of kilometers in diameter. Al-though no one knows the exact rate of forma-tion of impact craters on the Jovian satellites,geologists on the Voyager Imaging Team estim-ate that it would require several billion years toaccumulate the number of craters found onCallisto. They therefore conclude that Callistohas been geologically inactive almost since thetime of its formation.

Although superficially similar to the heav-ily cratered surfaces of the Moon and Mercury,Callisto is far from identical to these rockyworlds. One of the most obvious differences isa lack of craters larger than about 150 kilo-meters on Callisto, together with a tendency forlarge craters to have much shallower depths.

The concentric rings surrounding Valhalla are perhaps the most distinctive geological feature on Callisto This Voyager 1close-up shows a seg-nent of the ridged terrain. The presence of superposed impact craters shows that the rings formed earlyin Callisto's history; however, the density of craters is less here than on other parts of the satellite, where the surface is older.IP-221941

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The IRIS instrument measured the temperature of spots on the surface of Callisto as each Voyager sped past The measure-ments shown here were all made at equatorial latitudes (between 10° and 25°). Shown are very low predawn temperatures( 190° C) followed by an increase to a noon-time mayimum of about 120° C, and then a drop again as the Sun sets.[260-735)

Apparently the ice-rock composition of Callistoalters the ability of the crust to retain largecraters. Geologists speculate that the ice flowsover many millions of years, filling in craterfloors and gradually obliterating the largestcraters, There is also a conspicuous absence ofmountains on Callisto, again suggestive of aweak, icy crust.

The most prominent features in the Voy-ager pictures are the ghost remnants of whatmust have been immense impact basins. Thelargest of these, the "bullseye" of the Voy-ager I images, has been named Valhalla, forthe home of the Norse gods. These ghost basinshave lost nearly all their vertical relief. What re-mains is a central, light-colored zone (probablythe location of the original crater), surroundedby numerous concentric rings of subdued,bright ridges. Such features had never been seenbefore on any planet, and they appear to be thecharacteristic geologic feature of an ice-lockplanet.

Little is known about the composition ofCallisto's surface, the material from which sun-light is reflected. It appears to be primarily darkrock or soil, but it lacks diagnostic spectral fea-tures, except for one infrared band due to watermolecules bound in the soil. The many lighterspots and arcs that outline craters in the high-resolution pictures may be regions in which theice is showing through, but these cover only a

very small fraction of the exposed surface. (Itshould he noted that, although Callisto is thedarkest of the Galilean satellites, the term"dark" is relative, for even Callisto is brighterthan Earth's moon.)

The daytime surface temperature of Cal-listo, observed both from the ground and byVoyager, is about 118° C. The Voyager in-frared interferometer spectrometer also deter-mined the minimum temperature, reached justbefore dawn, of 193° C. No atmosphere isexpected at these cold temperatures, and nonewas seen.

Analysis of Voyager images provided animproved diameter for Callisto of 4840 kilo-meters, yielding an average density of 1.8 gramsper cubic centimeter. As noted previously, it isthis low density that leads to the conclusion thatice or water is an important component of theinterior of Callisto. The ice has never beendetected directly, but the peculiar nature of thecraters seen by Voyager adds strong circum-stantial support to this conclusion.

Callisto, with its heavy cratering, is themost familiar-looking of the Galilean satellites;if all of them had turned out to be as geologi-cally dead as Callisto, planetary geologistswould certainly have been disappointed. How-ever, each satellite, progressing in towardJupiter, presents increasing evidence of internalactivity.

145

Gam mede

The largest of the Galilean satellites (5270kilometers in diameter), Ganymede was ex-pected to be similar to Callisto in many ways.Both ha% e low densities (for Ganymede, 1.9grams per cubic centimeter), indicating a bulkcomposition of about halt' rocky materials andhalf water. In addition, their diameters differby only eight percent, and both are far enoughfrom Jupiter to escape the severe pounding loreceives from magnetospheric charged parti-cles. Thus it was with great interest that Voy-ager scientists looked at the differences thatemerged between these two satellites.

The surface of Ganymede as revealed bythe Voyager cameras is one of great diversity,indicating differing periods of geologic activity.At one extreme there are numerous dark areasthat resemble the surface of Callisto in bothalbedo (reflectivity) and crater density. Thelargest of these, Regio Galileo, stretches fromthe equator to latitude + 45° and is 4000 kilo-meters across, nearly as large as the continentalUnited States. This ancient terrain even pre-serves the remnants of a Callisto-type impactbasin in the form of a system of parallel, curv-ing, subdued ridges about 10 kilometers wide,100 meters high, and spaced about 50 kilo-meters apart. The central part of this ghost

Shaded rehef map of Canhede (260-6731

180° 150° 120° 90° 60°

basin is missing, however; it was presumablydestroyed by subsequent geologic activity.

Other regions of the surface of Ganymedeare clearly the product of intense internalgeologic activity. Generally, these regions areof higher albedo and consist of many straightparallel lines of mountains and valleys. TheVoyager geologists call these the grooved ter-rain because of their appearance from a greatdistance. Typically, these mountain ridges are10-15 kilometers across and about 1000 metershigh, similar in scale to some sections of theAppalachian Mountains in the Eastern UnitedStates. No higher relief exists, presumably forthe same reason it is absent on Callisto. Inmany places the grooved terrain forms betweenareas of the older, darker surface, giving theappearance of mountains extruded betweenseparating plates of ancient crust. In otherareas the relationships are much more complex,with curved systems of grooves and ridges over-lying each other, displaying intricate crosscut-ting relationships. Apparently Ganymede hasexperienced - series of mountain-buildingevents.

The grooved terrains show a substantialrange in age, as indicated by the crater densi-ties. The oldest have nearly the same densityas the ancient, dark plains, suggesting that for-mation of the gi ooved terrain began early, per-

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The hemisphere of Ganyinede that faces away from the Sun displays a great variety of terrain. In this Voyager 2 mosaic,photographed at a range of 300 000 kilometers, the ancient dark area of Regio Galileo hes at the upper left. Below it, thelighter grooved terrain forms bands of varying width, separating older surface units. On the right edge, a prominent craterray system is probably caused by water-ice, splashed out in a relatively recent impact, (260-6711

147

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The sinuous nature of some of the narrower Ganymede groove systems can be seen in this oblique view, obtained on March 5

by Voyager I . The area shown is about the size of California, with features visible as small as 5 kilometers across. The ridges

appear to be the result of deformation of the crust of Ganymede. [13-2I235)

haps 4 billion years ago. The youngest groovedterrain has only about one-tenth as manycraters, but this is still as many as are seen in the3.5-billion-year-old lunar plcins. The Voyagergeologists believe that even in these areasgeologic activity ceased billions of years ago.

Other t, pes of surfaces are seen on Gany-mede. Some regions are lightly cratered andsmooth, with no indication of mountain build-ing. In one place, there is a rough mountainousarea that looks more like the jumbled lunarmountains than the long ridges and valleys ofthe rest of Ganymede. Many of the largercraters are distinguished by brilliant white halosand rays that suggest that impacts may havesplashed large quantities of water or ice overthe surface.

Many of the geologic features seen onGanymede appear to have been caused bybreaking, faulting, or spreading of the crust. In

148

a few cases, there even seem to be indications oftransverse, or sideways, motion along faults.This evidence is extremely exciting to geolo-gists, since similar crustal motion on Earth isassociated with the drift of continental plates,drawn by convection currents deep in the man-tle. Such activity has never been seen before onanother planet.

Astronomers on Earth had known since1971 that about half the surface of Ganymedewas covered with exposed water ice and abouthalf with darker rock. An examination of thealbedo variations in the Voyager pictures sug-gests that the ice is exposed near large cratersand, to a lesser extent, in the grooved terrain,but no direct measurements were made by Voy-ager of the composition of different parts of thesurface.

The presence of ice on the surface sug-gested to many astronomers that Ganymede

The complex patterns of the grooved terrain on Ganymedeare apparent in high-resolution images. This picture, takenby Voyager 1 on March 5, has a resolution of about 3 kilo-meters and shows a region about the size of the state ofPennsylvania. The mountain ridges are spaced about 10 to15 kilometers apart and rise about 1000 meters, similar tomany of the mountains of Pennsylvania. The transectionsof different mountain systems indicate that they formed atdifferent times. A degraded crater near the left center of thepicture is crossed by ridges, indicating that it predated theperiod of crustal deformation and mountain building.[P-212791

Ray systems of exposed water-ice are visible in this high-resolution mosaic of Ganymede, obtained by Voyager 2onJuly 9 at a range of about 100 000 kilometers. The roughmountainous terrain at lower right is the outer portion of alarge fresh impact basin that postdates most of the otherterrain. At the bottom, portions of grooved terrain transectother portions, indicating an age sequence. The darkpatches of heavily cratered terrain (right center) are prob-ably ancient icy material formed prior to the grooved ter-rain. The large rayed crater at upper center is about 150kilometers in diameter. IP-21770B/W)

might have a very tenuous atmosphere of watervapor or oxygen, which might be released bythe breakdown of water vapor by sunlight.During the Voyager 1 flyby, a sensitive test foran atmosphere was made by the ultraviolet in-strument from observations of the star KappaCentauri as it was occulted by Ganymede. Nodimming of the starlight was seen, yielding anupper limit for the surface pressure of the gasesoxygen, water vapor, or carbon dioxide of10-11 bar, or one hundred-billionth the at-mospheric pressure at Earth.

