Questions & Answers - NASA - ER : Home · PDF fileQuestions & Answers ... “stars” to be other worlds. ... planets formed was cold enough for ice to solidify out of the gas

Saturn Educator Guide • Cassini Program website — http://www.jpl.nasa.gov/cassini/educatorguide • EG-1999-12-008-JPL

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Questions & Answers

Saturn

1. When did we discover Saturn?

We don’t know exactly when ancient observersfirst recognized what we now call the planetSaturn. Many ancient civilizations were awareof five wandering points of light in the nightsky, which were later named for gods of Romanmythology — Mercury, Venus, Mars, Jupiter,and Saturn. These planets were easily observedwith the unaided eye. It was not until the timeof Galileo, who first looked at the sky through atelescope, that we observed these wandering“stars” to be other worlds.

The planets outside Saturn’s orbit — Uranus,Neptune, and Pluto — were discovered usingmore modern telescopes, beginning with the dis-covery of Uranus by William Herschel in 1781.The discovery of each of these outermost plan-ets generated great public interest at the timeand made the discoverers famous.

2. How did Saturn get its name?

Nearly all ancient human cultures creatednames for, and stories about the Sun, the Moon,the planets and the stars. Like so many things inhistory, the naming of the planets happened byaccident. For example, the ancient Babyloniansrecognized five specks of light moving across thenight skies and believed that these specks werethe images of their most important gods. Notsurprisingly, they named the moving lights afterthese gods. When the Greeks (probably thePythagoreans, in the 5th century B.C.) cameinto contact with the Babylonian sky-lore, theyassigned to the five lights the names of thoseGreek gods corresponding most closely to theappropriate Babylonian deities. The Greeknames of the five planets were: Hermes,Aphrodite, Ares, Zeus, and Kronos. In due

course (perhaps in the 2nd century B.C.), theRomans became acquainted with Greek as-tronomy and the Greek planetary names. TheRomans then rendered the Greek names to fittheir own gods. This is the origin of the namesMercury, Venus, Mars, Jupiter, and Saturn, thenames still in use 2,000 years later.

3. Where is Saturn located?

Saturn is the sixth planet out from the center ofour Solar System. It orbits the Sun at a distanceof about 1.4 billion km (870 million mi). Sat-urn is about 9.6 times as far from the Sun as isEarth. Saturn is almost twice as far from theSun as is Jupiter, the fifth planet in the SolarSystem.

The Sun around which Saturn and the otherplanets orbit is one of hundreds of billions ofstars in the Milky Way galaxy. The nearest starto the Sun, called Alpha Centauri, is more than

Saturn, the fearsome Roman god.


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40 trillion km (25 trillion mi) away. Our SolarSystem is located a little more than half wayfrom the center of the galaxy. The whole SolarSystem (Sun and planets) orbits the center ofthe galaxy once about every 225 million years.The Milky Way galaxy is only one of an esti-mated 100 billion galaxies in the known Uni-verse. Each of these galaxies may have billions ofstars with planets around them — perhaps evensome like Saturn.

4. How old is Saturn?

Astronomers believe that the Sun and the plan-ets of our solar system first formed from a swirl-ing cloud of gas and dust about 4.6 billion yearsago. The Milky Way galaxy is older still, and theUniverse itself is believed to be between 10 and20 billion years old. Astronomers predict thatthe Sun will burn steadily for about another5 billion years or so. Thus, in astronomicalterms, the Sun and the planets orbiting aroundit could be called “middle-aged.”

5. How big is Saturn?

Radius and Volume: Saturn is the second-largestplanet in our solar system; Jupiter is the largest.Saturn measures about 60,000 km (37,200 mi)from its center to its equator, but is only

54,000 km (33,480 mi) from its center to itsnorth or south pole. Its volume is almost 760times that of Earth.

Mass: Saturn’s mass is about 95 times Earth’smass. We know this by observing the orbitalmotions of Saturn’s many moons. UsingNewton’s Law of Gravity and Kepler’s Laws, wecan calculate a mass figure for Saturn thatwould create the gravitational force for themoons to move as they do.

6. If Saturn is so much more massive thanEarth, why is it said that Saturn could floatin water?

Even though Saturn is much more massive thanEarth, this mass is spread throughout a much

The Solar System (not to scale).

JUPITER URANUS

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larger volume than Earth and so Saturn is lessdense than Earth. Saturn is the least dense of allthe planets in the Solar System!

Saturn’s average density is only 0.69 g/cm3,which is less than that of water (1.0 g/cm3).This means that a volume of water equal to thevolume of Saturn would weigh more than Sat-urn does. So if we imagine a titanic tub of waterbig enough to hold Saturn, Saturn’s weightwould be supported by the water and Saturnwould float! Of course, when the plug waspulled and the water drained away, Saturnmight leave... a RING!

8. Could we breathe Saturn’s atmosphere?

No. Saturn’s atmosphere is composed mainly ofhydrogen and helium, the same gases out ofwhich the Sun and most stars are made. Suchan atmosphere is not poisonous to humans, butneither does it provide the oxygen needed tosustain human life.

9. Pictures of Saturn show that it sort offlattens out near the poles and is wider atthe equator. Why is that?

Indeed, Saturn is about 60,000 km (37,200 mi)from center to equator, but only 54,000 km(33,480 mi) from center to pole. The differ-ence is 6000 km (3,720 mi), which is about theradius of Earth! Saturn is thus said to be quiteoblate: the greater the difference between thepolar and equatorial distances, the more oblateis the shape of the planet.

Saturn spins around its polar axis much fasterthan Earth does. Though it is much larger thanEarth, Saturn rotates once in 10 hours 39 min-utes, over twice as fast as Earth rotates. Thisrapid rotation, combined with the planet’s gas-eous composition, forces material outward nearthe equator, just like a spinning merry-go-roundcan push its riders toward the outside. The ob-lateness of Saturn is actually less than would beexpected if it were composed only of hydrogenand helium. This has led astronomers to believethat Saturn may have a massive rocky core.

10. Why is Saturn so much larger and moremassive than Earth?

If we assume that the process of creation of theSolar System involved a spinning disk of gasand dust that slowly condensed to form the Sunand the planets, the question becomes, “Whywas there so much more planet-building stuff inthe orbit of Saturn compared to the orbit ofEarth?” One idea is that at Saturn’s distant or-bit, the cloud of gas and dust from which theplanets formed was cold enough for ice tosolidify out of the gas (instead of only rock).

7. What is Saturn made of?

Like the other Jovian planets (Jupiter, Uranus,and Neptune), Saturn is primarily a ball of gaswith no solid surface. According to studiesbased on Voyager’s spectrometer, Saturn is madeup of about 94% hydrogen, 6% helium, andsmall amounts of methane (CH4)

and ammonia(NH3). The primary components, hydrogenand helium, are the same gases out of which theSun and most stars are made.

Detailed analysis of Saturn’s gravitational fieldleads astronomers to believe that the deepest in-terior of Saturn must consist of a molten rockcore about the same size as Earth, but muchmore massive than Earth. This core is only asmall percentage of Saturn’s total mass.


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This extra ice makes a tremendous “seed” togather in more rock and ice, and to gravitation-ally attract enormously more of the light gaseslike hydrogen and helium to form planets andmoons.

11. Since Saturn does not have a solid sur-face, would I sink to the middle of the planetif I tried to walk there?

Saturn has an outer layer of clouds that we con-sider the “edge” of the planet. At the top ofthese clouds, the atmospheric pressure is thesame as that of air on Earth. Thus to walk therewould be like trying to walk on air. You wouldindeed sink — or fall — through the layers ofSaturn’s interior. As you went deeper throughthe planet’s atmosphere, the pressure would in-crease and eventually you would be crushed.

12. What’s gravity like on Saturn? Would Iweigh the same on Saturn as on Earth?

The gravitational acceleration at the cloud topsof Saturn is similar to that near the surface ofEarth — 10.4 m/sec2 for Saturn, compared to9.8 m/sec2 on the surface of Earth. Thus, itturns out you would feel about the same weightin an airplane flying through the cloudtops ofSaturn as you would feel on the surface ofEarth. If you flew deeper into Saturn’s atmo-sphere, gravity’s pull would increase and youwould feel heavier.

Your weight depends on your mass and on thelocal acceleration of gravity. Your mass is thesame no matter where you are, but the accelera-tion of gravity can be different. Therefore, yourweight depends on where you are in the SolarSystem. For example, your weight would beabout 1/6 as much on the Moon as on Earth be-cause the acceleration of gravity is six times lesson the Moon. Your mass, however, is exactly thesame on the Moon or on Earth.

13. What is the temperature on Saturn?

Astronomers have measured the temperaturenear the cloudtops of Saturn to be about

–143 °C (–225 °F). This temperature increaseswith depth because the gases are compressed todramatically greater pressures at depth. Com-puter models predict that Saturn’s core is as hotas 10,000 °C (18,000 °F).

Saturn is about 10 times as far from the Sun asEarth is, so Saturn receives only about 1/100th(1%) as much sunlight per square meter as doesEarth. Nevertheless, Saturn is warmer thanwould be expected if there were a balance be-tween the solar energy absorbed and the energyemitted. Mysteriously, Saturn emits 80% moreenergy than it absorbs from sunlight. Unlike therocky Earth and the more massive Jupiter, Sat-urn should not have any heat left over from itsoriginal formation. Thus, there must be asource of heat inside Saturn producing the ex-cess energy. One theory is that the energy comesfrom the friction of liquid helium raining downin the interior of the planet. Cassini scientistswill be exploring Saturn’s energy balance for an-swers to this puzzle.

14. Does Saturn have winds and storms?

Yes, but the winds and storms on Saturn arevery different from those on Earth. The Voyagerspacecraft measured a giant jetstream nearSaturn’s equator with a fantastic eastward speedof about 1,800 km/hr (1,100 mi/hr). By con-trast, Earth’s jetstream flows eastward at about400 km/hr (250 mi/hr).

Saturn also has “spots” which are like hurricaneson Earth, except they are longer lived and muchlarger. Saturn’s “spots” may last longer thanEarth’s hurricanes, which lose their source ofenergy when they move over a solid surface. Youmay notice from weather reports that hurricanesgenerally lose their power as they move overcontinents. Jupiter’s Great Red Spot is the mostnotable example of a long-lived hurricane onanother planet.


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15. Since Saturn and Jupiter are both madeup of mostly hydrogen and helium, why isn’tSaturn the same color as Jupiter?

Saturn is a world of white and pastel yellowcloud layers, perhaps somewhat reminiscent ofthe colors in a lemon meringue pie. Jupiter, bycontrast, displays bright yellows, oranges, andreds in exotic swirls and storms.

The colors of Saturn’s cloud layers are due tothe same basic cloud chemistry as on Jupiter.Near the top of the atmosphere, ammonia be-comes cold enough to crystallize into ice par-ticle clouds, like very high cirrus clouds inEarth’s skies. But Saturn is colder than Jupiter,so the colorful ammonia cloud layers on Saturnare deeper in the atmosphere. The hazy atmo-sphere above the cloudtops is much thicker onSaturn, so we do not see the more colorfulchemical layers below as we do on Jupiter. Al-though bands and storms (like Jupiter’s GreatRed Spot) do exist on Saturn, they are harderto see because the colors do not contrast asmuch.

16. Is there life on Saturn?

Probably not. Extreme temperatures, and lackof adequate water and oxygen make it highlyunlikely that life as we know it exists anywherein Saturn’s atmosphere.

