NASA Facts IRAS - Mapping the Infrared Sky

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IRAS: Mapping the Infrared Sky

ORIGINAL PACE

COLOR PHOTOGRAPH

Each day as it looks outward from Earth orbit into deep

space, the Infrared Astronom ical Satellite ( IRAS) discoversthousands of new sources never before seen in the infrared.One source might be a distant galaxy whose radiation is toofaint to appear to all but this most sensitive of infrared tele-scopes. Another might be a dying star whose visible flame isburning out, but whose warm infrared glow is still enough tobe detected by IRAS.

The image shown here was reconstructed from data sentback by the international IRAS sates; to during the first weeksof its operaticn. In a scan across the Large Magellanic Cloud,the nearest galaxy to our own, WAS recorded dozens ofinfrared sources as seen in four separate wavelength bands.The short wavelength scan reveals many individual hot stars

in the region. (Hotter regions appear lighter.) As the observing

wavelength gets higher, though, the scan reveals coolerobjects, until we see an extended cloud o% cool dust and gas.This cloud is part of a neb ula called 30 Dorad us (for its loca-

tion ;n the constellation Dorad o), nicknamed the T arantula byastronomers. The cloud is a giant region of ionized hydrogengas and dust, whose existence was known before IRAS, butwhich had never before been seen in such illuminating detail.

Scanning the whole sky from Janu ary to December, 1983,the goal of the IRAS project is to find whatever is "out there"radiating infrared it the universe. When the survey is finished,IRAS may, as only a few projects really can, contribute to afundamental change in our understanding of nature.



ORIGINAL PAGE

C O L O R P H O T O G R A P HBecause our vision is tuned to the particular lighting condi-

tions of our own planet Earth, we see only a few of the many"colors" of the universe. Most of the electromagnetic spec-trum is invisible to the human eye—radio waves, ultraviolet,x-rays, gamma rays r nd the infrared wavelengths just belowthe threshold of signt.

Although the energy of radian! heat glows all around us, itwas not until a simple, intuitive experiment by the astronomerSir William Herschel in 1800 that it was recognized as a natu-ral part of the continuous spectrum of electromagnetic energy.While experimenting with the heating properties of differentcolors, Herschel noted that his thermometer measured thehighest temperatures when he placed it beyond the red regionof a prism spectrum, in an area where apparently there wasno light. He had discovered "calorific rays"—what we knowtoday as infrared.

In H erschel's own century astronomers used thermo-couples—devices that convert heat to electric current—todetect this invisible infrared radiation from space. The Moonwas first observed in this way in 1856, and by the early twen-tieth century most of the bright visible objects in the sky hadalso been observed in parts of the infrared. By the 1960s, thesame decade that saw a boom in radio, x-ray, and ultravioletastronomies, infrared observers began to benefit from new

techniques, particularly the use of supercooled (cryogenic)detectors. Infrared telescopes were moved to higher and drier

locations, and observers lofted their sensors by balloon,rocket, and airplane above the infrared-absorbing water of ouratmosphere.

A few preliminary surveys of the infrared sky, beginning in1968 with the California Institute of Technology short-wave-length survey at 2 microns for northern latitudes, cataloguedmany new sources. A similar, though less complete, surveyfrom a New Zealand observatory in the same year revealedsome of the brightest infrared objects in the southern sky.These were to be followed in the 1970s by the U.S. Air Forcesurvey with rockets at longer wavelengths, up to about 30microns, and, with the Naval Research Laboratory. the FarInfrared Space Experiment which observed at the still longerwavelength of 100 microns.