The differences between the geologic his-tories of Ganymede and Callisto are surpris-ingly large. No one knows the reason. Perhapsonly a small increase in internal temperature isnecessary to initiate geologic activity in an icyplanet, and for some reason Ganymede crossedthis threshold for a part of its history, whereasCallisto did not

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The many variants of smooth and grooved terrain on Ganymede suggest a complex geologic history for this satellite. Fourhigh-resolution views by Voyager 2 are grouped together. (Top left) A band of low mountain ridges has apparently been cut

and offset by a fault (Top right) Multiple sets of mountain ridges transect at nearly right angles Some impact craters vv ere

formed before, and some after, the grooved terrain. (Bottom left) Many short parallel ridge, butt into each other, making acrazy-quilt pattern. (Bottom right) In the center of this frame is an unusual smooth area, perhaps the result of flooding of the

surface by material that filled in the grooves 1260-678A]

EuropaAs one proceeds inward through the Gali-

lean satellites, these worlds become less and lessfamiliar to the planetary geologist. This was anunexpected effect. Callisto and Ganymede wereexpected to have unusual properties as a resultof their large percentage of ice. The densities ofEuropa and lo are more normal for the smaller,terrestrial-type planets, and before Voyagermany scientists expected these two satellites

150

might look much like the Moon, which theyresemble in size.

Europa, with a diameter of 3130 kilo-meters, is about 15 percent smaller than theMoon. Its density is 3.0 grams per cubic centi-meter, indicating a basically rocky composi-tion. However, cosmic mixtures of rocky andmetallic materials are often a bit denser thanthis, leaving room for a substantial componentof ice or water. Calculations indicate that if all

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the ice were at the surface, it might form a crustup to 100 kilometers thick.

Telescopic observations of Europa demon-strated long before Voyager that this satellite 1.,

almost completely covered with ice. It is awhite, highly reflecting body, looking, from agreat distance, like a giant snowball. In theearly Voyager pictures, Europa always showeda bland, white disk, in striking contrast to thespottiness of Ganymede or the brilliant colorsof 10.

Voyager 1 never got closer to Europa than734 000 kilometers, and at that distance it re-mained a nearly featureless planet, with no ob-vious impact craters or other familiar geologicstructures. What did show in the Voyager 1 pic-tures, however, were numerous thin, straightdark lines crisscrossing the surface, some ex-tending up to 3000 kilometers in length. To themembers of the Imaging Team, these featureswere "strongly suggestive of global-scale tec-tonic processes, induced either externally (as bytidal despinning) or internally (as by convec-tion)." It was with the greatest interest that theVoyager 2 images, taken from about four timescloser, were anticipated.

The spectacular pictures obtained of thesatellite in July were perhaps more confusingthan clarifying. Europa is entirely covered withdark streaks that vary in width from several

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kilometers to approximately 70 kilometers andin length from several hundred to several thou-sand kilometers. Most streaks are straight, butothers are curved or irregular. The streaks lieon otherwise smooth, bright terrain, featurelessexcept for numerous random dark spots, mostless than 10 kilometers in diameter.

Voyager 2 photos showed, in addition tothe smooth terrain with its dark streaks, legionsof darker, mottled terrain. This mottled terrainappears rough on a small scale, and it may con-tain small crater s just on the limit of resolution(about 4 kilometers). Only three definite impactcraters have been identified, each about 20 kilo-meters across. This small number of crater,suggests either that the surface is iclatiel!,young or that craters are not presery ed for longin the icy crust.

Although the dark streaks give Europa acracked appearance, the streaks themselves arcnot obviously cracks. They are not depressedbelow their surroundings; in tact, they have notopographic structure whatever. Europa is ex-traordinarily smooth, and the dark steaks lookrather like marks made with a felt-tipped penon a white billiard ball. The streaks al c not evenvery dark; the contrast with adjacent smoothten am is only about 10 percent.

One of the most remarkable geologic phe-nomena discovered by Voyager is the light

1 62151

streaks that appear on Europa. These aresmaller than the dark streaks, only about 10kilometers in width, but much more uniform.Seen at low Sun angle, they also show verticalrelief of less than a few hundred meters. Theselight ridges are seen best at low Sun and tend tobe invisible at higher illumination angles.

The most amazing thing about the lightridges is their form. Instead of being straight,they form scallops or cusps, with smooth curvesthat repeat regularly on a scale of 100 to a fewhundred kilometers. In some of the low-Sun-angle pictures, the surface of Europa seems tobe covered with a beautiful network of theseregular curving lines. The impression is so biz-zare that one tends not to believe the reality ofwhat is seen. Nothing remotely like it has everbeen seen on any other planet.

At present the geology of Europa remainsbeyond our understanding. Presumably there isa thick ice crust, perhaps floating on a liquidwater ocean. Presumably there is sufficient heatcoming from the interior to have producedcracking or motion in the ice crust, and the lightand dark streaks preserve a pattern in some wayrelated to this internal activity. However, theactual mechanisms for producing the observedfeatures so lightly traced on this smooth whiteworld remain for scientists to decipher.

to

The most spectacular of the Galilean satel-lites is lo. Even in low-resolution images, itsbrilliant colors of red, orange, yellow, andwhite set it apart from any other planet. Thedramatic scale of its volcanic activity confirmsthat lo is in a class by itself as the most geologi-cally active planetary body in the solar system.

The diameter of Io is 3640 kilometers, andits density is 3.5 grams per cubic centimeter.Both values are nearly identical to those of ourMoon. Were it not for Io's proximity to Jupi-ter, it would probably be a dead, rocky worldmuch like Earth's satellite.

Careful examination of all the Voyager im-ages of lo, some of which have resolutions asgood as 1 kilometer, has failed to reveal a singleimpact crater. Yet the flux of crater-producingimpacts at I o must be even greater than for theother Galilean satellites, because of the focus-ing effect of Jupiter's gravity. The absence ofcraters alone would indicate that lo has an ex-tremely young and dynamic planetary surface,even without the observation of active volca-noes. Calculations indicate that craters on lo

152

Europa looks like a cracked egg in this computer mosaic ofthe best Voyager 2 images. In this presentation, the varia-tion of surface brightness due to the angle of the Sun hasbeen removed by computer processing, so that surface fea-tures can be seen equally well at all places. The many broaddark streaks show up well. but this presentation does notbring out the much fainter and more enigmatic lightstreaks. These pictures were taken from a distance of about250 000 kilometers and show features as small as 5 kilo-meters across. 1260-6861

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One of the most remarkable of all the Voyager discoveries was the arcuate white ridges on Europa. Visible only at very lowSun angle, these curved bright streaks are 5 to 10 kilometers wide and rise at most a few hundred meters above the surface.Their graceful scalloped pattern is unique to this planet and has defied explanation Also visible in this view, taken byVoyager 2 on July 9 at a range of 225 000 kilometers, are dark bands, more diffuse than the light ridges, typically 20 to 40kilometers wide and hundreds to thousands of kilometers long [P-217661

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153

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Shaded relief map of lo as it appeared in early March 1979 1260-634K]

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The great erupting volcanoes on lo produce &linens e surface markings. The top row shows three stews of Prometheus (PO.

The bright ring on the surface ums the areas of fallout from the plume. and it is probably an area in which sulfur dioxidefrost is being deposited on the surface. The bottom %loss are of t oki (P,) as seen by Voyager I (Bottom left) The assymetric

structure of the plume can be seen. (Bottom right) An ultras iolet image has been used to produce a false-color composite; thelarge ultraviolet halo above the sio:le-light plume may be due to scattering from sulfur dioxide gas rather than solid

particles. 1260-451)

41"

1

Perhaps the most spectacular of all the Voyager photos of lo is this mosaic obtained by Voyager 1 on March 5 at a range of400 000 kilometers. A great variety of color and albedo is seen on the surface, now thought to be the result of surfacedeposits of various forms of sulfur and sulfur dioxide. The two great volcanoes Pele and Loki (upper left) are prominent.[260-464]

:

166 155

must be filled in or otherwise obliterated at arate corresponding to the deposition of at least100 meters per million years, and quite prob-ably a factor of ten greater, or 1 meter everythousand years.

In place of impact craters, the surface of lohas a great many volcanic centers, whichgenerally take the form of black spots a fewtens of kilometers across. In a few cases, high-resolution pictures show the characteristicshapes assoc;ated with volcanic calderas onEarth and Mars, .nd, if the other volcaniccenters are similar, about 5 percent of the entiresurface of lo is occupied by calderas. These areextremely black, reflecting less than 5 percentof the sunlight; often they are surrounded by ir-regular, diffuse halos nearly as black as the cen-tral spot. The calderas seem more like theValles caldera in New Mexico, which is associ-ated with vents that produced large quantitiesof ash, than with those of Hawaiian-type shieldvolcanic mountains.

There is evidence in many of the Voyagerphotos of extensive surface flows on lo. Theseoriginate in dark volcanic centers and eitherspread to fan shapes, typically 100 kilometersacross, or else snake out in long, twisting tenta-cles. Some of the flows are lighter than thebackground and some are darker. Most are redor orange in color, often outlined by fringes ofcontrasting albedo.

The equatorial regions of lo are quite flat,with no vertical relief greater than about 1 kilo-meter high; indeed, many of the volcaniccenters do not appear to correspond to moun-tains or domes at all. There are, however, anumber of long, curvilinear cliffs or scarps andnarrow, straight-walled valleys a few hundredmeters deep. These appear to be places in whichthe crust has broken under tension, somewhatsimilar to terrestrial faults and the valleys calledgraben. A few rugged mountains of uncertainorigin are visible in low Sun elevation pictures.