17. Does Saturn have a magnetic field likeEarth’s?

Yes. Deep inside Saturn, probably in the deepestlayers of liquid hydrogen and helium, some-thing is causing Saturn to act like a giant mag-net. The same sort of thing happens in the hotliquid iron core of Earth. On Earth, this mag-netism causes compass needles to align withEarth’s magnetic poles. The north-seeking endof a compass needle used on Earth would actu-ally point toward the south pole at Saturn! ThePioneer 11 and Voyager spacecraft discoveredand explored Saturn’s substantial magnetic field.Some of Cassini’s instruments will make a moreextensive exploration of Saturn’s magnetic field.The Hubble Space Telescope has observed auro-ras on Saturn. Auroras are caused when particlesstreaming from the Sun interact with Saturn’smagnetic field.

Voyager image of storms on Saturn.

The Hubble Space Telescope took this ultraviolet image ofSaturn’s aurora.

18. How long is a day on Saturn?

A day on Earth is 24 hours long — the time ittakes Earth to make one complete rotation rela-tive to the Sun. Saturn’s day is 10 hours 39 min-utes — the time it takes Saturn to make onecomplete rotation on its axis. Even though Sat-urn is much larger than Earth, a Saturn day isless than half as long as an Earth day.


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19. How long is a month on Saturn?

A calendar month on Earth is a bit longer thanthe time it takes for the Moon to completely or-bit Earth and go through a full set of moonphases — about 29.5 days. Saturn has manymoons whose times to orbit Saturn vary fromhalf an Earth day to more than an Earth year;therefore, no Saturn month has been formallyestablished. If a Saturn month were to be basedon its largest moon, Titan, which takes about16 Earth days to orbit Saturn, then a Saturnmonth would be 36 Saturn days long.

20. How long is a year on Saturn?

A year is the time it takes for Earth to make onecomplete trip around the Sun, or about 365Earth days. It takes Saturn 29.5 Earth years totravel once around the Sun, so one Saturn yearis about 30 Earth years. If you were 15 Earthyears old, you would only be half a Saturn yearold because Saturn would only have made halfan orbit around the Sun since you were born!(15/30 = 1/2 = 0.5 of a Saturn year old)

21. Does Saturn have seasons like Earth’s?

Yes, sort of. Earth has seasons because of the tiltof its axis. Imagine a line drawn through Earthfrom the North Pole to the South Pole. This linealways points toward the distant star Polaris, nomatter where Earth is in its orbit of the Sun.

The photographs show the changes in Saturn’s appearance from Earth as Saturn moves around the Sun.

This means that during Earth’s trip around theSun each year, sometimes the northern hemi-sphere is tilted toward the Sun, making daytimelonger so that the northern hemisphere receivesbrilliant, direct light that causes warmer tem-peratures (summer). Six months later, whenEarth is on the other side of the Sun, the north-ern hemisphere is tilted away from the Sun,thus receiving less direct sunlight, causinglonger nights and colder temperatures (winter).In both cases, the line between Earth’s polespoints toward the star Polaris.

Saturn has seasons in the sense that there aretimes of its year when the northern hemisphereis tilted toward the Sun, and times of its yearwhen the northern hemisphere is tilted awayfrom the Sun (with great ring viewing fromEarth). The series of photos above shows howSaturn’s appearance changes as viewed fromEarth. But Saturn does not have dramatic sea-sonal differences in temperature in the northernand southern hemispheres as on Earth, nor dothe temperatures cool down during the night onSaturn. Saturn’s internal heat source and theway Saturn’s thick atmosphere retains heat makethe temperatures of Saturn’s atmosphere less de-pendent on where the Sun is presently shiningon it.

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On Earth, a season only lasts about 3 months.But since Saturn takes almost 30 Earth years togo around the Sun, a Saturn season lasts morethan 7 years — about the same amount of timeit will take the Cassini–Huygens spacecraft toget to Saturn!

Some people have the misconception that sea-sons are caused by a planet changing its distancein relation to the Sun. Although both Saturnand Earth change distances to the Sun slightlyduring their orbits, this has a very small effecton their temperatures. Changing the distance tothe Sun does not explain why it’s winter inNorth America while it’s summer in Australia:Earth is slightly closer to the Sun in Januarythan it is in July.

Rings

22. How did we first find out about Saturn’srings?

In 1610, Galileo Galilei observed Saturnthrough a small telescope that magnified objectsabout 30 times. He saw stationary “bulges” oneither side of the planet that looked like

Huygens’ drawing of Saturn, 1683.

A 1616 drawing by Galileo.

“handles” or “ears.” Galileo speculated thatthe bulges were features of the planet or per-haps moons (he called them “lesser stars”). By1616, he could draw more clearly the shapeshe saw around Saturn, but did not knowwhat they were. Galileo died not realizingthat he had been the first human to observeSaturn’s rings.

In 1655, Christiaan Huygens, a Dutch as-tronomer, observed Saturn using a bettertelescope than Galileo’s. Using his observa-tions, and by studying the observations ofothers, he proposed that Saturn has a flat ringaround its equator. Unlike Galileo, he couldsee that the ring did not touch the planet. Inthe same year, Huygens discovered Saturn’slargest moon (named Titan 200 years after itsdiscovery).

In 1676, Jean-Dominique Cassini, an Italian-French astronomer, used an even more pow-erful telescope to discover that the ring thatHuygens saw had a gap in the middle. Theinner ring would later come to be called theB ring, and the outer one the A ring. The gapbetween these brightest rings of Saturn was

Early drawings of Saturn, originally from Systema Saturnium byHuygens, 1659.

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named the Cassini Division. Cassini discoveredfour moons of Saturn in addition to the onethat Huygens found: Iapetus (in 1671), Rhea (in1672), and Tethys and Dione (in 1684). Themoons were named later. No one yet knew whatthe rings were made of, how thick they were, orif they were permanent features around Saturn.

23. What are the rings of Saturn made of?Are they solid?

Saturn’s rings are made up of millions and mil-lions of orbiting particles interacting with each

This artist’s concept represents a thinly populated locale in Saturn’s rings. The rendering shows rings viewed from just above the ringplane. The largest ring particles shown here are house-sized. The large bodies are irregularly shaped and lie roughly in a flat layer;smaller particles are scattered among them.

other and with Saturn’s moons. These particlesare composed mostly of water ice and somerocky material. The icy particles’ sizes rangefrom that of specks of dust to “ringbergs” thesize of houses.

We first began to learn about the nature of therings when in 1857 James Clerk Maxwell, aScottish scientist who developed the theory ofelectromagnetism (which predicted the speed oflight), proved mathematically that the rings en-circling Saturn could not be a single, solid disk.

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He proposed that the rings are instead made upof many small particles. Observations by Ameri-can astronomer James Keeler in 1895 provedthe truth of his prediction.

24. How many rings are there?

Saturn’s ring system has been divided into sevenmain ring groupings. Astronomers have namedthese main ring groupings the A, B, C, D, E, F,and G rings. (See the illustration on page 234.)The rings are not located in alphabetical orderbecause they were named in the order of theirdiscovery. From inner to outer, the rings arenamed D, C, B, A, F, G, and E. One possibleway of remembering this is: Daring Cassini Be-gins A Far Greater Exploration.

The two brightest rings easily seen through tele-scopes from Earth are the A and B rings, whichare separated by the Cassini Division. The Bring is closer to the planet than the A ring. Thefaint C ring inside the B ring is barely visiblefrom Earth. The D ring is closest to the planet

A Voyager image showing the clumped and braided F ring.

and has only been seen clearly by Voyager as thespacecraft was looking back as it left the Saturnsystem.

Saturn also has three other rings. Just outsidethe A ring is the narrow F ring, discovered bythe Pioneer 11 spacecraft when it flew by Sat-urn in 1979. This mysterious ring sometimescontains clumps or “braids.” The clumps move,and come and go with time. Next is the faint Gring. This ring isn’t very bright and it is difficultto see, even from Saturn. In fact, the G ring isso thin that Voyager 2 actually flew through theedge of the ring without damaging the space-craft! Finally, there is a broad, faint ring calledthe E ring. The moon called Enceladus mayhave ice volcanoes or geysers that supply the Ering with tiny ice particles. The Cassini missionhopes to find out if this is true!

Depending on how you define an individualring, Saturn’s rings number in the thousands.Voyager observed that the main rings (A, B, C)


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CASSINI DIVISION ENCKE GAP

D RING

B RING

F RING

C RING

A RING

G RING E RING

APPROXIMATE DISTANCE BETWEEN EARTH AND ITS MOON

EARTH MOON

Saturn and its ring system shown to scale.

Note: Saturn’s

rings consist of

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Voyager close-up image of Saturn’s rings.

are composed of ringlets so numerous that thering system looks much like a phonographrecord, with thousands of thin grooves in it.

25. Do the rings move?

Yes, but not like a solid would move. The bitsand boulders of Saturn’s rings each orbit liketiny moons. They move around Saturn in thesame direction as Saturn rotates and the samedirection as all the known moons (except theoutermost moon, Phoebe, which orbits in theopposite direction). The ring particles closest toSaturn whiz around the fastest, and those far-ther away travel around at a slower speed (inkeeping with Kepler’s Laws).

The orbital speed of a ring particle dependsonly on its distance from Saturn’s center; it doesnot depend on the particle’s mass or size. Forexample, if a house-sized ring particle and asugar grain–sized ring particle are both orbitingat the inner edge of the B ring, they move at thesame speed around Saturn. However, ring par-ticles at the outer edge of the B ring orbit at aslower speed. The particles of the innermostrings are moving around Saturn faster than Sat-urn rotates.

Ring particles can also move randomly in otherdirections, such as perpendicular to the rings.These motions are usually damped out quicklyby collisions or gravitational interaction be-tween the particles. If you wait a much longertime (millions of years), some of the rings mayspread out and eventually disappear.

26. In the opening sequence of the TV showStar Trek: Voyager, a ship passes throughthe rings of Saturn. Do the rings containmore empty space or more solid particles?

Most of the volume of the rings is empty space.Each of the main rings (A, B, and C) containdifferent densities of particles. The B ring is thedensest, followed by the A ring, and then the Cring. The A and B rings are separated by a gap

that is 4,600 km (2,900 mi) wide, called theCassini Division, but close-up images by Voy-ager showed that the Cassini Division containsmany more ring particles than expected. Thereis a region in the Cassini Division near theouter edge of the B ring that is relatively free ofring particles. Near the outer edge of the A ringthere are two gaps almost empty of material: the330 km (210 mi) Encke Gap, and the 35 km(22 mi) Keeler Gap. The best places to flythrough the rings might be in gaps such asthese.

The rings have more empty space than ringparticles, even in the brightest B ring, whichcontains most of the mass of Saturn’s rings(2.8 × 1019 kg). But this doesn’t mean youcould fly through from top to bottom withouthitting any of the particles. Even if you had atiny spacecraft a few centimeters across, itwould be unlikely that it could pass through theA or B rings without being hit by ring particles.The particle impacts would not be gentlenudges, but chunks hitting the spacecraft atrelative speeds “faster than a speeding bullet”! Atthis speed, even a millimeter-sized particlemight be enough to damage Cassini, but the


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spacecraft will not be flying in regions of therings where many particles like these exist. Theprobability of Cassini being hit by such a par-ticle is less than 1%. When Voyager 2 flewthrough the edge of the G ring in 1981, instru-ments recorded hits by tiny dust particles lessthan 1 mm in size. Voyager made it throughwithout any damage.

27. How big are the rings?

Saturn’s ring system is both very broad and verythin. The inner edge of the rings begins ap-proximately 6,700 km (4,200 mi) from thecloudtops of Saturn. The outer edge of the Aring is at approximately 76,500 km (47,600 mi)from the cloudtops of Saturn. The outer edge ofthe outermost ring (the E ring) has been mea-sured out to about 420,000 km (260,000 mi)from Saturn’s cloudtops. This distance is greaterthan that between Earth and the Moon —384,000 km (242,000 mi). It is likely that therings extend even farther from Saturn in anever-diminishing zone of fine, icy dust.

In terms of thickness from top to bottom, thebright A and B rings may be as thin as a hun-dred meters — the length of a football field. Atits outer edge, the E ring increases in thicknessto several thousand kilometers.