Until 1983, however, there was no attempt to take a com-plete inventory of the major infrared emitters in the universe.This is the task of the Infrared Astronomical Satellite (IRA S),launched on January 25, 1983. An international effort involvingthe United States. United Kingdom, and The Netherlands, IRASis performing the first all-sky survey in a w ide range of infra-red wavelengths with a sens itivity 100 to 1000 times greaterthan any previous work. At the end of nearly a year of obser-vation from Earth orbit, IRAS data will be used to produce acomprehensive catalog and maps of significant infraredsources in the universe

An artist's conception of the IRAS satellite in orbit shows on e of its solar panels and its Earth-pointing communications antenna.



ln temat ionat MAS sc ienc:s team

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D r . F r a i l G ille tt K itt Pe a k Nat iona l Obse rva

D r . M i c h a e l H a u s e r G o d d a r d S p a c e A g m cen

Dr. J a me s H o u c k Corre l l Un ivers i ty

Dr . F rank LowU t ivers i ty o f Ar izona

D r . G e r r y Nc u g e b a u e r C a l if o rn ia I n s t it u t e o f Te c h n o l o g yC o - c h a i r m a n , J IS W G

Cal i fomia Ins t i tute o f T i t c l - w u y

Jam ieson Eng inee r ing

Univers i ty o f Groningen

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D r . B . T o m S o i f e r

Dr . Russe l l Wa lker

T H E N E T H E R L A N D S

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D r . J a n B o r g m a n

Dr. Tei ie de Jong

Dr . Ha rm Hab ing

Dr . S tewa r t Po t tasch

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U N I T E D K IN G D O M

Dr. Peter C legg

Pro f . R ichard Jenn ingsPro f . Ph i l li p Ma rsden

3 m m ar a y s

Vi r tua l ly eve ry th ing rad ia tes i n the i n f ra red A s t ronomica lob jec ts gene ra l l y emi t the ir ene rgy ove r a w ide range o f wav e-lengths , and the hot ter an objec t is , the mo re i ts energy ou tputi s concent ra ted a t th-3 shor t end o f the spect rum. H ot s tarsthe re fo re appear b lue (sho r t waves ) wh i le coo le r s ta rs a re red .W hen a n ob jec t i s no t qu i te ho t enough to sh ine i n v i s ib le l igh ti t em i ts the bu lk o f its energy in the inf rared, l ike a s tove burne rbefore i t beg ins to g low red hot . Inf rared as tron om y is thus thes tudy o f re la t i ve ly coo l ob jec ts be low a bout 60 00° Ke lv in(10,000°F) that as t ronom ers be l ieve accou nt fo r a s igni f i-

can t am ount o f the u n ive rse 's to ta l ene rgy ou tpu t .As ide f rom the ab i l it y to de tec t coo l ob jec ts , the re a re o the radvantages to obse rv ing the un ive rse i n the i n f ra red . Be tweenthe s ta rs o f our ga laxy the re i s a l a rge am ount o f cosm ic dus ttha t e f fec ti ve l y b locks ou t l i gh t a t v i s ib le w ave leng ths , becau sethese par t icu la r waves a re about the sam e s i ze as the dus tpar t ic les The inters te l lar dus t is espec ia l ly th ick in the planeof cu r galaxy , so that v is ib le l igh t com ing f rom the Mi lky W ay 'scen te r i s reduced by a fac to r o f ten b i l li on by the t ime i treaches Ear th . No t so w i th the longe r wa ves o f i n f ra red , w h ichare reduced by on ly one ten th . Because o f th i s re la t i ve t rans-parenc y, inf rared as tron om y is idea l fo r s tudy ing the br ightand den se co re o f the M i l ky Way .

In add i t i on , ce r ta in change s in ene rgy s ta te w i th in ho tgasses an d in te rs te ll a r c louds o f mo lecu les have the i r s igna-

ture in the inf rared spectrum. By s tudy ing emiss ions f romthese reg ions i t is poss ib le to reconstruc t the type o f chemis t rytak ing place there .