Near the poles of lo the terrain is more ir-regular. There are few volcanic centers, butmore mountains, some with heights of severalkilometers. In addition, there are regions thatappear to be made of stacked layers of mate-rial. These so-called layered terrains are re-vealed when erosion cuts into them, exposingthe layers along the cliff or scarp. The largestsuch plateau or mesa has an area of about100 000 square kilometers. The scarps some-times intel sect each other, suggesting a complexhistory of deposition, faulting, and erosion.Voyager geologists believe that these scarps

156

may be areas in which the release of liquid sul-fur or sulfur dioxide has undercut cliffs, analo-gous to internal sapping by groundwater atsimilar scarps on Earth.

Perhaps the most distinctive surface fea-tures on lo are the circular or oval albedo mark-ings that surround the great volcanoes. The firstof these to be seen was the 300-kilometer-widewhite donut of Prometheus, on the equator a',longitude 150°. Much more spectacular is thehoofprint of Pele, about 700 by 1000 kilome-ters. These symmetric rings mark the locationsof the kinds of eruptions that generate largefountains or plumes, and may be produced bycondensible sulfur or sulfur dioxide rainingdown from the volcanic fountain. At least onenew ring appeared during the four months be-tween the Voyage' encounters, centered atlongitude 330°, latitude + 20°, but by the timeVoyager 2 photographed this area, no plume re-mained active.

During the Voyager 1 flyby, temperaturescans of the surface of lo were made with theinfrared interferometer spectroinetei JRIS). Anumber of localized warm regions were found,the most dramatic being just south of the vol-cano Loki. Here the images showed a strange,U-shaped black feature about 200 kilometersacross. The IRIS team interpreted its data to in-dicate a temperature for the black feature of17° C (or room temperature), in contrast to thesurrounding surface at 146° C. Perhaps thedark feature was some sort of lava lake, eitherof molten rock or molten sulfur. The meltingpoint of sulfur is 112° C. If there were a scumof solidifying sulfur on top of the "lake," thisinterpretation might well be the correct one.

The brilliant reds and yellows of the sur-face of lo immediately suggest the presence ofsulfur. When heated to different temperaturesand suddenly cooled, sulfur can assume manycolors, ranging from black through variousshades of red to its normal light-yellowappearance.

Even before Voyager, laboratory studieshad shown that sulfur matches the overall prop-erties of the spectrum of lo, including the lowalbedo in the ultraviolet and the high reflectiv-ity throughout the infrared. Contemporarywith the Voyager flybys, additional telescopicobservations and laboratory studies by FraserFanale at JPL and Dale Cruikshank at the Uni-versity of Hawaii identified another componenton lo, sulfur dioxide. Sulfur dioxide is an acridgas released from terrestrial volcanoes, where itcombines with water in the Earth's atmosphere

4

Close-ups of lo reveal a wide variety of volcanic phenomena. This Voyager I view of an equatotial region near longitude300° shows several large surface flows that originatein volcanic craters or calderas. At the rise cage is a light floss about 250kilometers long. Another dark, lobate flow with bright edges is Just left of center, with an en..-edingly dark caldera to its left.[260-468A]

to produce sulfuric acid. At the temperature ofthe surface of lo, sulfur dioxide is a white solid.Researchers guessed that the extensive brightwhite areas in the Voyager pictures of lo mightbe covered with sulfur dioxide frost or snow.The presence of this material on lo was con-firmed when the infrared IRIS instrument ob-tained a spectrum of sulfur dioxide gas over theerupting volcano Loki during the Voyager 1

encounter.The discovery of the ongoing eruptions on

lo, made shortly after the Voyager 1 flyby, didmuch to clarify the confused evidence pouringin concerning the apparent youth of Io's sur-face. Here, under the very eyes of Voyager,eruptions were taking place on a scale thatdwarfed anything ever seen before. The discov-ery picture alone, taken from a distance of 4million kilometers, showed two eruptions (Peleand Loki), each of which was much larger thanthe most violent volcanic eruption ever re-corded on Earth.

Voyager 1 found eight giant eruptions,with fountains or plumes rising to heights ofbetween 70 and 280 kilometers. To reach thesealtitudes, the material must have been ejectedfrom the vents at speeds of between 300 and1000 meters per second, several times greaterthan the highest ejection velocities from terres-trial volcanoes. Although widely spaced inlongitude, these volcanoes were concentratedtoward the equator; seven of the eight were atlatitudes between + 30' and 30°, and theeighth at 44 °.

When Voyager 2 arrived four monthslater, it was able to reobserve seven of the eightvolcanoes. (To be identified reliably, the vol-canic plumes must be silhouetted against darkspace et the edge of the disk.) Six of thesewere still erupting; one, Pele, the largest plumeseen by Voyager 1, had cer c-xl activity. Theplume associated with Loki ad also changedmarkedly, increasing in height from 100 to 210kilometers in visible light. (All the plumes ap-

157

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Differences in surface elevation can clearly be seen in a few of the lo close-ups from Voyager 1. This remarkable picture is ofthe center of the great volcano Pele, at latitude 15°S and longitude 224°. A low mountain with flow features can he seen. Inthe background, there are several large irregular depressions with flat floors that appear to he the result of collapse. The

diffuse dark features in the center are probably the meta plumes being erupted from the Pele vent. 1P-21220B/1V]

158

1L

4

At the highest resolution obtained by the Voyager cameras, lo revealed some landscapes that looked familiar to terrestrialgeologists This picture, taker. by Voyager 1 at a range of only 31 000 kilometers, shows a region about the size of the state ofMaryland at a resolution of 3(X) meters Clearly seen is a volcano not too different from ,ome of those on the Earth or Mars.At the center is an irregular composite crater or aldera about 50 odometers in diameter with dark flows radiating from itsrim The style of solLanism illustrated here is quite different from the esplosne plumes or fountains with their associatedrings of bright material deposited on the surface This solcano is located at about longitude 330", latitude 70 °S [260-502]

170 159

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In addition to its giant volcanic plumes or fountains, lo possesses other indications of current volcanic activity. One of thesetakes the form of intermittent blue-white patches that may be caused by gas venting from the interior. In this pair ofphotographs, the same region of the surface is shown about six hours apart. On the right, there is an arcuate bright gascloud; on the left the same region is black. It is believed that the venting gas is sulfur dioxide, and that the condensation ofthis gas produces fine particles of "snow" that look blue. [260-5081

Some of the most dramatic changes in the surface of lo between March and July took place in the vicinity of the volcanoLoki, at longitude 3100 and latitude I5°N. On the left is r. Voyager I view; on the right, one from Voyager 2. The "lavalake" assn ated with Loki has become less distinct, apparently as a result of deposits that fell on the northern part of thedark U-shaped feature. Perhaps the surface had also cooled between these photos. In the upper left center, a new darkvolcanic caldera with bright spots near it and a large, faint bright ring had appeared by July, although it was not active at thetime Voyager 2 flew by. 1260-687ACI

pear larger when viewed in the ultraviolet.)Loki had developed a more complex structure;in March it appeared to originate near the southend of a 250-kilometer-long dark feature, but inJuly there was a double plume, with activity atboth ends of the dark feature.

No new eruptions were seen by Voyager 2;between them, the two spacecraft effectivelysurveyed the whole surface of 10 for plumesdown to 40 kilometers height. Interestingly, thesmallest plume seen was 70 kilometers high;there appeared to be a real absence of smallereruptions.

From the number and size of the observederuptions, it is possible to calculate the resur-facing rate for lo due to these plumes. Theresult is that each plume is erupting about10 000 tons of material per second, or morethan 100 billion tons per year. This quantitycorresponds to about 10 meters of depositionover the whole surface in a million years. Whenadditional note is taken of surface flows, thedeposition rate could easily be ten times higher,or 100 meters per million years, in agreementwith the rate estimated from the absence of im-pact craters.

Energy for the lo Volcanoes

Clearly, something extraordinary is hap-pening to lo to generate the observed level ofvolcanism. The primary heat source for theinteriors of the terrestrial planets is the decay ofthe long-lived radioactive elements thorium anduranium. But lo would have to be supplied witha hundred times its quota of these elements toexplain the observed activity.

A way out of this difficulty was providedby a theoretical investigation carried out byStanton Peale of the University of California atSanta Barbara and Pat Cassen and Ray Reyn-olds at the NASA Ames Research Laboratory.Working in the months before the first Voyagerflyby, they calculated that the tidal effects ofJupiter on lo could generate large-scale heatingof the satellite. lo is about the same distancefrom Jupiter as the Moon from Earth, but themuch greater mass of Jupiter raises enormoustides in its satellite. These tides distort its shape,but no other effect would be present if lo re-mained at a constant distance from Jupiter.What Peale, Cassen, and Reynolds realized wasthat the distance of lo from Jupiter varies as theresult of small gravitational perturbations fromthe other Galilean satellites. Therefore the tidaldistortions also vary, in effect squeezing and

unsqueezing lo each orbit. Such flexing pumpsenergy into the interior of lo in the form ofheat; theorists calculated that the heat suppliedcould be as high as 1013 watts. They predicted,in a paper published just three days before theVoyager flyby of lo, that "widespread andrecurrent surface volcanism might occur," andthat "consequences of a largely molten interiormay be evident in pictures of lo's surfacereturned by Voyager."