28. How much stuff is in the rings?

If all the material in Saturn’s rings were col-lected together, it would form a moon aboutthe size of Saturn’s moon Mimas, which is392 km (244 mi) across. This is about 9 timessmaller than the diameter of Earth’s Moon.Scientists estimate the mass of Mimas to be4 × 1019 kg, or about 1,800 times less massivethan Earth’s Moon.

The majority of the mass of the rings is in par-ticles from a few centimeters to a few meters insize — neither very large nor very small. This isbecause there are very few really big particles inthe rings, and the mass of the dust is very small,so the biggest and smallest particles do not con-tribute very much to the overall mass.

There is far less mass in Saturn’s rings than isfound in the asteroid belt between the orbits ofMars and Jupiter. Furthermore, the asteroids aremade of rock and iron, substances which aremany times denser than the water ice out ofwhich Saturn’s rings are made.

29. Do ring particles collide?

Yes, there are several types of forces that can al-ter a ring particle’s normal orbital path, causingit to collide with neighboring particles. Saturn’srings represent an ever-changing, dynamic sys-

APPROXIMATE DISTANCE

EARTH MOONNote: Saturn’s

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tem of particles which evolves with time. Un-derstanding that change is one of Cassini’s goalsas it studies the rings.

Saturn’s moons are important gravitational in-fluences that can cause ring particles to moveout of their normal circular orbits. The orbit ofa ring particle becomes circular once againthrough collisions with other ring particles, andthe ring particle may end up in a slightly differ-ent orbit from the one it was in before it got a“kick” from the moon. These “kicks” occur atspecific locations in the rings and can actuallycause large “waves” or “knots” to form in therings.

Saturn’s magnetic field can also cause the small-est ring particles to alter their orbits, causingcollisions with other ring particles. The smallestparticles are easiest to electrically charge, andonce charged, they can be lifted above the ringsby Saturn’s magnetic field. This is one of themost popular theories for what causes the mys-terious B ring “spokes” that were discovered bythe Voyager spacecraft.

30. Why does Saturn have rings? How werethe rings made?

The rings probably formed primarily becauseone or more small moons broke up close to Sat-urn. This breakup could have been the result of

A Voyager image of “spokes” in Saturn’s rings.

a collision with a comet or asteroid, or with an-other moon in close orbit around Saturn. Overmillions of years, the moon bits and cometaryice chunks spread out into the complex ring sys-tem that exists today.

Ring formation can also start or be sped up if alarge enough moon, asteroid, or comet comesclose enough to Saturn that the gravitationalpull of Saturn gently breaks it apart. This canhappen when the difference between gravity’spull on the side facing Saturn and its pull onthe side facing away from Saturn is enough totear the object apart. After it is pulled apart, thesmaller particles will begin to collide with eachother and with preexisting ring particles. Thisslowly grinds them down in size and spreadsthem out, gradually adding to Saturn’s rings.

31. How old are the rings? Has Saturn alwayshad rings? Will it always have rings?

Using data collected by NASA’s Pioneer andVoyager spacecraft, scientists have estimatedthat Saturn’s rings are “only” 100 million yearsold, a small fraction of the 4.6 billion years theSolar System has existed. It is possible that therings come and go. They might be constantlyrenewed and reformed over time by internalcollisions and by the addition of comets or as-teroids that are captured as they pass close toSaturn. If we were to come back and visit Sat-urn in a billion years, the faint rings may begone completely, and an entirely new ring sys-tem formed in their place.

The E ring may be younger than the A and Brings. Scientists have speculated about how ma-terial in the E ring might come from small dustor ice particles knocked loose from Saturn’s icymoons. It is also possible that Enceladus mayhave water volcanoes or geysers constantly feed-ing the E ring with new dust-sized ice particles.Enceladus’ bright young surface and apparentepisodes of surface melting are evidence of heatsources that could also generate volcanism.


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The Cassini spacecraft will learn more about theunusual ring system of Saturn. It is now appar-ent that Saturn’s rings are complex, dynamic,and constantly evolving things.

32. Are there other planets with rings?

Yes! The Voyager spacecraft saw rings at Jupiter,Uranus, and Neptune. However, the rings ofthese planets are all much fainter than Saturn’sand have only recently been discovered. In1977, observers using a telescope aboard an air-plane flying high over the South Atlantic Oceandiscovered a series of narrow rings around Ura-nus. These rings appear to be made of very darkparticles, unlike the bright, icy particles ofSaturn’s rings. In 1979, Voyager 1 discovered afaint, dusty ring around Jupiter. Neptune’s ringsystem was first detected in 1984, when as-tronomers on the ground noticed that the lightfrom a distant star dimmed slightly as the Nep-tune system moved in front of it. This suggesteda ring system or unknown moons orbiting theplanet. Voyager 2 later flew by Uranus and Nep-tune and returned stunning images of theseplanets’ rings.

Although all the giant gaseous planets are nowknown to have rings, none has a ring system

that compares with the size and grandeur ofSaturn’s. Saturn really is “Lord of the Rings”!Some scientists believe that Mars may have avery faint ring system associated with its tiny,dusty moons Phobos and Deimos. A futurespacecraft mission to Mars might be able to de-tect these rings, if they exist.

33. Why doesn’t Earth have rings?

The small inner planets may have had rings inthe past and may have rings again in the future.One prevalent theory says that ring systems aremuch younger than the age of the Solar System,and as such, may come and go with time. IfEarth had rings in the past — for example,from the breakup of a comet or asteroid thatcame too close — then those rings may havespread out and disappeared long ago. Earthmight have a ring in the future if a comet or as-teroid passes Earth in just the right orientationto be broken apart by Earth’s gravity rather thandisintegrating in Earth’s atmosphere.

Our Moon is too distant to provide material fora future Earth ring; however, the Moon mayhave formed from a huge, very short-lived ringof material encircling Earth. Such a ring wouldhave been formed if a huge body — the size of

A Voyager image of Saturn casting its shadow across the rings.


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Mars or larger — hit Earth, spewing hugeamounts of debris into orbit around our planet.In a very short time, this debris would gathertogether to form the Moon.

34. If Earth had rings like Saturn’s, whatwould they look like from the ground?

If you were to scale down Saturn and its ringsystem so that Saturn was the size of Earth, theouter edge of the A ring would stretch about6,400 km (4,000 mi) from Earth’s surface. Therings would look quite different in the sky de-pending on the latitude at which you werestanding, the time of year, and the time of day.

If you were standing on Earth’s equator, youwould have to look directly overhead to see therings. The rings would only appear as a thinline running across the sky from east to west,because the rings would orbit nearly overEarth’s equator. If you were standing at a higherlatitude in the northern hemisphere, you wouldhave to look to the south to see Earth’s rings.The rings would form an arching band acrossmuch of the sky. If you were standing all theway at the North Pole, you would not be ableto see any rings at all, because they would bebelow your southern horizon.

If you were standing about halfway between theequator and the North Pole, the rings wouldappear different depending on what time ofyear you looked. In the summer, Earth’s tiltwould orient the rings toward the Sun and youwould see sunlight reflected directly off the topside of the rings. But during the winter, theSun would be shining more directly on Earth’ssouthern hemisphere, and thus the rings wouldbe lit up from the bottom side! In that case,you would see some light pass through therings, but they wouldn’t appear as brilliantly litas during summer.

You would also have to be sure to look at therings at a time of day when the Sun was shiningon them. Noontime would be best. Earlier orlater in the day, the east or west edges of therings might be blocked by Earth’s shadow fallingon them. If you looked at midnight, the ringswould be mostly blacked out from being inEarth’s shadow. Many Voyager pictures showSaturn’s shadow on the rings.

So, if Earth had rings like Saturn’s, they wouldappear exceptionally beautiful on the summersolstice from midlatitudes at noon.

Moons

35. How many moons does Saturn have?

At least 18 moons orbit Saturn. Until recently,Saturn was the moon champion, recognized ashaving more than any other planet in the SolarSystem. However, astronomers have announceddiscoveries of additional moons around Uranus,bringing that planet’s total to 20 if the observa-tions are confirmed. There are probably manyadditional small moons amidst Saturn’s rings,and the Cassini mission may find some of them.

Saturn’s 18 moons are listed in the table onpage 240, along with their distances from thecenter of Saturn, orbital periods (time to go oncearound Saturn), sizes, and notable features. Thesmallest moons (Pan, Atlas, Prometheus,Pandora, Epimetheus, Janus, Telesto, Calypso,Helene, Hyperion, Phoebe) have irregularshapes. For these moons, an average “radius” hasbeen calculated by averaging measurements oftheir sizes in three dimensions. The larger moons(Titan, Rhea, Iapetus, Dione, Tethys, Enceladus,Mimas) are very close to being spheres.

Just like Earth’s Moon, most of Saturn’s moonsrotate at the same rate as they revolve aroundSaturn, and thus keep the same face toward Sat-urn. This also means the moons always have one


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Saturn’s Moons

Distancefrom Center Period of Averageof Parent Orbit Around Radius

Name (km × 1,000) Parent (hours) (km) Special Features or Behavior

Pan 133.6 13.85 10 Orbits in Encke Gap, sweeping it clean.

Atlas 137.6 14.42 16 May keep outer edge of A ring well-defined.

Prometheus 139.4 14.71 53 Shepherd moon; helps keep the F ringnarrow.

Pandora 141.7 15.07 43 Shepherd moon; helps keep the F ringnarrow.

Epimetheus 151.4 16.68* 60 Irregular; may have been joined withJanus.

Janus 151.5 16.68* 90 Irregular; trades orbits with Epimetheus.

Mimas 185.5 22.61 199 Has giant crater, Herschel; looks like“Death Star” battle station from movie.

Enceladus 238.0 32.88 249 Icy, shiny; may have ice geysers that feedthe E ring.

Tethys 294.7 45.31 530 Has large trench, Ithaca Chasma; alsoa large crater, Odysseus.

Telesto 294.7 45.31 12 Co-orbital with Tethys, 60º behind.

Calypso 294.7 45.31 10 Co-orbital with Tethys, 60º ahead.

Dione 377.4 65.69 560 Cratered leading face; wispy features ontrailing hemisphere.

Helene 377.4 65.69 18 Co-orbital with Dione, 60º ahead.

Rhea 527.0 108.42 764 Largest icy satellite; densely cratered.

Titan 1,221.9 382.69 2,575 Largest moon of Saturn; second-largestmoon in Solar System; only moon witha dense atmosphere.

Hyperion 1,481.1 510.64 144 Irregular, dark surface; chaotic tumblingorbit.

Iapetus 3,561.3 1,903.92 718 Much darker leading hemisphere; lightertrailing hemisphere.

Phoebe 12,952 13,211 110 Retrograde orbit; may be capturedasteroid.

Earth’s Moon

The Moon 384.5 665.73 1,738 Rocky, cratered, mountainous; prominentflat, dark areas called maria on Earth-facing side (probably lava flows follow-ing huge, ancient cratering events).

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side that faces toward the direction of their mo-tion around Saturn, and one side that facesaway from their direction of motion. These arecalled the leading and trailing hemispheres.

36. Who discovered all these moons?

In 1655, Christiaan Huygens, a Dutch astrono-mer, observed Saturn through a telescope anddiscovered Saturn’s largest moon, Titan, al-though it wouldn’t be named this for another200 years. Titan’s diameter is about 5,150 km(3,200 mi), or 1.5 times as large as than thediameter of Earth’s Moon — 3,500 km (2,200mi). Titan is the second-largest moon in theSolar System — Jupiter’s Ganymede is the larg-est. Both Titan and Ganymede are larger thanthe planets Mercury and Pluto. Titan andGanymede are defined as moons because theyorbit planets, while Mercury and Pluto are de-fined as planets because they orbit the Sun.