W i th a l l o f th is sc ien t i fi c i n fo rmat ion con ta ined in pho tons o finf rared l igh t i t is anno y ing, at leas t fo r as t ronom y, that af ter b i l -l ions o f m i les o f gene ral ly unimped ed t rave l f rom the far par tso f the un ive rse mos t o f them are b locked jus t as they reachEar th . Wate r and carbon d iox ide in the a tmosphere a bso rb thebu lk o f in f ra red rad ia t ion f rom spa ce . On ly a few w ave leng thsm ake i t th rough to the g round in nar row o bse rva t iona l w in-d o w s c e n t e r e d a t 1 . 2 5 , 1 .6 5 , 2 . 2 , 3 . 5 , 4 . 7 5 , 1 0 . 5 , 1 9 . 5 , 3 5 , 3 5 0 ,and 800 m ic rons . Even a t these t ransparen t w indow s the reare prob lem s— the a i r above us g low s b r igh t ly i n the i n fra red ,so even on the darkes t n igh ts i n f ra red as t ronomers m us treso r t to techn iques l i ke nodd ing the i r te lescopes back and

fo r th to so r t ou t sky "no ise " f rom as t ronom ica l sources .The so lu t i on i s to o rb i t an i n f ra red te lescope abo ve the

a tmosp here w he re i t is exposed to pure , un f i lt e red rad ia t ionand can survey the w ho le sky , no r th and sou th , even a t wave-leng ths be tween the g round -based w indows . Th is i s the pur-pose of the In f rared Ast ronomica l Sate l l i te .

T h e I R A S P r o je c t

L ike ma ny o f the ven tu res in to space p lanned fo r the 1980sand 1 990s , IRAS fea tu res i n te rna t iona l coopera t i on . TheNethe r lands A e rospace Agency (N IVR ) supe rv i sed the des ignand m anufac tu re o f the spacec ra f t bus tha t suppo r ts andpowers the m a in te lescope , and the U n ive rs it y o f Gron ingenprov ided a Du tch Add i t iona l Expe r iment package .

The Am es Research Cente r and the Je t P ropu ls ion Labo ra-tory (JPL), both in Cal i fo rn ia, deve loped the inf rared te lescopefo r NAS A. JPL i s a l so processing IRAS da ta i n to f ina l i n f ra redca ta logs and ma ps . The Un i ted K ingdom con t r ibu tes to thepro jec t th rough i ts Sc ience and En g inee r ing Research Coun -c i l by t rack ing the sa te l l it e and rece iv ing i ts rad ioed da ta .

T h e I n fr a r e d S p e c t ru m

The e lec t romag ne t i c spec t rum i s d i v ided in to seve ra l ca te -gor ies o f rad iat ion, each wi th d i f fe rent w ave leng ths (F igure 3) .A t one end a re the low-ene rgy rad io wa ves w i th wave leng thsup to tens o f thousands o f m e te rs . A t the o the r end a re theg a mm a r a y s w h o s e w a v e l e n g t h s a re s m a l le r t h a n t h e d ia -me te r o f an a tom. The sma l le r the wa ve leng th , the g rea te r theenergy o f the rad iat ion.

Be tween these two ex t rem es l i es the i n fra red reg ion , w i thwav e leng ths f rom one m i ll im e te r ( the sho r tes t rad io waves ) toapprox imate ly 0 .8 microns ( .0008 mi l l imeters) , the longestwa ves o f v i s ib le red l i gh t. The fam i li a r i n f ra red he at pho to-g raphs a re m ade b y f i lms sens i ti ve to on ly the sho r tes t wavesc losest to v is ib le l ight

The E l e c trom a gn e t ic Spe c t rum10 cm1 C m1 mm1 mm10 microns1 micron1000 A100 A10 A1 b11000 microns),(100 microns)

O R I G IN A L P A G EC O L O R P H O T O G R A P H



A technician checks out the satellite prior to launch. Thespacecraft bus containing electronics and communicationsequipment (left) is separated from the telescope (middle) andits conical sunshade by thick insulation.

Baff le

SecondaryMirror

Pr imaryMirror

Foca l P laneAssemb ly

Dutch Addit ionExper imen tElectronics

HorizonSensor

Electronics

nPir.INAL PAGE)TOGRAPH

Superf luidHel ium Tank

Exper imentElectronics

Dutch A ddi tiona lExper iment

As an international project, the satel l ite is m anag ed by aJoint IRA S Project Executive G roup made up of represe nta-t ives from the participating age ncies, institutes, and industries.Similarly, the I RA S science team draws i ts 18 me mbers f romthe three cooperating nations.