Voyager 2 obtained beautiful views of the volcaniceruptions during its ten-hour to volcano watch on July 9.On the edge of the crescent image are the volcanoesAmirani (Ps) and below it Maui (P6), each sending upfountains about 100 kilometers above the surface. The bluecolor is probably the result of sunlight scattered by tinyparticles of sulfur dioxide snow condensing in the eruptingplume. 1P-217801

172

161

VOLCANIC ERUPTIONS ON 10

Plume Number NameLocation

(latitude/longitude)

Height DuringVoyager 1 Flyby

(kilometers)Activity DuringVoyager 2 Flyby

1 Pe le 200/255° 280 ceased2 Loki +20° /300° 100 increased3 Prometheus 50/155' 70 increased4 Volund +200/175° 95 no data5 Amirani +25"/120° 80 similar6 Maui +20°/120° 80 similar7 Marduk 25°/210° 120 similar8 Masubi 40° / 50° 70 similar

Between the two encounters, the volcanic eruption at Loki (P,) changed character. The single plume emanating from thewestern end of an apparent dark fissure seen in March was joined by a second fountain of similar size about 100 kilometersto the east. The plume also increased in height, from about 120 kilometers in the Voyager I image (left) to 175 kilometers inthe Voyager 2 image (right). 1260-662A and 1-31

162

The detailed structure near the volcano Loki is like nothing seen elsewhere on lo. (Top) When this Voyager 1 picture wastaken, the main eruptive activity (P2) came from the lower left of the dark linear feature (perhaps a rift) in the center. Belowit is the "lava lake," a U-shaped dark area about 200 kilometers across. In this specially processed image, detail can be seenin the dark surface of this teature, possibly due to "icebergs" of solid sulfur in a liquid sulfur lake. (Bottom) The IRIS onVoyager 1 found this "lava lake" to be the hottest region on lo, with a temperature about 150° C higher than that of thesurrounding arca (260-642B1

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One model for the structure of to indicates that an ocearrof liquid sulfur with a solid sulfur crust covers most of the satellite.Heat escapes from the interior in the form of lava, which erupts beneath the sulfur ocean. Secondary eruptions in the sulfurocean heat liquid sulfur dioxide, which is mixed with solid sulfur in the crust. The rapid expansion of sulfur dioxide gas thenproduces the great eruptive plumes, which consist of a mixture of solid sulfur, sulfur dioxide gas, and sulfur dioxide snow.

The Voyager observations appear to con-firm the theoretical calculations. The tidal heatsource has presumably been acting since lo wasformed more than 4 billion year ago. With atotally molten interior and continuing large-scale volcanism, lo has had an opportunity tothoroughly sort out its composition. In the pro-cess it would have lost all the volatile gases suchas water and carbon dioxide, explaining why lonow has no appreciable atmosphere in spite ofthe outpouring of material from the interior. Inaddition, most of the sulfur from the interiorcould have risen to the surface, where it wouldbe constantly recycled through volcanic activ-ity.

The presence of large amounts of sulfur onthe surface may help explain the extraordinarynature of the lo volcanoes. One model consid-ered for the satellite postulates that it is coveredby a sea of liquid sulfur several kilonrters

164

deep, with a crust of solid sulfur and, below thesurface, liquid sulfur dioxide. Calculations bySue Kieffer of the U.S. Geological Survey andothers indicate that the expansion of the sulfurdioxide in such a model can explain the ob-served eruption velocities of up to a kilometerper second.

The volcanic plumes on lo appear to bemade primarily of sulfur and sulfur dioxide.Both are molten as they emerge from the vent,but they quickly cool as the plume rises 100 ormore kilometers into the near vacuum of space.Unlike terrestrial volcanoes, there is almost nogas in the plumes. It requires about half anhour for the fine particles of solidified sulfurand sulfur dioxide snow to fall back to the sur-face, where they form the colorful rings thatmark the major eruptive sites.

Almost all the roughly 100 000 tons ofmaterial erupted each second by the lo vol-

Sulfur dioxide gas cloud

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Direct evidence of an atmosphere on to was obtained during the Voyager I flyby by the IRIS. In the region near the volcanoLoki and its associated "lava lake," infrared spectra clearly showed the signature of sulfur dioxide gas It is not knownwhether this gas was a temporary feature associated with the eruption of Loki or if it might be present on to more generally.Other evidence, however, points to the sulfur dioxide atmosphere as a transient feature. A small amount of the sulfur dioxideescapes and is broken apart by sunlight to provide the oxygen and sulfur tons observed in the lo torus.

canoes snows back to the surface. But appar-ently a partperhaps 10 tons per secondescapes from lo and is captured by the Jovianmagnetosphere. Another part contributes to anionospherea tenuous atmosphere of electronsand ionsthat surrounds lo. The injection ofseveral tons of particles each second into themagnetosphere has dramatic consequences thatcan be seen even from Earth.

The Io Torus

Surrounding Jupiter at the distance of lotsa donut-shaped volume, or torus, of plasmathat originates at the satellite. At first, theatoms escaping from to expand outward as agas, but soon they are stripped of electrons andbecome electrically charged. Some of thesegases, such as sulfur dioxide, apparently origi-nate in the large volcanic eruptions; other, such

as the sodium cloud being studied with Earth-based telescopes, result from sputtering of sur-face materials by energetic particles in themagnetosphere. After they are ionized by theloss of one or more electrons, the atoms arecaught by the spinning magnetic field of Jupiterand become a part of what is called a co-rotating plasma, spinning at 74 kilometers persecond with the same ten-hour period as Jupiteritself.

The to torus was easily detected onVoyager by the ultraviolet spectrometer, evenfrom a distance of ISO million kilometers. Thestrongest ultraviolet radiation comes fromtwice-ionized sulfur (atoms that have lost twoelectrons) (S Ill), emitting a wavelength of 69nanometers or about one-eighth the wavelengthof visible light. The spectrometer also detectedglows from atoms of triply ionized sulfur (S IV)and twice-ionized oxygen (0 111).

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At the time of the Voyager I encounter, the most abundantheavy ions in the Jovian magnetosphere were sulfur andoxygen. Multiply ionized sulfur and oxygen both emitstrongly in the ultraviolet, where they could be observed bythe ultraviolet spectrometer. This spectrum of the lo torusregisters the tremendous amount of ultraviolet energy(about a million million ,,,atts) being radiated To emit sostrongly, the temperatures in he torus must be near100 000 K.

Scans across the torus showed that it had athickness of 1.0 R , and w as centered at adistance of 5.9 R

Jfrom Jupiter. Tne torus is

centered on the magnetic, rather than the rota-tional, equator of the planet. To produce theintense glow observed, the electron tempera-tures in the ton's must be 100 000 K, with anelectron density of about 1000 per cubic cen-timeter. The brightness in the ultraviolet cor-responds to a radiated power of more than amillion million (1012) watts. This enormousamount of energy must be continuously sup-plied by the magnetosphere.

The ultraviolet emissions from the lo torusseen by Voyager were dramatically differentfrom those seen in 1973 and 1974 with thesimpler ultraviolet instrument on board Pio-neers 10 and 11. These changes correspond tomore than a factor of 10 in brightness. As notedby the Voyager Team, "Because of the remark-able differences we conclude that the Jupiter-loenvironment has changed significantly since

166

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Direct measurement of the heavy ions associated with the totorus were made by the Voyager I LECP instrument. Herethe amounts of various elements are shown for two cases:the Jovian inner magnetosphere (solid line) and a typicalsolar event Both the Jovian and the solar particles havebeen scaled to show similar amounts of oxygen, but thesolar particles are also rich in carbon and iron, whereasJupiter has a great deal of sulfur. Evidence such as thisdemonstrates that the sulfur does not come from the Sun;rather, the sulfur and most of the oxygen appear to be theproduct of the lo sulfur dioxide volcanic eruptions

December 1973. The observed differences areso spectacularly large that this conclusi .. does

not depend on a detailed comparison of the twoinstruments, their calibrations, or the observinggeometry." The reason for this change, or thedegree to which it reflects a large-scale variationin the volcanic acti' of lo, is one of themajor questions arising from the Voyager mis-sion.

The lo torus can also be observed from theground. In 1976, spectra showed the glow ofsingly ionized sulfur (S II), and in 1979 singlyionized oxygen (0 11) was detected. The obser-%ations of S II and 0 II are particularly inter-esting because they provide a measure of thedensity and temperature of the plasma. In April1979, between the two Voyager flybys, CarlPilcher of the University of Hawaii succeededin obtaining a telescopic image of the torus inthe light of S II. He measured nearly the samering diameter (5.3 5.7 R .1) as had Voyager; in-

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The emission of light from sulfur ions in the lo torus is sostrong it can be measured from the Earth In these moures,UnlYersity of Hawaii astronomer Carl Pitcher photo-graphed the torus in the light of ionized sulfur on the nightof April 9, 1979 As the planet rotates, the torus is seen firstpartly opened, then edge-on, and again opened in the oppo-site direction The dark band on the right of each image isdue to light from Jupiter scattered in the telescope, asshown in the bottom moure, \stitch contains the scatteredlight only

terestingly, both agree that the sulfur torus iscentered slightly inside the )rbit of to (6.0 19.

Direct measurements of the torus weremade from Voyager 1 as the spacecraft passedtwice through this region, once inbound andonce outbound. The low energy charged parti-cle instrument and the cosmic ray instrument

both determined that the Lomposition of theions in the to torus was primarily sulfur andoxygen. Ionized sodium was also observed. Forseveral years, ground-based telescope observa-tions had revealed a cloud of neutral sodiumaround lo; the Voyager instruments picked upthese atoms after they had each lost an electronand become trapped in the magnetosphere.These instruments also derived the electron andion density (about 1000 per cubic centimeter)and confirmed that the ions were co-rotatingwith the Inner magnetosphere.