Later in the 1600s, the Italian–French astrono-mer Jean-Dominique Cassini discovered fourmore moons of Saturn: Iapetus (1671), Rhea(1672), and Tethys and Dione (1684). Not sur-prisingly, these moons were Saturn’s next larg-est, with diameters ranging from 1,400 km(870 mi) for Tethys to 1,500 km (960 mi) forRhea. Cassini noted that when Iapetus was onone side of Saturn, it could be easily seen; how-ever, when it was on the other side of its orbit,it was invisible. He correctly deduced thatIapetus was keeping the same side always to-ward Saturn, and that one side of the moon (itsleading hemisphere) was much darker than theother side (its trailing hemisphere).

In 1789, William Herschel in England discov-ered moons of Saturn that would later benamed Mimas and Enceladus. These moonshave even smaller diameters: about 500 km(310 mi) for Enceladus and 400 km (250 mi)for Mimas.

Hubble Space Telescope images of the 1995 ring-plane crossing.

In 1848, astronomers William Bond andGeorge Bond (father and son) of Harvard Col-lege discovered Hyperion, with a diameter of290 km (180 mi). The very same night, Will-iam Lassell of England also discovered it withhis telescope. In 1898, William Pickering, alsoof Harvard, discovered Phoebe, with a diameterof 220 km (140 mi). Phoebe was the first moondiscovered using photography, rather than bylooking directly through a telescope’s eyepiece.

The innermost four moons (Pan, Atlas,Prometheus, and Pandora), which are inter-twined with Saturn’s A and F rings, were notdiscovered until Voyager 1 flew past Saturn in1980. Pan, in fact, eluded discovery until evenafter Voyager. It was not until 1991 that as-tronomer Mark Showalter searched throughVoyager images of the narrow, clear Encke Gapin Saturn’s A ring and found Pan.

The rest of Saturn’s currently known moonswere discovered by observers on Earth duringthe 1966 and 1980 ring-plane crossings, whenSaturn’s thin rings were seen edge-on fromEarth. With the rings temporarily not visiblefrom Earth, faint objects near the planet areeasier to see. During the 1966 ring-plane cross-ing, Audoin Dollfus discovered Janus, and JohnFountain and Steve Larson discovered its com-panion, Epimetheus. Telesto, Calypso, andHelene were discovered by three different


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groups of astronomers during the 1980 ring-plane crossing.

Ring-plane crossings occur about every15 years. Like Earth’s North Pole, Saturn’snorth pole is tilted with respect to the plane ofits orbit, and this causes our view of the ringsto change as Saturn travels in its 30-year orbitaround the Sun. Ring-plane crossings occurnear Saturn’s equinoxes when the planet’s tilt isneither toward nor away from the Sun.

In 1995, astronomers using the Hubble SpaceTelescope announced that they had discoveredtwo — perhaps even four — previously un-known moons. Amanda Bosh and AndrewRivkin spotted what appeared to be new moonsof Saturn in photographs they made during thering-plane crossing. Two of the “newly discov-ered moons” in the photos turned out to be thepreviously known moons Atlas and Prometheus,but they were at different positions than pre-dicted by previous estimates of their orbits.

Careful analysis of the remaining two moonsshowed that they were at the distance of theF ring, and appeared to change shape as they or-bited Saturn. These objects are now believednot to be moons, but rather “clumps” of ringmaterial within the F ring. Bosh was not disap-

The gravity of Prometheus and Pandora keeps the F ringparticles confined to a very narrow ring.

pointed that the “moons” turned out not to bemoons after all. Astronomers were still excited,because this was the first time the F ring clumpshad been seen from Earth.

37. How did the moons get their names?

In the mid-1800s, English astronomer JohnHerschel (son of William Herschel, who haddiscovered two moons of Saturn) wrote that itwould not be right to name the moons of Sat-urn after Saturn’s children. In Roman mythol-ogy, Saturn ate all his children. Instead, saidHerschel, Saturn’s moons should be named afterSaturn’s brothers, the Titans, and Saturn’s sis-ters, the Titanesses. These were mythologicalgiants who were believed to rule in the heavensbefore Jupiter conquered them. Astronomersaccepted Herschel’s suggestion for naming themoons. Because the moon discovered byHuygens was so much larger than the rest, theychose to name it Titan rather than naming it af-ter one of the giants. For more information, seeDiscussion 4 — Mythology of Saturn, page 219.

38. Are Saturn’s moons like Earth’s Moon?

Yes and no. Many of them are covered with im-pact craters like our Moon, but the moons ofSaturn are made up of much more water icethan Earth’s moon. Earth’s Moon may have verytiny patches of ice, but almost all its mass isrock. Because of this, most of Saturn’s moonsare about one-third the density of our Moon.Titan’s density is a little higher, but still onlyabout half as dense as Earth’s Moon. Saturn’soutermost moon, Phoebe, may be a captured as-teroid, in which case it would likely have amuch higher density.

Titan is the only one of Saturn’s moons that islarger and more massive than our Moon. All therest are significantly smaller and less massive,and many of them are irregularly shaped ratherthan spherical. (See the table on page 240 forcomparisons.) Also, unlike our Moon, and un-


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like all of the other 60 or so moons in the SolarSystem, Titan has a thick atmosphere.

39. Why does Saturn have so many moons,but Earth has only one?

Here again, astronomers can make some edu-cated guesses. There is more planet-building“stuff ” at Saturn’s orbital distance than at Earth’sorbital distance. This is because Saturn’s orbit isso far from the Sun that ice becomes a substan-tial source of planet-building material. Themore planet-building material, the more mate-rial for forming moons around the planet. Manyof Saturn’s moons and rings are composedlargely of ice. Saturn may also have moons thatare captured asteroids. This is the most likelyorigin for Saturn’s outermost moon, Phoebe,and the Cassini spacecraft will make an investi-gation of this on its way into the Saturn system.

Earth’s Moon is unusually large relative to thesize of its parent planet. The diameter of Earthis less than 4 times larger than the Moon’s diam-eter. By contrast, Saturn’s diameter is nearly 25times greater than Titan’s diameter. Thus, Earth’sMoon is far too large, compared with the size ofEarth, to have been formed as an original moonfrom the spinning disk of gas and dust thatformed the planet. The present Moon was mostlikely formed as a result of a tremendous colli-sion between Earth and a huge asteroid the sizeof Mars or larger, which broke apart Earth andcreated the Moon. This impact may have actu-ally created multiple moons around Earth,which later collided with each other or Earth,and now we are left with one large moon.

40. Are Saturn’s moons in the rings? Do themoons collide with the ring particles?

One small moon has been found orbiting withinthe main rings (A, B, C). This moon, namedPan, orbits in the Encke Gap, near the outeredge of the A ring. Pan sweeps the gap clear ofthe smaller ring particles, thereby maintainingthe gap. If Pan disappeared, so would the Encke

Gap. Scientists suspect that other moons arelurking in the Saturn system, creating some ofthe other gaps in the main rings. Cassini mayfind these moons during its mission.

Collisions between ring particles occur fre-quently in the main rings, and ring particlescan easily be knocked into a new orbit by thesecollisions. If a moon collided with a largeenough ring particle, the moon could be frac-tured or lost within existing ring material. In ei-ther case, it might no longer be identifiable as aseparate moon.

Saturn’s E, F, and G rings orbit outside themain rings. The outermost ring — the E ring— is the most extended. Enceladus movesthrough the E ring, and it may have ice volca-noes that are responsible for producing thering’s tiny particles of ice. The G ring is so thinthat it would probably disappear quickly if itdid not have several small moons orbitingwithin it and producing particles. No one hasyet seen these probable G ring moons — per-haps the Cassini spacecraft will!

41. What’s the difference between a moonand a ring particle?

The rings are nothing more than a dense swarmof tiny, interacting moons. In principle, youcould find an orbit for every ring particlearound Saturn if the particles did not interactwith one another and change orbits slightly.Different kinds of forces act on ring particles ofall sizes and modify their orbits. If you cannottrack an object and predict its orbital path, youmight call it a ring particle rather than a moon.

The orbits of the largest particles in the ringsprobably change the least. However, we do nothave much data on this question since the larg-est “ring particle” Voyager imaged was Pan, andwe don’t know how Pan’s orbit may be chang-ing with time. Cassini will provide a wealth ofnew data on Pan and will probably discover


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new moons embedded in the rings. You mightask: At what size something is no longer a moonbut is just a “ring particle”? There’s no clear dis-tinction. Suppose the ring particles sometimesstick together in larger collections of many par-ticles, and sometimes break apart via collisions.In this case, do some “moons” come and go?

There really is no sharp cutoff between a moonand a ring particle (or “ringberg”). The smallerthe moon, the harder it is for it to maintainan empty gap around Saturn. We think thatsmaller “moons” might clear small areas that arethen filled in with ring particles after the“moon” has passed by. However, maintaining agap depends in part on the density of ring par-ticles in the region in which the moon orbits.Denser regions like the A or B rings would re-quire a larger moon to maintain a gap than amuch more diffuse region such as the C ring.Hence, defining a moon as an object that main-tains a gap in the rings would produce differingcutoffs in moon sizes for each ring region.

It will be interesting to see just what definitionsevolve once Cassini begins making its closer ex-amination of Saturn’s rings!

42. What’s gravity like on Saturn’s moons?Could we walk there?

Titan’s gravity is a bit less than that of Earth’sMoon, which has 1/6 the surface gravity ofEarth. Someone who weighs 110 pounds onEarth would weigh only 20 pounds on theMoon, and 15 pounds on Titan. Just think howeasy it would be to jump over a 6-foot highfence! The person’s mass, 50 kg, would be thesame on both Earth, the Moon, and Titan.

To compute the surface gravity for a moon, youneed to know the moon’s size and mass. For sev-eral moons, we only have guesses for these num-bers. Many moons are oddly shaped, sodepending on where you stood, you wouldweigh a different amount.

44. How cold are Saturn’s moons?

Saturn’s moons and rings are even colder thanSaturn, with surface temperatures ranging from–145 °C to –220 °C (–230 °F to –365 °F). Thebrightest moons are the coldest, because theyreflect almost all the Sun’s light, rather than ab-sorbing it.

45. Do any of Saturn’s moons have anatmosphere? Could we breathe it?

Among Saturn’s moons, only Titan has a thickatmosphere. Titan’s atmosphere is mainly nitro-gen, like Earth’s, but it does not have enoughoxygen for humans to breathe. Gerard Kuiper[KOY-per], a Dutch-born American astrono-mer, first discovered Titan’s atmosphere in 1944using a spectrometer that detected infrared light(heat). This instrument was attached to a tele-scope with an 82-inch mirror. Many gases arehard to detect using visible light, but mucheasier to detect using infrared. Kuiper detected

43. Are there volcanoes on any of Saturn’smoons?

Although the evidence is circumstantial, it ispossible that Saturn’s moon Enceladus has watervolcanoes or geysers, active today or in the re-cent past. Cassini plans close flybys ofEnceladus to search for direct evidence of suchvolcanoes. Some scientists believe that volcanoeson Enceladus are the source of particles inSaturn’s E ring. Because Titan is so large, it ispossible it may have a warm, active core thatalso causes volcanoes on its surface. TheHuygens probe will help us see if any volcanoesexist on Titan.

Artist’s rendition of ice geysers on Enceladus.


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the presence of methane gas. Before Voyager 1’sflyby of Titan in 1980, only methane and a fewother simple chemicals called hydrocarbons hadbeen detected on Titan.

Observations from the Voyager spacecraft usingradio waves and infrared light indicated thatTitan’s deep atmosphere was composed mostlyof nitrogen — at least 90%, compared to theEarth’s 79%. Most of the remainder of Titan’satmosphere is methane. On Earth, methane isfound bubbling out of marshes or swamps. Voy-ager 1 also determined that Titan’s atmosphere isnearly 10 times as deep as Earth’s. However, be-cause Titan’s gravity is weaker, the atmosphericpressure on Titan is only about 50% higher thanon Earth.