IRAS was launched by a Delta rocket from the WesternMissile & Space C enter in California on January 25 , 198 3 andplaced in a ne ar-polar circular orbit (Florida launch sites areused for east-wes t orbits) at 90 0 ki lometers (560 miles) alt i-tude, v. el l above the atmosphere b ut below the Va n Allen

radiation belts. The 1,0 76-kilogram (2,36 5-pound) cylindricalsatell i te is about the size of a passe nger van, m easuring 3.6meters (12 feet) in length and 216 me ters (7 feet) in d i a m e t e r .I ts main component, a 57-ce ntimeter (2 2.4-inch) Ritchey-Chretien-type ref lecting telescope, has a n array of infrareddetectors m ounted at the focal plane. At its ba se the telescopeis attached to a Dutch-buil t spacecraft Lu g that contains al lthe electronic "housekee ping" equipm ent for computing,powe r distribution, tape recording, communications, a nd tele-scope pointing control . A large visor-l ike sun sha de cutsdown the a mount of stray l ight reaching the telescope, andtwo solar pane ls catch sunl ight for converting to e lectricity.

As w ith infrared telescopes on Earth, the IRA S instrume nt 'sheat-sensit ive detectors are kept at very low tem peratures. Butfor the purposes of this highly sensit ive surve y, the telescope

itsel f must also be super-cooled to preve nt its infrared heat

Anatomy of the IRAS Satelli te



from interfering with the de tection of very faint astronomicalsources. For this reason, the optical assemb ly and detectorsare surrounded by a dew ar, or cooling vessel, shaped like adoughnut stretched In height. The vessel contains 475 liters(125 gallons) of extremely cold liquid helium , spread out in thenear zero-gravity of space into a film on the walls of its tank.The tank Is in turn mounted inside a main shell, but is separ-ated from the outer shell walls by a vacuum layer to furtherreduce heat flow to the telescope, like a giant thermos b ottleThick insulation at the telescope's base shields it from thewarmth o f the spacecraft electronics.

Because of this cryogenic system, the first ever flown on an

orbiting instrument, the temp erature at the IRAS focal plane,where the Infrared detectors are located, stays at a low 2°K(-455°F), or only two degrees above absolute zero—the low-est temperature theoretically possible and the point at whichall molecular motion comes to a halt. This 2 0 focal plane

temperature is required for IRAS' great sensitivity—the surveycan detect objects in the universe as cold as 15°K (-432°F),sources w hose energ y is as faint as one m illion-tril l ionth of awatt per square centimeter by the time it reaches Earth.

The same cryogen ics that allow IRAS to perform its mission,however, limit its lifetime. Th e liquid helium boils off even incold space at a slow, steady rate, escaping as vapor throughpores in a stainless steel plug in the tank. This leak rate dic-tates the useful life of the satellite, which was estimated to beapproximately 11 months at the time the IRAS survey b egan.

The IRAS orbit was chosen with several factors in mind.First, the heat-sensitive telescope mu st always point morethan 60° away from the Sun and m ore than 88° from thebrightness of the Earth's limb, or edge. The pow er-producingsolar panels m ust also receive sunlight at least part of thetime. The sa tellite therefore follows a nearly polar orbit closelyaligned with the Earth's term inator, or sunrise-sunset line.The orbit is Sun-synchronous— it shifts approximatelyone degree each d ay to keep the sam e attitude relativeto the Sun as the Earth travels its seasona' journey. Horizonsensors on the satellite warn it away from bright objects andkeep it looking at dark space. They also help in the compu ter-guided, gyro-assisted attitude control system. S tar sensors areused to achieve a telescope pointing accuracy down to only a

few seconds of arc.The purpo se of all this caution and engineering is to maxi-mize the am ount of radiation striking the IRAS telescope'ssensitive infrared detectors on each s urvey scan. There a re 62detectors in all. sensitive in four wavelength band sRectangular in shape, they average about the size of amedium -length printed word on this page. They work on theprinciple that exposure to infrared radiation reduces the elec-trical resistance of their crystals by a know n amou nt, so thatthe amo unt of radiation reaching the telescope's focal planecan be read directly as an increase in current. The Band 1detectors, observing at shorter wavelengths, typically revealthe emissions of hotter point sources such as stars. Band 4,on the other hand, observes cooler or more extended objectssuch as dust clouds with its longer wavelength sensitivity.