Another lo-associated phenomenonsearched for by Voyager was the lo flux tube.As a conductor moving through the Jovianmagnetic field, lo generates an electric current,estimated to have a strength of about 10 million(107) amperes and a power of the order of amillion million (1012) watts. The region ofspace through which this current flows from thesatellite to Jupiter is called the flux tube.

Voyager I was targeted to fly through theto flux tube. This was an important decision,since this option precluded the possibility of ob-taining occultations by either lo or Ganymede.The event was to take place on March 5, justafter closest approach to 10. The effects of theflux tube were clearly observed by the magneto-meter, the LECP instrument, and other particleand field instruments; however, subsequentanalysis indicated that the spacecraft had notpenetrated the region of maximum currentflow; it probably missed the center of the fluxtube between 5000 and 10 000 kilometers.

The flux tube is not the only connectionbetween lo and Jupiter. Radio emissions fromthe atmosphere are triggered by Io's orbitalposition, and the aurorae that illuminateJupiter's polar regions are the result of chargedparticles falling into the planet flom the totorus. Other charged particles can occasionallyescape outward and be detected as far away asEarth.

lo is unquestionably a remarkable world.The only planetary body known to be ecologi-cally more active than the Earth, it pros idesmany extreme examples to test the theories ofgeoscientists. Its Intimate interconnections withthe Jovian magnetosphere and the planet itselfprovide a unifying theme to the complex pro-cesses taking place to the inner parts of the Jo-

ran sy stem.

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CHAPTER 10

RETURN TO JUPITER

A Successor to Voyager

The spectaculL discoveries of the Voy-agers did not exhaust our interest in the Joviansystem. Both the giant planet and its system ofsatellites will almost certainItt play a central rolein any future program of solar system explora-tion and research. Thus, even as the two Voy-ager spacecraft directed their attention furtheroutward toward Saturn, NASA had begun de-velopment of the next Jupiter mission, namedGalileo.

Galileo is an ambitious, multiple-vehicleplanetary mission. It has two major interlock-ing elements: a probe to be placed in the atmo-sphere of Jupiter and an orbiter to exploreJupiter, its satellites, and its magnetosphere. Byusing individual satellite flybys to alter its orbit,the Galileo spacecraft can carry out a satellite"tour" consisting of flybys if the Galileansatellites at different geometries and a deeppenet, 'tion into the magnetosphere in the unex-plored region of space behind Jupiter.

Bo h an atmospheric probe for the planetand a long -li "ed orbiter to study the satellitesand magnetosphere are logical successors toVoyager. In 1974, three years before Voyagerlaunch, the Space Science Board of the Na-tional Academy of Sciences was already empha-sizing the scientific advantages of both of theseapproaches. In suggesting goals for 1975-1985,the Board wrote, "We recommend that a sig-nificant effort in the NASA planetary programover the next decade be devoted toward theouter solar system. Jupiter is the primary object

of outer solar system exploration." Looking atspecific mission goals, the Board recommendedthat "the primary objectives in the explorationof Jupiter and its satellites for the period1975-1985 in order of importance are (1) deter-mination of the chemical composition andphysical state of its atmosphere, (2) the chem-ical composition and physical state of thesatellites, and (3) the topology and behavior ofthe magnetic field and the energetic particlefluxes. In order to carry out this program, itwill be necessary to utilize orbiting spacecraftand probe-deliverir.g spacecraft."

In the same period NASA carried outstudies of two possible orbiter and probe mis-sions. Working through the Ames ResearchCenter, a scientific panel chaired by James VanAllen explored the adaptation of the Pioneer 10and 11 spinning spacecraft to carry a probe toJupiter and to carry out an orbiter missionemphasizing magnetospheric studies. WilliamB. Hubbard of the University of Arizonachaired a JPL-based panel investigating the useof a Mariner-class fully stabilized spacecraftsimilar to Voyager to carry out a satellite-oriented orbiter mission. In 076 these conceptswere combined in a study, again chaired by Dr.Van Allen, of a Voyager-type orbiter withprobe-carrying capability. This mission conceptwas given the name JOP, for Jupiter OrbiterProbe, and lead responsibility was assigned byNASA to JPL, with Ames carrying out thedesign of the probe.

In 1977, as Voyager activity was buildingtoward autumn launch, a struggle was under-

The Galileo Probe will make a fiery entry into the Jovian atmosphere, carrying a payload scientific instruments for thefirst direct sampling of the atmosphere of a giant planet Shown here is the moment, at a pressure level of about 0.1 bar,whet 'he parachute is deployed and the still-glowing heatsfueld drops free from the Probe

169

way in Washington to obtain approval for thenew Jupiter orbiter and probe mission. Budg-eting authority was requested in the President'sFiscal Year 1978 budget, but only after exten-sive testimony and several Congressional soteswas the mission appros ed. The official newstart for JOP, soon to be renamed Galileo, wasset for July 1, 1977, and the scientific investi-gators and their instruments were selected inAugust.

At JPL, many members of the VoyagerTeam made a smooth transition to the GalileoProject. Much of the knowledge that had goneinto the design of the Voyager spacecraft andits subsystems was now incorporated intoGalileo. Similor!y, at Ames the knowledgegained from thi. design of the Pioneer Venusprobes, which were launched to Venus in 1978,a year after Voyager launch, was applied todesign of a Jupiter probe. Among the in-dividuals who brought their Voyager experienceto Galileo were John Casani, who left the posi-tion of Voyager Project Manager to becomeGalileo Project Manager, and TorrenceJohnson of the Voyager Imaging Team, whobecame Galileo Project S6eutist.

The Scientific Capabili1) of Galileo

The investigations of Jupiter and its systemplanned for the Galileo Project . :presentedsubstantial advances over those carried out byVoyager. In part, this was the result of newspacecraft capabilities, particularly the at-mospheric entry probe. It also representedincreasing sophistication in scientific instru-nentation over the seven-year interval betweenthe selection of the payloads for the two mis-sions.

The main emphas.s in the study of Jupiteritself is on direct measurements with the Probe.For the first time it will be possible to examinedirectly the atmosphere of a giant planet. Bymeasuring the temperature and pressure as itd,:scends through the clouds, the Probe candetermine the structure of tl- e atmosphere withmuch higher precision than could ever be ob-tained from remote observations. The struc-ture, in turn, provides information ondynamicsthe circulation and heat balance ofthe Jovian atmosphere. In addition, the Probecan make direct measurements of the composi-tion of the gases, with sensitivity in some casesto quantities as low as a few parts per billion. Inaddition to the elemental abundance, the

170

amount of different isotopes can also bemeasured.

Direct studies of the clouds of Jupiter canbe made from the Galileo Probe. With a deicecalled a nephelometer (literally, cloud-meter),the sizes and compositions of individual aerosolparticles will be determined. An infrared instru-ment will determine the temperatures of thecloud layers and measure the amounts of sun-light deposited in different regions of the at-mosphere. Another instrument will search forlightning; it has the ability to detect both theflash of light and the radio static generated byeach bolt.

Additional studies of the atmosphere,similar to those of Voyager, can be carried outfrom the Galileo Orbiter. Television pictures,ultraviolet and infrared spectra, and measure-ments of the polarization of reflected light willall be obtained with the same scan platform in-struments that are used to study the surfaces ofthe satellites.

A full battery of fields and particles in-struments is planned for the Galileo Orbiter.Many of these aie direct descendants ofVoyager instruments. In general, theircapabilities hate been improved, particularlytheir ability to determine the composition ofcharged particles. There is a steady progressionfrom Pioneer to Voyager to Galileo: The earlymeasurements were concentrated on particleenergies but more sophisticated instrumentsyield the composition of the ions and the detailsof their motion.

Many of the advances expected from Gali-leo in magnetospheric studies result from theOrbiter's ability to explore many p ?rts of theenvironment of Jupiter. The Pioneer andVoyager spacecraft made single cuts throughthe magnetosphere, and often it was difficult todistinguish temporal from spatial effects.Galileo will repeatedly swing around Jupiter,sampling conditions at many distances from theplanet os er a time span of two years or more. Inaddition, it is planned to adjust the orbit ofGalileo to swing out into the magnetotail, theturbulent region of the magnetosphere thatstretches "downwind" from Jupiter for hun-dreds of Jupiter radii. No flybys can reach themagnetotail; an orbiting spacecraft is required.

The Galilean satellites naturally will be aprimary focus of Galileo science, particularlyafter Voyager. It is planned to have as many asa dozen individual encounters, most of them atmuch closer range than the Voyager flybys. Totake adsantage of these opportunities, the

The Orbiter section of the Galileo spacecraft will carry both remote sensing and direct measuring instruments for the study ofJupiter, its satellites, and its magnetosphere. Several remote sensing instrumentsan imagery system, a near infraredmapping spectrometer, an ultraviolet spectrometer, and a photopolarimeterfradiometerwill be mounted on a scanplatEirm. The particles and fields instruments will be on a spinning section of the spacecraft. The Orbiter is expected toopet ate for at least two years around Jupiter, providing one close flyby of to and several each of Europa, Ganymede, andCallisto. [P-20772]

Galileo scan platform will carry two newremote sensing systems.