Some of Saturn’s other moons may have ex-tremely thin atmospheres, but these have not yetbeen detected.

46. Is there water on Titan?

If there is water on Titan, it is probably fro-zen solid at the bottom of lakes or oceans ofliquid hydrocarbons like ethane and meth-ane. However, like most of Saturn’s othermoons, much of Titan’s interior is probablywater ice.

47. Is there life on Titan?

With detectable organic compounds likemethane in the atmosphere, it is very naturalto wonder whether life exists there now, ex-isted there in the past, or might yet existthere in the future. Few people believe thatlife as we know it currently exists on Titan,because of the extreme cold and the lack ofoxygen and liquid water. However, the envi-ronment is in some ways similar to that ofthe early Earth, and it is possible that Titancould teach us something about how life be-gan on Earth.

Comparison of atmospheric cross-sections of Earth and Titan. Note the difference in vertical scale — while both Earthand Titan have atmospheres composed primarily of nitrogen and the surface pressures are similar, the atmosphere ofTitan is much more extended because of its low gravity.

NITROGENOXYGENARGON

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NITROGENMETHANEARGON (?)

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THIN HAZE LAYER

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METHANE

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48. What is the weather like on Titan?

We know it is very cold on Titan, and that theatmospheric pressure at the surface is 1.5 timesthat of Earth, but we are not at all certainabout the motions (winds and storms) inTitan’s atmosphere. Titan turns very slowly, soa day on Titan is almost 16 Earth days long.There are 192 hours of dim sunlight followedby 192 hours of darkness. Temperatures prob-ably do not change much from day to night.Titan is nearly 10 times as far from the Sun asEarth is, and temperatures there hover around -180 °C (–292 °F)!

Several of the experiments on the Huygensprobe, which will descend through Titan’s at-mosphere during the Cassini mission, are de-signed to detect various aspects of Titan’sweather, such as temperature, pressure, andwind speed.

49. Cassini carries a probe that is going toTitan and not Saturn or any of the othermoons. Why Titan?

Saturn’s atmosphere is mostly hydrogen andhelium. While this is interesting in its ownright, Titan, with its nitrogen atmosphere andmysterious surface features, is extraordinarilyintriguing for several reasons. Titan and theEarth are the only two bodies in the Solar Sys-tem with thick nitrogen atmospheres. Titanis the only body where cold, exotic lakes ofethane and methane are believed to exist. Liq-uid ethane exists between the temperatures of–183 °C (–297 °F), where it freezes, and–89 °C (–128 °F), where it boils. Some scien-tists believe Titan’s environment is similar tothat of Earth before life began on our planet.

Titan is a unique place, and a great way to ex-plore it is to visit it! In previous flyby missions,Pioneer 11 visited the Saturn system in 1979,Voyager 1 visited in 1980, and Voyager 2 vis-ited in 1981, but none of these spacecraftcould see through the haze in Titan’s atmo-

sphere to determine what the surface lookedlike. We now anticipate what the Cassini mis-sion might be able to do with the cameras andinstruments aboard the Huygens probe. Wewonder what Titan’s landscape will be like. Willit have mountains of ice? Mysterious lakes? Or-ganic goo covering its surface?

50. Will there be a mission that takes hu-mans to Titan in the near future?

None are currently planned. Because of Saturn’sgreat distance from Earth, we would be morelikely to send humans to nearer destinations,such as the Moon, Mars, or an asteroid, beforesending humans to Titan.

Observing Saturn in the Sky

51. Can I see Saturn in the sky at night?

Yes! Saturn normally appears as an unflickeringyellowish point of light about as bright orbrighter than stars in the Big Dipper. FromEarth’s northern hemisphere, Saturn appears tothe south, slowly moving along the same arc inthe sky as do the Moon, the Sun, and the rest ofthe planets. There are always times of the yearwhen Saturn is not visible in the night sky. Thisis because Earth is on the other side of the Sunfrom Saturn, and so Saturn is in the sky duringthe daytime. During the cruise phase of theCassini mission (1997–2004), Saturn will be

Artist’s concept of the Huygens probe landing on Titan.


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best visible in the sky during the winter. Youcan use astronomy magazines and many WorldWide Web sites to find information about lo-cating Saturn in the night sky. See the Appen-dices for information on resources.

In Appendix 3, there is a table of informationabout where Saturn appears in the night skyover the course of the Cassini mission (1997–2008). At Cassini’s launch date of 15 October1997, Saturn, Venus, and Jupiter were easilyvisible in the night sky, but when Cassini ar-rives at Saturn on 1 July 2004, Saturn will notbe visible in the night sky. On its way to Sat-urn, Cassini does two flybys of Venus, one ofEarth, and one of Jupiter. (These flybys providegravity assists to help the spacecraft reach Sat-urn by July 2004.) Venus was easy to see in thenight sky during the Cassini flybys, and Jupiterwill also be visible in the sky when Cassinipasses by in December 2000. After the Sun andMoon, these two planets are the next-brightestobjects in the sky.

52. Can I see Saturn’s rings from Earth?

You cannot see the rings with the unaided eye,but the rings are easily visible if you peerthrough a telescope with a magnification of 30times or more. Such a telescope typically uses amirror or lens several inches across to focus thelight from Saturn. You can purchase a qualitytelescope for several hundred dollars, or buildyour own from kits available through catalogs.(Beware of cheap telescopes.) A larger telescopeand a more powerful eyepiece would enableyou to view more detail, such as some of thelarger moons (which appear as small points oflight near Saturn), the Cassini Division be-tween the A and B rings, and bands in the at-mosphere of Saturn.

There are times when Saturn is observable, butthe orientation of Saturn’s tilt is such that itsrings seem to disappear. When Saturn is at aplace in its orbit around the Sun where this tilt

has the north pole tipped toward the Sun, therings are illuminated on the north (top) side.When Saturn is at a place in its orbit where thistilt has the south pole tipped toward the Sun,the rings are illuminated on the south (bottom)side. At the beginning of Saturn’s summer andwinter (i.e., at the solstices), when the poles aremost tipped toward the Sun, the rings are mostopen as seen from Earth. The rings close withrespect to the Sun at the beginning of Saturn’sspring and autumn (i.e., the equinoxes). Atthese times, the poles are tipped neither towardnor away from the Sun, and the rings appear ex-actly edge-on, becoming nearly invisible toEarth observers for a short time. This event iscalled a ring-plane crossing, and it is a goodtime to look for Saturn’s moons and to measurethe thickness of the rings.

The last ring-plane crossing occurred in 1995–96. The Earth-orbiting Hubble Space Telescope,as well as telescopes all over the world, took ad-vantage of these few days to observe Saturnfrom this rare perspective. Saturn ring-planecrossings occur about every 15 years, but thenext two ring-plane crossings will be difficult toobserve from Earth, because Saturn will be inthe sky mostly during the day. Earth observerswill not have another good edge-on view of Sat-urn until 2038–39.

53. What do I do if I want to see Saturn’srings, but I don’t have a powerful enoughtelescope?

Try to find someone who does have one. Inmany communities there are clubs of amateurastronomers, most of whom own their own tele-scopes. These clubs often hold star parties inwhich anyone is invited to come out and ob-serve through the telescopes. Some universitiesor communities have larger observatories thathold periodic open houses for the public. More-over, amateur and professional astronomers inyour area may be willing to conduct star partiesin conjunction with school open houses.


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Science museums and planetariums can be aplace to start for information about trackingdown an opportunity to view Saturn through atelescope. National Astronomy Day, held everyyear in April or May, is a good time to be on thelookout for opportunities. Also, you can look inthe calendar sections of magazines such as As-tronomy or Sky & Telescope for regional star par-ties near you. The same two magazines may beuseful for finding good quality, secondhandtelescopes at bargain prices. See the Appendicesfor resource information.

54. If I were on Saturn or Titan, could I seeEarth and its Moon? Would I need a tele-scope?

The astronomer Christiaan Huygens sawSaturn’s moon Titan from Earth with a 17th-century telescope, using lenses a few inches ac-ross. Earth’s Moon is about the size of Titan,and Earth is about twice as large as Titan, so atfirst glance it seems you could certainly detectEarth and its Moon from Saturn with a tele-scope of modest power. However, if you were totry to look at Earth from Saturn, you’d be look-ing almost directly at the Sun. Even thoughEarth and the Moon would be large enough tosee in a dark nighttime sky, they would be verydifficult to detect in the Sun’s glare.

55. If I were standing on Titan, how wouldSaturn look?

To see Saturn, you would need to be standingon the side of Titan that always faces Saturn.But even if you were doing that, Titan’s thick,hazy atmosphere would prevent you from seeingSaturn. If somehow you could see through theclouds, the rings of Saturn would stretch acrossabout 15° of the sky. If you reach out your armfully and spread your fingers toward the sky, theangle between your pinkie and your thumb isalso about 15°. Saturn and its rings would ap-pear almost 30 times wider in the sky than theMoon does from Earth!

The Cassini–Huygens Mission

56. Why are we sending a spacecraft and notpeople to Saturn ?

Spacecraft are robots that represent humans inspace. Neither humans nor robots can surviveunprotected in the dangerous environment ofouter space, but humans require a greater degreeof protection and safety. Robotic spacecraft likeVoyager and Cassini have shields to protectthem from extremes of heat and cold, from in-tense radiation, from the vacuum of space, andfrom collisions with small particles. Astronautsin the Space Shuttle also have these protections,but even so they cannot stay in space very long.The additional needs of people for long jour-neys — for instance, oxygen, water, food, andartificial gravity — make human space travelcost a great deal more than robotic space travel.Also, safety measures taken for human space-flight are generally more costly than thoseneeded for robotic space travel.

Even if humans were more easily accommo-dated as travelers in space, the Space Shuttle isnot designed to travel out of Earth orbit. TheSpace Shuttle flies only about 600 km (370 mi)above Earth’s surface. This is barely the distancebetween Los Angeles and San Francisco. Bycontrast, it is over 1 billion miles to Saturn fromEarth. We no longer have the ready capabilityto send humans to Earth’s Moon, let alone to amore distant planet. Time is also a consider-ation. Even if the Cassini spacecraft were largeenough to carry humans, it would need to carryenough food, water, and oxygen for the 7-yeartrip to Saturn, plus several years exploring, andfinally the return to Earth.

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them? Robotic spacecraft offer an alternative tohuman spaceflight and can more easily be builtto endure in the harsh environment of space.

57. What will the Cassini robot do?

The Cassini spacecraft will make a 4-year scien-tific tour of the Saturn system. The Cassini or-biter will conduct long-term, detailed, close-upstudies of Saturn, its rings, its moons, and itsspace environment. Cassini is the best-equippedspacecraft we have ever sent to another world.The Cassini orbiter carries six instruments to“see” in four kinds of light (visible, infrared, ul-traviolet, and radio). There are also instrumentsfor detecting dust particles, magnetic fields, andcharged particles such as protons and electrons.

The Cassini orbiter will release the Huygensprobe, which will parachute through Titan’shazy atmosphere to the surface. The Huygensprobe will carry a suite of instruments to mea-sure various properties of Titan’s atmosphereand surface. One of the probe’s instruments willmake more than 1,000 images of Titan’s surfaceand clouds — sights never before seen by hu-man beings!

58. What spacecraft have been to Saturn?How have we gathered information aboutSaturn up until now?

Saturn was first visited by Pioneer 11 in 1979and later by Voyager 1 in 1980 and Voyager 2in 1981. These spacecraft passed through theSaturn system and made many extraordinaryobservations and discoveries. Scientists have alsoused the Hubble Space Telescope (HST), whichNASA placed in orbit around Earth in 1990, tostudy Saturn. HST has observed storms inSaturn’s atmosphere and detailed structure in itsrings. Using infrared cameras, HST has also de-tected large bright and dark regions deep be-neath Titan’s veil of haze. Scientists don’t knowyet what these features are — perhaps conti-nents and ethane oceans?

There are many aspects of Saturn that cannotbe studied or detected by remote sensing tech-niques such as using a telescope. For example,only a spacecraft flying within the Saturn sys-tem can directly sense Saturn’s magnetic field.Also, only from a spacecraft beyond Saturn canwe look back at the night side of Saturn (or itsmoons) to collect data on such things as night-time temperatures or how much sunlight isblocked by the dust in Saturn’s rings.

59. What will Cassini learn that we do notalready know from Voyager and HubbleSpace Telescope data?

The Pioneer 11 and Voyager flybys were an ini-tial reconnaissance of Saturn. The Hubble SpaceTelescope (HST) has been used to detect pos-sible continents or other large features on Titan’ssurface. Cassini is a follow-on to these missions,but instruments on the Cassini orbiter are ca-pable of much more detailed observations of theplanet, moons, and rings than either Voyageror HST. The Cassini spacecraft will also have4 years to study the Saturn system instead of afew days as with a flyby mission like Voyager ora few hours every few months as with HST.

In addition, the Huygens probe will parachuteinto Titan’s atmosphere to the surface, and in-struments on the probe will observe detailedproperties of an atmosphere and surface thatVoyager and Hubble could never have seen.Cassini’s scientific objectives cannot be com-pleted by HST because of HST’s huge distancefrom Saturn and the very different instrumentsthat HST and Cassini carry.

60. Why care about the Cassini mission?

The Cassini spacecraft is a robotic ambassadorfor all of humanity. It is an extension of oursenses to a distant, magnificent world full ofmysteries. Solving some of these mysteries hasthe power to teach us about ourselves and ourplace in the Universe. The Cassini mission is anexpression of our deep desire to learn — to


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Areas shown in gray represent the states and countries participating in the Cassini–Huygens mission.

cross the boundary between the known and un-known. Thanks to the world’s myriad possibili-ties for communicating what Cassini is doing,through the Internet, newspapers, television, ra-dio, and classrooms, it is possible for all of us toshare in this extraordinary adventure! How ex-citing it will be at last to unveil some of themysteries of Saturn and Titan — and certainlycreate just as many new mysteries as well.

61. Why is NASA’s mission to Saturn calledCassini?

The Cassini spacecraft is named after the Ital-ian–French astronomer Jean-Dominique Cassini(or Giovanni [Gian] Domenico Cassini), whofigured prominently in the earliest discoveriesabout the Saturn system. The astronomerCassini made his observations of Saturn fromthe Paris Observatory in the late 17th century.He used a series of increasingly larger telescopesto discover four of Saturn’s major moons:Iapetus, Rhea, Tethys, and Dione. In 1675,Cassini discovered that Saturn’s ring was splitinto two parts by a division about 4,600 km(2,900 mi) wide. The gap between the two partsof the ring would become known as the CassiniDivision, and the rings were given separatenames.

62. How much does the Cassini mission cost?Who pays for it?

The total cost for the Cassini mission is about$3.2 billion. This includes the Cassini orbiter,the Huygens probe, the Titan IV launch ve-hicle, and the United States’ portion of missionoperations and data analysis.

NASA projects are funded by the U.S. govern-ment, and thus much of Cassini is paid for bythe taxpayers of the United States. The Cassinimission involves extensive international collabo-ration. The Huygens probe, the high-gain an-tenna on the Cassini orbiter, and portions ofthree science instruments were built in Europe,and were paid for by the people of Europe. Weall have a vested interest in the success of theCassini mission!

63. How long does it take to plan and carryout a mission like Cassini?

About 5 to 8 years are required from approvalto launch for a sophisticated mission likeCassini. For example, Voyager 2 was approvedin May 1972 and launched in August 1977.Cassini was approved in October 1989 andlaunched in October 1997. Cassini is the last ofNASA’s series of giant missions to the outer


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planets of our Solar System. Smaller spacecraftare being developed and launched in 2 to 3years. In the case of Cassini, the mission is de-signed to last at least 11 years after the launch:7 years traveling to Saturn, and 4 years investi-gating the Saturn system.

Planning and carrying out a mission likeCassini requires several phases, which arenamed as follows: Phase A, Concept Study;Phase B, Definition and Preliminary Design;Phase C, Detailed Design; Phase D, Develop-ment through Launch and Instrument Check-out; and Phase E, Mission Operations and DataAnalysis.

NASA defines the end of a spacecraft missionaccording to a specific plan, but this doesn’tmean the spacecraft won’t outlive that plan. Forexample, although the grand scientific tours ofVoyager 1 and 2 are complete, flight controllersare still in touch with the two Voyager space-craft as they hurtle away from the Solar Systemtoward interstellar space. Astronomers hope thatthe Voyagers will eventually return data aboutthe heliopause, which is the boundary betweenthe region of space influenced by our Sun andthe region of space influenced by other stars.

The Spacecraft

64. How big is the Cassini spacecraft?

Height and Width: Cassini is the largest inter-planetary spacecraft ever built by the UnitedStates. It is about the size of a school bus. Thespacecraft is about two stories tall (6.8 m, or22 ft), and 4 m (13 ft) across. It would takeabout 7 large adults with arms outstretched toencircle Cassini.

Mass and Weight: The mass of the Cassinispacecraft is 5,650 kg, which includes theHuygens probe (370 kg), the Cassini orbiter’s

science instruments (370 kg) and 3,130 kg ofpropellant. Before launch, more than half ofCassini’s weight was rocket propellant! At thesurface of Earth, the Cassini spacecraft wouldweigh about 12,400 pounds, or approximately6 tons. That’s about the weight of three or fourmedium-sized cars!

65. How much wire is used in the Cassinispacecraft?

Engineers estimate that Cassini uses approxi-mately 12 km (7.5 mi) of wiring to intercon-nect its electrical components.

66. Is the Cassini spacecraft really all coveredwith gold?

Much of Cassini is covered with gold-coloredmaterial, but it’s not really gold. It’s a multilayerfabric attached to the spacecraft like clothing toprotect it from extremes of hot and cold andfrom impacts by small space rocks and dust inspace called micrometeoroids. The fabric looksgold because the top layer is a translucent amber

Cassini–Huygens in High Bay 2 at the Jet Propulsion Laboratory.


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material called KAPTON®, which has a coating ofshiny aluminum. Together, they look like shinygold foil.

In addition, a large portion of Cassini’s protec-tive layers are graphite-filled blanketing. Thisblack covering protects Cassini’s science instru-ments without interfering with their operations.For example, gold blanketing near one ofCassini’s cameras might cause unwanted reflec-tions to appear in the images it makes.

reliably for many years at a distance that is 9.6times farther from the Sun than Earth is. Thispower must also be supplied while keeping thespacecraft small enough and light enough to belaunched from Earth.

If the Cassini spacecraft were equipped with thehighest efficiency solar cells available, such asthose developed by the European Space Agency,it would make the spacecraft too heavy forlaunch to Saturn. The resulting solar arrayswould cover an area larger than two tenniscourts! RTGs are thus the only feasible powersystem for the Cassini–Huygens mission.

The RTGs start the mission providing about820 watts of power, and end the mission pro-viding about 650 watts. The power output de-clines because RTGs generate energy from aradioactive substance called plutonium that de-cays over time. It is important to know thatCassini’s three RTGs have nothing to do withthe launch or propulsion of the spacecraft.

68. How does an RTG work? If it involvesplutonium, is it dangerous?

An RTG uses the heat energy from a radioactivesource, plutonium (Pu-238). The radioactivitygenerates heat, which in turn is converted toelectrical energy that powers Cassini’s instru-ments, radios, and computers.

Although plutonium is indeed a very toxic sub-stance if breathed into the lungs, Cassini’sRTGs contain a heat-resistant, ceramic form ofit called plutonium dioxide. These ceramicmodules are designed and packaged to preventthe formation of fine dust particles of pluto-nium that would be harmful if breathed intothe lungs. Years of extensive safety testing andanalyses have demonstrated that RTGs are ex-tremely rugged and resistant to a release of theplutonium dioxide fuel, even in severe accidentenvironments. In October 1968, an Atlas rocketcarrying an RTG was destroyed shortly after

67. Will the spacecraft use solar panels toprovide power to the instruments on Cassini?

No. Cassini uses radioisotope thermoelectricgenerators, or RTGs. To identify the most ap-propriate power source to run Cassini’s instru-ments, radios, and computers, NASA’s JetPropulsion Laboratory conducted an in-depthanalysis of the available electrical power systems,including systems that use solar energy. Cassini’sinstruments, radios, and computers require 600to 700 watts. For comparison, the power de-mand for an average American residential homeis about 1,400 watts. The challenge for Cassini’sdesigners was that this power must be produced

Cassini–Huygens in the test chamber at the Jet PropulsionLaboratory.


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launch from Vandenberg Air Force Base. Theplutonium-containing portions of the RTG fellinto the ocean intact, and all the plutonium wasrecovered and reused in a subsequent mission.

69. How well can Cassini aim its instru-ments?

Some of Cassini’s instruments must be aimedprecisely to gather data. They do not swivel bythemselves but require the entire spacecraft topoint in the desired direction. The spacecraftcan point the instruments with an accuracy ofabout 0.06° (1/17th of a degree). Once pointed,the Cassini spacecraft is extremely stable.

The Science Instruments

70. What kinds of instruments does theCassini orbiter have? What do they do?

In some ways, the Cassini spacecraft has sensesbetter than our own. For example, Cassini can“see” in wavelengths of light and energy that thehuman eye cannot. (See the Appendices for an il-lustration of the electromagnetic spectrum.)The instruments can “feel” things about mag-netic fields and tiny dust particles that no hu-man hand could detect. The Cassini spacecrafthas been designed with 18 major science instru-ment packages: 12 on the Cassini orbiter, andsix on the Huygens probe.

Even without knowing the details of all of theinstruments and the nature of what they aremeasuring or detecting, it is still possible to dis-cern several things about them from their de-scriptions. For example, you can classify thescience instruments in a way that enables you tomake a comparison with the way your ownsenses operate. Your eyes and ears are “remotesensing” devices because you can receive infor-mation from remote objects without being indirect contact with them. Your senses of touchand taste are “direct sensing” devices. Your nosecan be construed as either a remote or direct

sensing device. You can certainly smell the applepie across the room without having your nose indirect contact with it, but the molecules carry-ing the scent do have to make direct contactwith your sinuses. The Cassini instruments are:

1. Imaging Science Subsystem (ISS)Makes images in visible light, and some infraredand ultraviolet light. The ISS has a camera thatcan take a broad, wide-angle picture and a cam-era that can record small areas in fine detail. En-gineers anticipate that ISS will return hundredsof thousands of images of Saturn and its ringsand moons! [Remote sensing / sight]

2. Radio Detection and Ranging (RADAR)Produces maps of Titan’s surface and measuresthe height of surface objects (like mountainsand canyons) by bouncing radio signals off ofTitan’s surface and timing their return. This issimilar to listening for the echo of your voiceacross a canyon to tell how wide the canyon is.Radio waves can penetrate the thick veil of hazesurrounding Titan. In addition to bouncing ra-dio waves, the RADAR instrument will listenfor radio waves that Saturn or its moons may beproducing. [Remote active sensing / listening toecho; Remote passive sensing / sight]

3. Radio Science Subsystem (RSS)Uses radio antennas on Earth to observe theway radio signals from the spacecraft change asthey are sent through objects, such as Titan’s at-mosphere or Saturn’s rings. RSS uses radio re-ceivers and transmitters at three differentwavelengths. This gives detailed information onthe structure of the rings and atmosphere. [Re-mote sensing / sight or hearing]

4. Ion and Neutral Mass Spectrometer (INMS)Analyzes charged particles (like protons andheavier ions) and neutral particles (like atoms)near Titan and Saturn to learn more about theiratmospheres. [Direct and remote sensing /smell]


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The Cassini–Huygens spacecraft.