Instead of the v'dicon television camera onVoyager, Galileo imaging will be done with anew solid-state detector called a charged cou-pled device (CCD). The CCD has a wider spec-

tral response and greater photometric accuracy.In addition, its increased sensitivity permitsshorter exposures, so that even on very closeflybys the pictures will not be blurred byspacecraft motion. Substantial coverage at aresolution of 100 meters s!.ould be possible,

171

NASA

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Galileo will be launched by the Space Shuttle, the central element of the new NASA Space Transportation System. Togetherwith its upper stage launch rocket, Galileo will be placed into Earth's orbit in the large Shettle bay, about 20 meters long and5 meters in diameter. After releasing Galileo, the Shuttle will be piloted back to a landing at Cape Canaveral, to be usedagain for many future flights. 15-78-23599]

The Space Shuttle and the Inertial Upper Stage of Galileo as they will appear in Earth orbit. The upper stage has beenreleased from the Shuttle bay and is being prepared for launch to Jupiter.

172

compared to Voyager's best resolution of 1kilometer for lo and 4 kilometers for Europa.

To determine the composition of satellitesurface materials, Galileo will also carry a near-infrared mapping spectrometer (NIMS). Thisinstrument will obtain measurements over thevisible and infrared spectra of areas as small as 10kilometers across. With NIMS, it should be pos-sible to investigate the composition of individualfeatures as small as the volcanic calderas on loor the ejecta blankets of Ganymede's craters.

Galileo Mission Design

The Galileo Orbiter and Probe are to belaunched with NASA's new Space Shuttle andInertial Upper Stage. To carry the maximumpossible payload to Jupiter, a close flyby ofMars is planned en route. The gravitational

field of Mars will give a boost to Galileo, just asthat of Jupiter was used by Voyager to swing onto Saturn.

The exact launch date and trajectory forGalileo have not yet been specified, but if allgoes well, the Orbiter spacecraft will approachJupiter from the dawn side of the planet some-time in the mid-1980s. It will not be moving asfast as Voyager, since it must be placed intoorbit around Jupiter rather than flashing paston its way to the outer solar system. On its ini-tial trajectory, Galileo will probably co-me with-in 5 Rj of Jupiter, slightly closer than Voyager1. At this time it will fire its rocket engines (sup-plied by the Federal Republic of Germany in acooperative program with NASA) to shed ex-cess speed and let itself be captured by Jupiter'sgravity. The first pass will also be the time for aclose flyby of lo.

The most critical period of the Galileo flight will be the Probe entry at Jupiter. The Probe must strike the atmosphere atprecisely the correct angle and speed to be slowed down without being destroyed. At a pressure level of about 0 1 bar therapid deceleration period ends and the heat shield is released. A parachute is deployed to slow the descen. further, and theProbe then has a period of nearly an hour to study the atmosphere and clouds of Jupiter. The Probe mission ends when itsbatteries run down or when it is crushed by the pressure of the Jovian atmosphere near the 20-bar level, whichever comesfirst. [SL78-545(3)]

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184

GALILEO PROBE SCIENCE INVESTIGATIONSProbe Scientist: L. Collin, NASA Ames

Investigation Principal Investigator Primary Objectives

Atmospheric structure

Neutral mass spectrometer

Helium abundance interferometer

Nephelometer

Net flux radiometer

Lightning and radio emission

Energetic particles

A. Seiff, NASA Ames

H. B. Neimann,NASA Goddard

U. von Zahn, Bonn U.(Germany)

B. Ragent, NASA Ames

R. W. Boese, NASA Ames

L. J. Lanzerotti, Bell Labs

H. M. Fisc:her, U. Kiel(Germany)

Measure temperature, density, pres-sure, and molecular weight to deter-mine the structure of Jupiter'satmosphere.

Measure the composition of the gasesin Jupiter's atmosphere and the varia-tions at different levels in the atmos-phere.

Measure with high accuracy the ratioof hydrogen to helium in Jupiter'satmosphere.

Determine the sizes of cloud particlesand the location of cloud layers inJupiter's atmosphere.

Measure energy being radiated fromJupiter and the Sun, at different levelsin Jupiter's atmosphere.

Measure lightning flashes in Jupiter'satmosphere, from the light and radiotransmissions from those flashes.

Measure energetic electrons and protons in the inner regions of the Jovianradiation belts and determine theirspatial distributions.

Because of the intense radiation environ-ment, the Galileo Orbiter will not be able tospend much time in the inner magnetosphere,near the orbit of lo. To do so would riskdamage to the spacecraft electronics and apremature end to the mission. Additionalthruster firing during the first orbit can be usedto raise the periapse to 10 R or greater. Nomore close passes by lo will be possible, butstudies of this satellite can be made on eachsubsequent orbit with imaging resolutions ofabout 10 kilometers, sufficient to see details ofthe volcanic eruptions and monitor volcano-associated changes in the surface.

At each k,ibsequent orbit, Galileo will beprogrammed for a close flyby of one of theother satellites. Several passes each of Callisto,Ganymede, and Europa should be possible.The satellite tour does not need to be fullyplanned in advance; by adjusting the spacecrafttrajectory with small bursts of the thrustermotors, navigation engineers can modify theorbit to permit adaptation to scientific needs.As the Orbiter mission (,.ogresses, the space-craft will also sample many parts of the magne-

tosphere, including one long excursion, at least150 Rj, into the magnetotail.

The total duration of the Orbiter mission isplanned to be at least 20 months. Additions tothe basic mission are possible if the spacecraftremains healthy and fuel reserves are adequate.In contrast, the Galileo Probe mission lastsonly a few hours.

As the Probe approaches the atmosphereof Jupiter at the awesome speed of 26 kilo-meters per second, it will be traversing a regionof space never before explored. An energeticparticle detector will investigate the innermostmagnetosphere before the entry begins. Tnen,wit tin a period of just a few minutes, frictionwith the upper atmosphere must dissipate theProbe energy until it is falling gently in theJovian air.

Jupiter, being the largest planet, presentsthe most challenging atmospheric entry missionever undertaken by NASA. The design of theGalileo Probe calls for a massive heat shield toprotect the instruments during the high-speedentry phase. After the Probe has slowed to sub-sonic velocities, a parachute will be deployed,

GALILEO ORBITER SCIENCE INVESTIGATIONSProject Scientist: T. V. Johnson, JPI.

In estigation Principal Investigator Primary ObjectivesSolid state imaging M. J. S Belton, Provide images of Jupiter's at-

I.'tt Peak Observatory mosphere and its satellites; study at-(Team Leader) mospheric structure and dynamics on

Jupiter; investigate the compositionand geology of the satellite surfaces;study the active solcanic processes on10.

Ultraiolet spectrometer W. Hord, U. Colorado Study composition and structure of theupper atmospheres of Jupiter and itssatellites

Near-int rared mapping R W Carlson, .1131_ Provide spectral images and reflectedspectrometer (NIMS) sunlight spectra of Jupiter's satellites,

indicating the composition of their sur-laces; measure reflected sunlight andthermal emission from Jupiter'satmosphere to study composition,cloud structure, and temperatureprofiles; monitor hot spots on 10.

Photopolarimeter/ radiometer I L Hansen, Measure temperature profiles andNASA Goddard energy balance of Jupiter's atmos-

phere; measure Jupiter's cloud charac-teristics and composition.

'Magnetometer inKt% Mediate agneta. fields and the waysUC Los Angeles they change near Jupiter and its satel-

lites; measure variations r.atzied by thesatellites interacting with Jupiter'sfield.

Plasma particles I A. l rank, U. lot, a Provide information on low-energyparticles and clouds of ionized gas inthe magnetosphere.

Energetic particles D .1. Williams, NOAA Measure composition, distribution,Space Emironment Lab and energy spectra of high-energy par-

ticles trapped in Jupiter's magneto-sphere.

Plasma sales D A Gurnett, U. Iowa In%estigate waves generated insidelupiter's magnetosphere and wavesradiated by possible lightning dis-charges in the atmosphere.

Dust detection I Grua, Max-Planck-Institut Determine size, speed, and charge of(Germans) small particles such as micrometeorites

near Jupiter and its satellites.Celestial meLhanks I. D Anderson. .11)1 Use the tracking data to measure the

( I eam I eader) gran its fields of Jupiter and its satel-lites; search for gravity vases propa-gating through interstellar spare.

Radio propagation II T. Flovaid, Stanford U. Use radio signals from the Orbiter and(Team I eader) Probe to study the structure of the at-

mospheres and ionospheres of Jupiterand its satellites.

Interdistaplinary Scientists. 1 P analc ( ), P. .1. Gierasch (Cornell U.), D. Ni. Hunten (U. Arizona),H Masursks (U S. Geological Sursey), M. B. McElroy (Harsard U.), D Mor-rison (U. Hawaii), CI S. Orion ( IPL), T. Owen iSU New York), I. B. Pollack(NASA Ames), ( T. Russell (UC t os Angeles), ( . Sagan (Cornell U.), F. L. Scarf( TRW), G Schubert (VU Los Angeles), C. P Sonett (U. Arizona), .1. A. Van Allen(U. lima)

106

13

During its two years in orbit, tile Galileo Orbiter wiii cal I v out flan y investigations of the planet, the Galilean satellites and

the Jovian magnetosphere. Repeated close flybys of the satellites are used to modify and shape the orbit to provide addi-

tional flybys at an optimum viewing geometry. Initially, the orbit is a long loop that extends in the general direction of the

sunset side of Jupiter. The orbit Is then contracted, and the encounters with the satellites rotate it behind the planet for a long

excursion into the magnetotail late in the tour.

and the heat shield, having done its job, will bedropped free.

The Probe will spend nearly an hour de-scending from a pressure level of about 0.1 bar,where the heat shield is jettisoned, to a depth of10-20 bars. During this time it will make mostof its scientific measurements, relaying themback to Earth via the Probe carrier. Designersexpect the Probe to sink through regions of am-monia clouds, ammonium hydrosulfide clouds,and ice and water clouds during this hour.