5. Visible and Infrared Mapping Spectrometer(VIMS)Makes pictures using visible and infrared lightto learn more about the composition of moonsurfaces, the rings, and the atmospheres of Sat-urn and Titan. VIMS also observes the sunlightand starlight that passes through the rings tolearn more about ring structure. [Remote sens-ing / sight]

6. Composite Infrared Spectrometer (CIRS)Measures the infrared light coming from an ob-ject (such as an atmosphere or moon surface) tolearn more about its temperature and what it’smade of. [Remote sensing / sight]

7. Cosmic Dust Analyzer (CDA)Senses the size, speed, and direction of tiny dustgrains near Saturn. Some of these particles areorbiting Saturn, while others may come from

other solar systems. [Direct sensing / touch ortaste]

8. Radio and Plasma Wave Science (RPWS)Receives and measures the radio signals comingfrom Saturn, including the radio waves givenoff by the interaction of the solar wind withSaturn and Titan. [Direct & remote sensing /many senses]

9. Cassini Plasma Spectrometer (CAPS)Measures the energy and electrical charge ofparticles such as electrons and protons that theinstrument encounters. [Direct sensing / touch,taste, smell]

10. Ultraviolet Imaging Spectrograph (UVIS)Makes images of the ultraviolet light reflectedoff an object, such as the clouds of Saturn and/or its rings, to learn more about their structureand composition. [Remote sensing / sight]

RADIO DETECTIONAND RANGING(RADAR)

RADIO SCIENCESUBSYSTEM (RSS)

DUAL TECHNIQUEMAGNETOMETER (MAG)ON BOOM

RADIO AND PLASMAWAVE SCIENCE (RPWS)

IMAGING SCIENCESUBSYSTEM (ISS)

VISIBLE AND INFRAREDMAPPING SPECTROMETER(VIMS)

COMPOSITE INFRAREDSPECTROMETER (CIRS)

ULTRAVIOLET IMAGINGSPECTROGRAPH (UVIS)

ION AND NEUTRALMASS SPECTRO-METER (INMS)

CASSINI PLASMASPECTROMETER(CAPS)

MAGNETOSPHERICIMAGINGINSTRUMENT (MIMI)


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11. Magnetospheric Imaging Instrument (MIMI)Produces images and other data about the par-ticles trapped in Saturn’s huge magnetic field,or magnetosphere. [Direct and remote sensing/ sight and smell]

12. Dual Technique Magnetometer (MAG)Measures the strength and direction of themagnetic field around Saturn. The magneticfields are generated partly by the intensely hotmolten core at Saturn’s center. Measuring themagnetic field is one of the ways to probe thecore, even though it is far too hot and deep toactually visit. [Direct and remote sensing /touch and smell]

71. How well can the Cassini cameras see?

Cassini’s highest-resolution camera is able tosee a penny, 1.5 cm (0.5 in) across, from a dis-tance of nearly 4 km (2.5 mi).

72. How do we know what color a planet ormoon really is?

In several of Cassini’s cameras, color filters canbe placed in and out of the cameras so that thedetector sees only one color at a time. Each im-age is then transmitted to Earth. Image pro-cessing computers on Earth combine the datato recreate the image in its original colors. Oureyes work in a similar way: we can really seeimages just in three primary colors — red,green, and blue. Our brains combine thesethree colors to make other colors, such aspurple, yellow, and orange.

73. What does the Huygens probe do?

Soon after arriving in the Saturn system, theCassini orbiter will release the Huygens probe,which will descend into the atmosphere of Ti-tan — Saturn’s largest moon. The Huygensprobe, built by the European Space Agency,carries six instruments to collect data on Titan’sclouds, atmosphere, and surface.

The 320 kg probe is built a little bit like a clam:it’s hard on the outside, to protect the delicateinstruments on the inside. The shell must bebuilt so it can survive the 20,000 km/hr(13,000 mi/hr) rush when it first hits the atmo-sphere, and the 12,000 °C (22,000 °F) tempera-tures as the friction from Titan’s atmosphereviolently slows it down.

As the 2.7 m (8.9 ft) diameter probe entersTitan’s atmosphere, it will begin taking mea-surements in the haze layer above the cloudtops. During its 2.5-hour descent — first on amain parachute and later on a smaller “drogue”parachute — various instruments will measurethe temperature, pressure, density, and compo-sition of the atmosphere. As the Huygens probefinally breaks through the bottom layer ofclouds, a camera with 11 simultaneous viewingdirections will capture panoramic images ofTitan’s surface.

The probe’s instruments will also measure prop-erties of Titan’s surface as it descends and possi-bly after landing — if the probe survives theimpact with the surface. The probe lands rela-tively hard, at about 25 km/hr (15 mi/hr) andthus may not survive the landing.

Rendition of the Huygens probe being released above Titan.


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Instruments on the two sides of the Huygens probe.

74. What kinds of instruments does theHuygens probe have?

The Huygens probe carries six instruments. Asthe probe falls through the atmosphere towardTitan’s surface, some of the instruments will bebusily monitoring the atmosphere by lookingout windows in the probe or “sniffing” the at-mosphere through holes. Other instruments willstart working after the probe lands — or floats— on Titan. Radios on the probe will send backdata to the Cassini orbiter. These are the instru-ments on the Huygens probe:

1. Gas Chromatograph and Mass Spectrometer(GCMS)Analyzes the amounts of various gases in Titan’satmosphere. It will look for organic moleculesthat may indicate interesting chemistry happen-ing in Titan’s atmosphere, as well as simplermolecules that will help scientists understandhow Titan formed. [Direct sensing / smell]

2. Aerosol Collector and Pyrolyser (ACP)Detects the particles in Titan’s thick, hazyclouds. The ACP might help detect the gasesand clouds that would be spewed by any activevolcanoes on Titan. [Direct sensing / smell]

3. Descent Imager / Spectral Radiometer (DISR)Takes pictures of Titan as the probe descendstoward the surface. It also measures how Titan’sclouds dim the Sun’s light. This will help as-tronomers understand how Titan is heated bythe Sun. [Remote sensing / sight]

4. Huygens Atmosphere Structure Instrument(HASI)Watches for lightning and listens for thunder inTitan’s clouds. Using the probe’s batteries forpower, HASI will also create its own very tinylightning bolts to explore how Titan’s atmo-sphere interacts with electricity. [Remote sens-ing / sight, hearing; direct sensing / touch]

5. Doppler Wind Experiment (DWE)Measures the speeds of Titan’s winds. Does Ti-tan have huge hurricanes, or is it a relativelycalm place? Maybe, like Earth, it’s windy atsome altitudes and more calm at others. Thisinstrument is so sensitive that it might alsomeasure the probe gently swinging below itsparachute! [Direct sensing / touch, balance]

HUYGENS ATMOSPHERE STRUCTURE INSTRUMENT (HASI)

DOPPLER WIND EXPERIMENT (DWE)

AEROSOL COLLECTORAND PYROLYSER (ACP)

DESCENT IMAGER / SPECTRALRADIOMETER (DISR)

SURFACE-SCIENCEPACKAGE (SSP)

GAS CHROMATOGRAPH ANDMASS SPECTROMETER (GCMS)


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6. Surface-Science Package (SSP)This set of eight instruments examines theprobe’s landing site — whether rocks, snow,“goo,” or a lake. The SSP has detectors to mea-sure how hard the probe hits, the temperature,the speed of sound in Titan’s atmosphere, andthe type of liquid in which the probe may befloating. [Direct sensing / touch, hearing, taste]

75. What happens to the Huygens probe afterit lands on Titan?

The Huygens probe may survive landing onsolid ground, ice, or even liquid. Engineers de-signed it to float! Many scientists theorize thatTitan may be covered by lakes or oceans ofmethane or ethane, so the Huygens probe is de-signed to function whether it goes “splash” or“splat.” One instrument on board will tell us ifHuygens is bobbing in liquid, and other instru-ments on board will tell us what that liquid ismade of. If the battery-powered probe survivesits landing, it will send measurements fromTitan’s surface until its batteries die or theCassini orbiter flies out of radio contact — forup to 30 minutes.

After the probe runs out of battery power, itcould sit wherever it lands for thousands ofyears. It could be caught in a landslide or anavalanche, if such phenomena occur on Titan!Huygens could wash up on some frigid Titanicbeach. Or, it could land on a methane icebergand float endlessly on an ethane sea. Whereverit lands, organic chemical compounds fallingfrom Titan’s sky will likely rain down on it. Likea car parked outdoors in Los Angeles for toolong, Huygens eventually would be coated withthe residue of this light brown, smog-like goo.Maybe in the far future, we will return to Titanto find out what became of the Huygens probe.

76. If the Huygens probe were to sink, wouldthere be any way to send information back?

No. If the probe were to sink in cold liquidethane, which may well be present on Titan, the

batteries and radio would not operate well, andthe probe would not be able send informationback to the Cassini orbiter.

The People of the Cassini Team

77. How many people have worked onCassini?

At its peak, Cassini’s development involvedabout 4,500 people, including 3,000 in theU.S. and 1,500 in European countries. Thisincludes engineers, scientists, and many otherpeople at universities, research institutions, andin industry. These people worked in 32 U.S.states and 16 European countries.

A life-size model of Cassini–Huygens at JPL, with a few of thethousands of Cassini team members.

Another way of expressing the magnitude of thehuman effort involved is to consider the num-ber of workyears needed to prepare Cassini forlaunch. One workyear is equivalent to 1 personworking full-time for 1 year. Preparing theCassini mission for launch took about 13,000workyears of effort — almost half of what itmust have taken to build the Great Pyramid ofCheops at Giza!


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78. Who manages the Cassini Project?

The Jet Propulsion Laboratory (JPL), a divisionof the California Institute of Technology, man-ages the Cassini Project. JPL is under contractwith NASA to design and fly robotic space mis-sions. JPL is especially famous for its work inplanetary science, including the Voyager mis-sions to the outer planets and Pathfinder toMars.

79. What sorts of people work on a spaceproject like Cassini?

It is a monumental task to design, build, launch,and fly a sophisticated robot like Cassini. Agreat diversity of talented people are required tomake it happen. Most of those who work on theCassini project are scientists and engineers, butthe project also involves people such as com-puter programmers, educators, machinists, elec-tricians, secretaries, security guards, and travelagents.

80. How could I prepare for a career involv-ing a space project?

In preparation for careers involving a spaceproject, it is wise to take all the courses you canin school, especially math, science, and Englishcourses. Go to college if possible, and pick afield of study that particularly interests you. Sci-ence and engineering are the most likely path-ways to becoming involved in a spaceexploration project, but there are other ways aswell. Seek out someone who is already in aspace-related career and talk to them aboutwhat skills and attitudes they needed to be suc-cessful. It is helpful to become aware of, and be-gin to cultivate, some of the useful job skillsthat may not necessarily be taught in school. Inaddition to having basic mathematical skills andsome sort of technical training, it is helpful tobe enthusiastic, creative, able to learn newthings, speak and write well, use a computer,work well in a team, and persevere throughproblems.

END-END INFOSYSTEM

TELE-COMMUNICATIONS

SCIENCE

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Launch and Navigation

81. When was Cassini launched?

Cassini was scheduled to be launched on 6 Oc-tober 1997. Several weeks before launch, engi-neers detected a minor problem with insulationinside the Huygens probe, and program man-agers rescheduled the launch for 13 October1997. After the launch was postponed oncemore due to technical problems, Cassini wassent on its journey to Saturn at 4:43 in themorning on 15 October 1997, from CapeCanaveral, Florida.