By the time it has descended below thewater clouds, the increasing pressure will ex-ceed the strength of some Probe components.Engineers expect the Probe to have completedits mission, exhausted its battery power, andbeen crushed by the atmospheric pressurebefore the 20-bar level is reached. Lifeless, itwill then sink on into the thick, hot lower at-mosphere of Jupiter.

Beyond Galileo

After Galileo, the future cannot bepredicted. Perhaps there will no longer be a

176

program of planetary exploration. But ifhumanity still has the vision to seek a future inthe stars, there will surely Lc other Jupiter mis-sions.

Perhaps the next mission will concentrateon Jupiter itself. Probes could be built to with-stand pressures as high as several hundred bars,feeling their way deep into the murky depths ofthe planet. Or a hot-air ballon could be de-ployed from a probe to catty instruments forlong-term studies of the atmosphere. A numberof proposals have also been made for addi-tional satellite missions, including orbiters orlanders for Ganymede and Callisto. Or perhapsit 'sill be desirable to land a vehicle on one ofthe satellites and collect a sample and return itto Earth for laboratory analysis.

Whatever the future holds, it is clear thatthe Pioneer and Voyager missions blazed thepath to Jupiter and beyond. The little Pioneersproved that it could be done, and the Voyagersexpanded their vision, exploring and discover-ing new worlds more remarkable and excitingthan anyone could have imagined.

1 4.,) i

APPENDIX A

PICTORIAL MAPSOF THE

GALILEAN SATELLITES

These maps were prepared for the Voyager Imaging Team by the U.S. GeologicalSurvey in cooperation with the Jet Propulsion Laboratory, California Institute ofTechnology and the National Aeronautics and Space Administration. Copies areavailable from Branch of Distribution, U.S. Geological Survey, 1200 South EadsStreet, Arlington, VA 22202, and Branch of Distribution, U.S. Geological Survey,Box 25286, Federal Center, Denver, CO 80225.

177

Preliminary Pictorial Map of Callisto

Atlas of Callisto1:25,000,000 Topographic S-riesJc 25M 2RMN, 19791-1239

This map was compiled from Voyager 1 and 2 pictures of Callisto. Placement offeatures is based on predicted spacecraft trajectory data and is highly approximate.The linkage between Voyager 1 and Voyager 2 pictures is particularly tenuous.Placement errors as large as 10° are probably common throughout the map, and afew may be even larger. Feature names were approved by the International Astro-nomical Union in 1979. Airbrush representation is by P. M. Bridges.

Jc 25M 2RMN: Abbreviation for (Jupiter) Callisto, 1:25,000,000 series, secondedition, shaded relief map, R, v4ith surface markings, M, andfeature names, N.

+90°550,

1000 800

178

Scale 1 13 980 000 at 56' latitudePolar stereographic projection

+ 90"

+ 550600 400 200 0 200 400 600 8"O 1000

Kilometers

180°

0

V00

NU

Akverto

?d'

0

co

tar

North polar region

0°200

1600

180°South polar 'Peon

34o0

190

O

OOa

Callisto

180"70°t

60°

40°

160" 140"

111

North120" 100"

A s 9

20°

t

44116.

'

a r a

80" 60"

I

V A it. 14 A

20' 0'

a's

70"180"

180

160' 140" 120' 100' 80South

60' 40'' 20" 0'

North300" 280' 260" 240' 220" 200" 180"

00 340" 320" 300" 280" 260'' 240" 220" 200" 180`'700South

Scale 1.25 000 000 at 0° latitudeMercator projection

± 701000800 600 400 200 0 200 400

Kilometers

± 500

±30°± 10°

1 52

800 WOO_ + 70°

+ 500

±30°+ 10°

182

Preliminary Pictorial Map of Ganymede

Atlas of Ganymede1:25,000,000 Topographic SeriesJg 25M 2RMN, 19791-1242

This map was compiled from Voyager 1 and 2 pictures of Ganymede. Placement offeatures is based on predicted spacecraft trajectory data and is highly approximate.The linkage between Voyager 1 and Voyager 2 pictures is particularly tenuous.Placement errors as large as 10° are probably common throughout the map, and afew may be even larger. A large unresolved discrepancy exists in the area boundedby the 45° and 55° parallels between 120° and 180° longitude. Relative place-

ment of features is distorted in that area. Feature names were approved by the Inter-national Astronomical Union in 1979. Airbrush representation is by J. L. Inge.

Jg 25M 2RMN: Abbreviation for (Jupiter) Ganymede, 1:25,000,000 series, secondedition, shaded relief map, R, with surface markings, M, andfeature names, N.

Scale 1 13 980 000 at 56° latitudePolar stereographic projection

+ 90° + 90°

+ 55° + 55°

1000 800 600 400 200 0 200 400 600 800 1000Kilometers

1 0...: kj)

co

q00

180"

C:1°' 1'4 300

10:

0"North polar region

0'

34o

So

760i, .180°

South polar region

0

1D

' I .1 . 1

0° 340° 320° 300°

41110

00

A

4

North280°

I 1

340° 320' 300" 280°South

260° 240° 220° 200^ 180°.70°

I

2 60 ° 240°

60°

40°

20°

0°

-20°

-40°

-60°

A -70°220° 2 100°

Scale 1 25 000 000 at 0° latitudeMercator projection

Kilometers1000 800 600 400 200 0 200 400 600 800 1000±60° ±60°#40" 4 40°20" + 20°0' 0°

186185

Preliminary Pictorial Map of Europa

Atlas of Europa1:25,000,000 Topographic SeriesJe 25M 2RMN, 19791-1241

This map was compiled from Voyager 1 and 2 pictures of Europa. Placement offeatures is based on predicted spacecraft trajectory data and is highly approximate.Feature names were approved by the International Astronomical Union in 1979.Airbrush representation is by J. L. Inge.

Je 25M 2RMN: Abbreviation for (Jupiter) Europa, 1:25,000,000 series, secondedition, shaded relief map, R, with surface markings, M, andfeature names, N.

+90°

+55° r-1000

186

Scale 1 13 980 000 at b6° latitudePolar stereographic projection

+90°

+ 55°

800 600 400 200 0 200 400 600 800 1000

Kilometers

S

180°

2000

20c

2.0°

0°North polar region

0°

3.400

3400

1600180°

South polar region

g?

of

198

tDtN

EuropaNorth

7080" 60" 40" 20' O'' 340" 320" 300" 280" 260°°

60°.

40°- -t-

20°'

ge_

20°,

40°

60"

ss-

70"80" 60" 40" 20"

South

188

340' 320' 300" 280° 260°

260° 240° 220" 200°

North

180" 160° 140° 120" 100° 80°70°

+L.

1

0°

20°

-40°

70°260° 240' 220" 200' 180° 160^ 140" 120^ 100° 80°South

800± 70°

1500

± 30°

± 10°0°

Scale 1:25 000 000 at 0° latitudeMercator projection

Kilometers0600 400 200

200

200

firI /

400 600 800± 70°

60°

30°± 10°0°

190

Preliminary Pictorial Map of lo

Atlas of lo1:25,000,000 Topographic SeriesJi 25M 2RMN, 19791-1240

This map was compiled from Voyager 1 and 2 pictures of lo. Placement of featuresis based on preliminary control information provided by M. E. Davies of the RandCorporation, Santa Monica, California, and is probably accurate within 50 to 100km. Feature names were approved by the International Astronomical Union in1979. Airbrush representation is by P. M. Bridges.

Ji 25M 2RMN: Abbreviation for (Jupiter) lo, 1:25,000,000 series, second edition,shaded relief map, R, with surface markings, M, and featurenames, N.

+ 900

Scale 1 13 980 000 at 56° latitudePolar stereographic projection

:. .

+ 55° - -. -. + 55°1000 800 600 400 200 0 200 400 600 800 1000

Kilometers

+ 90°

o r 44, ..,, , v,A.

00

co

- 34o,

a°4,4

akit G4A--

.1 oft?

x-Slillrtart

Merv,

Va: -.4. ' -)i, '

ihmsa tt -'- 111 (10

t i

ifht.Pith A.

'64prth

180"

South poiar region

202

North

180" 160° 140° 120" 100" 80" 60'

SAMOA1111610

411

I

40" 20" 0°70"

60°

40°

TARSUS IN

41If

I.1

REGRX

M.