82. Which launch vehicle did Cassini use?

A Titan IV rocket launched Cassini on its wayto Saturn. Cassini has a mass of 5,650 kg, andthus it takes a mighty force to free the space-craft from Earth’s gravity. The Titan IV is themost powerful expendable launch vehicle in theUS fleet. It was built by Martin Marietta —now Lockheed Martin — under contract to theU.S. Air Force.

The Titan IV did not send Cassini by itself.Rather, the Titan launched both Cassini and anupper stage rocket called a Centaur. The Cen-taur helped to place Cassini into a temporaryorbit around Earth called a parking orbit. Fromthere, the Centaur waited until Cassini was inthe right position, and then fired its engines topropel Cassini away from Earth and toward Ve-nus for Cassini’s first gravity assist. After firingits engines, the Centaur disconnected fromCassini and the spacecraft began its long,unpowered coast through space. The Centaurwas developed by General Dynamics and is themost powerful upper stage in the world.

The Titan IV launch vehicle, including theCentaur, has a mass of 4.4 million kilograms,of which about 90%, or 4 million kilograms, isfuel! The 370 kilograms of Cassini’s scientificinstruments seems amazingly tiny next to themass of the launch vehicle.

83. How much rocket fuel does Cassini carryin order to complete its mission at Saturn?

Cassini carries about 3,000 kilograms of fuel(or propellant). Some of the fuel will be used todirect the spacecraft’s course on its way to Sat-urn, some will be used to slow down the space-craft as it arrives at Saturn, and some will beused while touring the Saturn system. Well over99% of Cassini’s trip, however, will be anunpowered coast through space.

84. When does Cassini arrive at Saturn?

Cassini is due to arrive at Saturn on 1 July2004. Just before Cassini’s closest approach toSaturn, the spacecraft fires its engines for overan hour to slow itself down enough to be cap-tured into orbit around Saturn. Cassini will bemoving so fast — 32 km/sec (71,000 mi/hr) —that if it didn’t fire its engines, it would cruisepast Saturn and never return.

The Titan IV vehicle launching Cassini.


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85. How long does the Cassini mission last?

The Cassini mission is planned to last a total of11 years: 7 years traveling from Earth to Saturn,and 4 years touring the Saturn system. However,the spacecraft could continue to send informa-tion back to Earth for many more years.

86. Why does it take so long to get toSaturn?

Cassini is an extremely heavy spacecraft — theheaviest interplanetary spacecraft ever launchedby the United States. Because it is so heavy, itwas not feasible to boost it to Saturn directly.Instead, the spacecraft was launched inward to-ward Venus, and has two Venus flybys, oneEarth flyby, and one Jupiter flyby on the way toSaturn. Each of these flybys increases the speedof the spacecraft using a gravity assist. (Thiskind of flyby is sometimes called a swingby.)Cassini will eventually get to Saturn, but ittakes time to speed up the spacecraft and get itgoing fast enough.

Artist’s concept of Saturn orbit insertion.

Cassini–Huygens’ interplanetary trajectory.

EARTHSWINGBY18 AUGUST 1999

LAUNCH15 OCTOBER 1997

VENUS 1SWINGBY26 APRIL 1998

VENUS 2 SWINGBY24 JUNE 1999

DEEP-SPACEMANEUVER3 DECEMBER 1998

JUPITERSWINGBY30 DECEMBER 2000

SATURN ARRIVAL1 JULY 2004

ORBIT OF EARTH

ORBIT OF VENUS

ORBIT OF JUPITER

ORBIT OF SATURN

PERIHELIA27 MARCH 1998 0.67 AU29 JUNE 1999 0.72 AU


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87. Couldn’t we get to Saturn faster if weflew directly to Saturn instead of wrappingaround other planets?

Yes, but we would need a much more powerfullaunch vehicle or a much smaller spacecraftthan Cassini. Given the launch technology andthe mass of Cassini, using gravity assists fromother planets is absolutely necessary to increaseCassini’s speed. Without using gravity assistsfrom other planets, or a larger launch vehicle, itjust wouldn’t be possible to get Cassini to Sat-urn at all, in any amount of time!

The increase in speed provided by the gravityassists from Venus, Earth, and Jupiter wouldotherwise require an additional 3.6 million kilo-grams of fuel. During Cassini’s 4-year tour ofSaturn, the 40 gravity assists from Titan willprovide the equivalent of another 49 million ki-lograms of fuel. This is over 12 times as muchfuel as the Titan IV launch vehicle carries!

88. What is gravity assist?

Gravity assist is a way of using the gravitationalpull of a massive planet on a spacecraft in orderto transfer momentum and energy from theplanet to the spacecraft that is flying (or “swing-ing”) by it. When the Voyager spacecraft flew byJupiter, it gained 16 kilometers per second ofspeed relative to the Sun, at a cost of initially re-ducing Jupiter’s orbital speed by about 30 cm(1 ft) per trillion years. Exploration of the outerplanets would not be possible without gravityassist, unless we were to use smaller payloadsand mightier rockets than currently exist.Cassini gained about 6 km/sec relative to theSun at the first Venus swingby, 7 km/sec at thesecond Venus flyby, 6 km/sec at Earth, and willgain about 2 km/sec at Jupiter.

89. How close does Cassini come to Earthduring its flyby?

Cassini flies about 1,170 km (720 miles) overEarth during its flyby on August 18, 1999 —higher than the Hubble Space Telescope’s

altitude of 600 km (370 miles). Cassini swingsby Jupiter at a much greater distance of almost10 million km (6 million mi).

90. Can we see the Cassini spacecraft fromEarth during its flyby of Earth?

Yes, but at Cassini’s closest approach to Earth, ittravels very fast and is just about to pass intoEarth’s shadow. It would be visible for about 30seconds as a moving point of light about asbright as stars of the Big Dipper. If you were ata location on Earth near Cassini’s path, Cassiniwould come from the west at dusk and traverseabout half the sky before passing into Earth’sshadow. Cassini would emerge from Earth’sshadow and reappear about 24 minutes later. Atthis point, Cassini would be much more distantfrom Earth and so would appear much dimmer,although you could still see it with binoculars ifyou knew where to look.

91. How far does Cassini travel from Earth toSaturn?

The direct distance from Earth to Saturn variesfrom about 1.2 billion km (750 million mi) to1.6 billion km (980 million mi), depending onwhere Saturn and the Earth are on their lapsaround the Sun. Due to its flybys of Venus,Earth, and Jupiter, Cassini actually travels morethan 3 billion km (2 billion mi) to reach Saturn.Once there, Cassini travels another 1.7 billionkm (1.1 billion mi) on its tour of the Saturnsystem.

Artist’s concepts of the Venus flyby (left) and theEarth flyby (right).


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92. How fast does Cassini go?

During the Cassini mission, the spacecraftreaches relative speeds of 13 km/sec (or about29,000 mi/hr) flying by Venus (equivalent toflying from Los Angeles to Boston in under5 minutes!), and 19 km/sec (43,000 mi/hr) fly-ing by Earth. While cruising to Saturn, Cassini’sspeed is as high as 32 km/sec (71,000 mi/hr). Atthis speed, even the gravity of Saturn is notenough to capture it — Cassini must fire its en-gines to slow down at Saturn, or it would con-tinue on past the planet and never return.

93. How close does Cassini fly to Saturn’scloudtops?

Upon reaching Saturn, Cassini swings close tothe planet, to an altitude only one-sixth thediameter of Saturn itself — about 20,000 km(12,000 mi). This begins the first of more than70 orbits during its 4-year mission, and it’s theclosest that Cassini ever gets to the planet.

94. What happens to Cassini after it com-pletes the Saturn tour?

After completing its tour, the spacecraft willcontinue orbiting Saturn. If all goes well duringthe mission, there should be enough attitude-control propellant and electrical power for thespacecraft to continue to relay data back toEarth for many years (just as Magellan did andthe two Voyagers still do). However, budgetconstraints may limit how long NASA is able tooperate the spacecraft after the end of the mis-sion in 2008. Cassini will continue to orbit Sat-urn until the spacecraft runs out of propellant.Before it runs out of propellant, flight control-lers will probably place Cassini in an orbit thatminimizes its chances of colliding with any ofthe moons for a long time.

A sample Saturn orbital tour. The view is from above Saturn’s north pole. This type of diagramis called a petal plot because each orbit resembles the petal of a flower. The range of orbitorientations allows a detailed survey of Saturn’s magnetosphere and atmosphere.

TO SUN


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Communications and Science Data

95. How long does it take for a radio signalto travel between Earth and Saturn?

A signal takes between 70 and 90 minutes toreach Earth from Saturn. The exact time de-pends on the ever-changing locations of Earthand Saturn in their orbital laps around the Sun.Radio waves travel at the speed of light, or300,000 km/sec (186,000 mi/sec).

96. Has anything been learned from thefailure of the high-gain antenna on theGalileo spacecraft which has altered thedesign of Cassini’s high-gain antenna?

The Galileo spacecraft’s high-gain (or main) an-tenna was designed to unfurl itself like an um-brella while in flight to Jupiter. When flightcontrollers commanded it to open, it openedpartially but not enough for it to be of usetransmitting data between the spacecraft andthe ground. The Cassini mission had already

planned to use a fixed antenna before the failureof Galileo’s folding antenna. The Cassini high-gain antenna, which was provided by the ItalianSpace Agency, has no moving parts. The failureof Galileo’s antenna triggered an intense analysisof the Cassini spacecraft to avoid other types ofmechanical failures.

97. How much power do Cassini’s radiotransmitters put out?

Cassini’s radio transmitters send out about20 watts of power — about the same amountof power it takes to operate a refrigerator’s lightbulb.

98. What is the Deep Space Network?

The Deep Space Network (DSN) is a collectionof huge, dish-shaped radio antennas distributedaround the world that send and receive mes-sages to and from spacecraft like Cassini.

Data flow from Saturn to Cassini to Earth.

SCIENTIFIC COMMUNITY

WORLDWIDEGENERAL

PUBLIC

JET PROPULSION LABORATORY

OBJECT OFINTEREST CASSINI

SPACECRAFT

DEEPSPACE

NETWORK


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If you could place one of NASA’s 70-m Deep Space Network antennas inside the Rose Bowl in Pasadena, California,it would look like this.

Cassini will regularly use the Deep SpaceNetwork’s largest antennas, which are 70 m(230 ft) in diameter — nearly the size of a foot-ball field. The DSN’s large radio dishes must bepointed to within a small fraction of a degree ofa spacecraft’s location to be able to communi-cate with it.

99. What if something goes wrong with thespacecraft? Do we have to wait an hour tolearn about it?

Yes, we would have to wait to learn about aproblem, but Cassini might be able to take careof itself in the meantime. Much planning hasbeen done for times when things don’t go asplanned. Many of Cassini’s parts have backupsthat can be activated from Earth, or in somecases turned on automatically by the spacecraft.For example, Cassini has two radio receivers. Ifone should fail, the spacecraft’s computerswould realize that they haven’t heard from Earthrecently, and automatically switch to the backupreceiver to listen for further instructions.Cassini’s capabilities for detecting and handlingproblems by itself are collectively called “faultprotection.”

100. How much science data will Cassinireturn?

On a busy day at Saturn, Cassini could trans-mit up to 500 megabytes (500,000,000 bytes,or about a CD-ROM’s worth) of informationto Earth. More than 300 gigabytes of sciencedata will be sent back to Earth during themission. This would fill more than 400 CD-ROMs — a stack of CDs that would behigher than 4 m (13 ft). It is also about theamount of information in 2,400 sets of theEncyclopedia Britannica.

101. How many pictures will be sent backfrom Cassini–Huygens?

Engineers estimate that the Cassini missionwill return as many as a million images of Sat-urn, the rings, Titan, and the other moons.This includes more than a thousand imagestaken by the Huygens probe of scenes neverbefore seen by humans.

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