I

180' 160" 140" 120" 100°

SouthBO"

gam& _to_60° 40°

Scale 1:25 000 000 at 0° latitudeMercator projection

1000 80Kilometers

600 400 200 0500

± 30°± 10°

0° t t

200

20°

4

0°

20°

40°

60°

70"

400 600 800 1000700

50°± 30°

±10°0°

APPENDIX B

VOYAGERSCIENCE TEAMS

Imaging Science

Bradford A. Smith, University of Arizona,Team Leader

Geoffrey A. Briggs, NASA HeadquartersA. F. Cook, Smithsonian InstitutivnG. E. Danielson, Jr., California Institute of

TechnologyMerton Davies, Rand Corp.G. E. Hunt, University College, LondonTobias Owen, State University of New YorkCarl Sagan, Cornell UniversityLawrence Soderblom, U.S. Geological SurveyV. E. Suomi, University of WisconsinHarold Masursky, U.S. Geological Survey

Radio Science

Von R. Eshleman, Stanford University, TeamLeader

J D. Anderson, Jet Propulsion LaboratoryT. A. Croft, Stanford Research InstituteGunnar Linda!, Jet Propulsion LaboratoryG. S. Levy, Jet Propulsion LaboratoryG. L. Tyler, Stanford UniversityG. E. Wood, Jet Propulsion Laboratory

Plasma Wave

Frederick L. Scarf, TRW Systems, PrincipalInvestigator

D. A. Gurnett, University of !owa

Infrared Spectroscopy and Radiometry

Rudolph A. Hanel, Goddard Space FlightCenter, Principal Investigator

B. J. Conrath, Goddard Fpace Flight Center

0 .A. '..) J

P. Gierasch, Cornell UniversityV. Kunde, Goddard Space Flight CenterP. D. Lowman, Goddard Space Flight CenterW. Maguire, Goddard Space Flight CenterJ. Pearl, Goddard Space Flight CenterJ. Pirraglia, Goddard Space Flight CenterR. Samuelson, Goddard Space Flight CenterCyril Ponnamperuma, University of MarylandD. Gautier, Meudon, FranceS. Kuman, University of Southern California

Ultraviolet Spectroscopy

A. Lyle Broadfoot, Kitt Peak NationalObservatory, Principal Investigator

J. L. Bertaux, Service d'Aeronomie duCNRS, France

J. Blamont, Service d'Aeronomie du CNRS,France

T. M. Donahue, University of MichiganR. M. Goody, Harvard UniversityA. Dalgarno, Harvard College ObservatoryMichael B. McElroy, Harvard UniversityJ. C. McConnell, York University, CanadaH. W. Moos, Johns Hopkins UniversityM. J. S. Belton, Kitt Peak National

ObservatoryD. F. Strobel, Naval Research LaboratorySushi! Atreya, University of MichiganWilliam R. Sande!, University of Southern

CaliforniaDonald Shemanski, University of Southern

California

Photopolarimetry

Charles W. Hord, Ur iversity of Colorado,Acting Princir: '.ivestigator

195

D. L. Coffeen, Goddard Institute for SpaceStudies

J. E. Hansen, Goddard Institute for SpaceStudies

K. Pang, Science Applications Inc.

Planetary Radio Astronomy

James W. Warwick, University of Colorado,Principal Investigator

Anthony Riddle, Radiophysics, Inc.Jeffrey Pearce, Radiophysics, Inc.J. K. Alexander, Goddard Space Flight

CenterA. Boischot, Observatoire de Paris, FranceW. E. Brown, Jet Propulsion LaboratoryT. D. Carr, University of FloridaSamuel Gulkis, Jet Propulsion LaboratoryF. T. Haddock, University of MichiganC. C. Harvey, Observatoire de Paris, FranceY. LeBlanc, Observatoire de Paris, FranceR. G. Peltzer, University of ColoradoR. J. Phillips, Jet Propulsion LaboratoryD. H. Staelin, Massachusetts Institute of

Technology

Magnetic Field:.

Norman F. Ness, Goddard Space FlightCenter, Principal Investigator

Mario H. Acuna, Goddard Space FlightCenter

K. W. Behannon, Goddard Space FlightCenter

L. F. Burlaga, Goddard Space Flight CenterR. P. Lepping, Goddard Space Flight CenterF. M. Neubauer, Technische Universitat,

F.R.G.

Plasma Science

Herbert S. Bridge, Massachusetts Institute ofTechnology, Principal Investigator

196

J. W. Belcher, Massachusetts Institute ofTechnology

J. H. Binsack, Massachusetts Institute ofTechnology

A. J. Lazarus, Massachusetts Institute ofTechnology

S. CI lbert,1".1assachusetts Institute ofTechnology

V. M. Vasyliunas, Max Planck Institute,F.R.G.

L. F. Burlaga, Goddard Space Flight CenterR. E. Hartle, Goddard Space Flight CenterK. W. Ogilvie, Goddard Space Flight CenterG. L. Siscoe, University of California, Los

AngelesA. J. Hundhausen, High Altitude

Observatory

Low-Energy Charged Particles

S. M. Krimigis, Johns Hopkins University,Principal Investigator

T. P. Arrnctrong, University of KansasW. I. Axford, Max Planck Institute, F.R.G.C. 0. Bostiom, Johns Hopkins UniversityC. Y. Fan, University of ArizonaG. Gloeckler, University of MarylandL. J. Lanzerotti, Bell Telephone Laboratories

Cosmic Ray

R. E. Vogt, California Institute ofTechnology, Principal Investigator

J. R. Jokipii, University of ArizonaE. C. Stone, California Institute of

TechnologyF. B. McDonald, Goddard Space Flight

CenterB. J. Teegarden, Goddard Space Flight

CenterJames H. Trainor, Goddard Space Flight

CenterW. R. Webber, University of New Hampshire

APPENDIX C

VOYAGERMANAGEMENT TEAM

NASA Office of Space Science

Thomas A. Mutch, Associate Administratorfor Space Science

Andrew J. Stofan, Deputy AssociateAdministrator

Adrienne F. Timothy, Assistant AssociateAdministrator

Angelo Guastaferro, Director, PlanetaryDivision

Rodney A. Mills, Program ManagerMilton A. Mitz, Program ScientistWalter Jakobowski, Viking and Flight

Support Manager

NASA Office of Space Tracking andData Systems

William Schneider, Associate Administratorof Space Tracking and Data SystemsAcquisition

Charles A. Taylor, Director, NetworkOperations and Communication Programs

Frederick B. Bryant, Director, NetworkSystem Development Programs

Maurice E. Binkley, Director, USN Systems

NASA Office of Space TransportationSystems

John F. Yardley, Associate Administrator forSpace Transportation Systems

Joseph B. Mahon, Director, ExpendableLaunch Vehicles

Joseph E. McGolrick, Chief, Small andMedium Launch Vehicles

B. C. Lam, Titan III Manager

Jet Propulsion Laboratory,Pasadena, California

Bruce C. Murray, Laboratory DirectorGen. Charles H. Terhune, Jr., Deputy

Laboratory DirectorRobert J. Parks, Assistant Laboratcry

Director for Flight ProjectsRaymond L. Heacock, Project ManagerEsker K. Davis, Deputy Project ManagerPeter T. Lyman, Deputy Project ManagerRichard P. Laeser, Mission DirectorGeorge P. Textor, Deputy Mission DirectorCharles E. Kohlhase Mission Planning Office

ManagerJames E. Long, Science Directorate ManagerCharles H. Stembridge, DeputyArthur L. Lane, Assistant Project Scientist

for JupiterFrancis M. Sturms, Sequence Design and

Integration Directorate ManagerRobert K. Wilson, DeputyMichael J. Sander, Development, Integration

and Test Directorate ManagerRobert G. Potansky, DeputyMichael W. Devirian, Space Flight Operations

Directorate ManagerRaymond J. Amorose, DeputyMarvin R. Traxler, Tracking and Data System

ManagerKurt Heftman, Mission Control and

Computing Center Manager

California Institute of Technology,Pasadena, California

Edward C. Stone, Project Scientist

197

23;

ADDITIONAL READING

TECHNICAL

Jupiter, T. Gehrels, Ed., U. of Arizona Press,Tucson, 1254 pages (1976).

Space Science Reviews, special Voyager in-strumentation issues, Vol. 21, No. 2, pgs.75-232 (November 1977); Vol. 21, No. 3,pgs. 234-376 (December 1977).

"Melting of lo by Tidal Dissipation," by S.J.Peale, P. Cassen, and R.T. Reynolds,Science, Vol. 203, pgs. 892-894 (2 March1979).

Science, special Voyager 1 issue, Vol. 204,pgs. 945-1008 (I June 1979).

Nature, special Voyager 1 issue, Vol. 280,pgs. 725-806 (30 August 1979).

"Jupiter's Ring," by T. Owen et al., Nature,Vol. 781, pgs. 442-446 (11 October 1979).

Science, special Voyager 2 issue, Vol. 206,pgs. 925-996 (23 November 1979).

Geophysical Resewch Letters, special Voyagerissue, Vol. 7, pgs. 1-68 (January 19801.

NONTECHNICAL

"The Solar System," special issue of ScientificAmerican, Vol. 223, No. 3 (September 1975).

"The Galilean Satellites of Jupiter," by D.P.Cruikshank and D. Morrison, ScientificAmerican, Vol. 234, No. 5, pgs. 108-116(May 1976).

Pioneer Odyssey, by R.O. Fimmel,W. Swinde'l, and E. Burgess, NASA SP-396,217 pages (1977).

Murmurs of the Earth: The Voyager InterstellarRecord, by Carl Sagan et al., RandomHouse, New York, 1978.

"Jupiter and Family," by J. Eberhart, ScienceNews, Vol. 115, pgs. 164-173 (17 March1979).

"The Far-Out Worlds of Voyager," by J.K.Beatty, Sky and Telescope, Vol. 57, pgs.423-427 (May 1979) and pgs. 516-520 (June1979).

"Return to Jupiter and Co.," by J. Eberhart,Science News, Vol. 116, pgs. 19-21 (14 July1979).

"Voyage to the Giant Planet," by C. Sutton,New Scientist, Vol. 83, pgs. 217-220 (19 July1979).

"Voyager Views Jupiter's Dazzling Realm,"by R. Gore, National Geographic, Vol. 157,No. I, pgs. 2-29 (January 1980).

"The Galilean Moons of Jupiter," by L.A.Soderblom, Scientific American, Vol. 242,No. 1, pgs. 88-100 (January 1980).

"The Great Red Spot," by D. Schwartzenburg,Astronomy, Vol. 8, pgs. 6- 13 (July 1980).

"Four New Worlds," by D. Morrison, Astron-omy, Vol. 8 (September 1980).

206199