Mounted w ith the array of detectors in the telescope's focalplane are three m ore instruments, known collectively as theDutch Additional Experiment. They include two photometersand a low-resolution spectrometer. The spectrometer is usedwith the main telescope to obtain spectra for strong pointsources em itting in the 7.4-23 micron range. This helps intheir classification. For statistically measu ring the distributionof infrared sources in areas of high stellar density there is ashort wavelength (4.1-8 microns) channel photometer, alsoused with the main su rvey telescope.

In addition, another long-wavelength photometer map s theareas of high an d low infrared radiation in large, extendedsources by giving data on re lative intensities. This instrument

is not used wh ile the survey is in progress, but requires a spe-cific "pointed" observation mo de.

Surveying the Infrared Sky

Following a two-week check-out of its telescope and opera-tional systems, IRAS began history's first infrared survey of theentire sky on February 9, 19 83. The first scan recordedinfrared sources in the constellations Taurus and H ercules inthe northern sky and Scorpius and E ridanus In the southern,among others, as the satellite traced a circle around the Earth.

For the purpose of the survey, m ission planners have

divided the celestial sphere into overlapping banan a peel-shaped segm ents called "lunes." Lunes are the areas slicedout between two great c+rcles of ecliptic (as opposed to Earth)longitude, and they each cover 30° of the celestial sphere. Thetelescope scans a thin ( 1 h° field of view) segment in each o ftwo lunes on e very north-south orbit—first rising through on e,then descending through the corresponding lune on the otherside of the Earth.

As IRAS scans the sky, the faint radiation from astronomicalsources is focused onto the array of detectors at its focalplane. These detectors are arranged so that the energy fromany source strikes two of each kind of detector at a time. Thesame se gment of the sky is then re-scanned on the next orbitto give a total of four data readings for each source in each ofthe four infrared channels. Then, later in the survey, the tele-

scope returns to do another double scan of that portion of thesky, so that every area is covered at least four times (someeven more), and every astronomical source has a possibleeight detections overall in each IRA S waveb and. This repeti-tion is needed to make sure that the survey is thorough, andalso to prevent transitory objects—asteroids, comets, evenglints of moonlight—from be ing wrongly interpreted as infraredsources in deep space.

O RBIT GEO METRY

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The IRA S orbital path is closely aligned w ith Earth's sunrise/sunset line, and shifts about 1 degree each day to maintainthat position throughout the changing seasons. Solar panelsacquire sunlight while the telescope (protected by a sun-shade) points outward to dark space.

RIG INAL PAG EDR PHOTOGRA



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ORIGIN A L

COLORT O G A p HP

focal plane assembly includes 62 small rectangular Infrared Band Typ e of Detector Num ber Spectral RangeIR Band 1 Silicon Arsenide 1 5 8.5-15 micronsIR Band 2 Sil icon Antimonide 16 19-30 micronsIR Band 3 Germanium Gall ium 16 40-80 micronsIR Band 4 Germanium Gall ium 1 5 8 3 -119 microns

62 total

IRA S circles Earth 14 t imes a day, once every 103 minutes,arth and out at

nly about 60% of i ts t ime wil l be spent on thearth's Van

l len radiation belts, which dip below the alt i tude of the IRA Ssouth A tlantic ocean. Trying

sensitive survey in this region would be point-

s would create too m any

So during these periods, the IRA S telescope is focused onc astronom ical objects that mission planners h ave tar-

most interesting for observing in the infrared.f these addit ional observations—of unusua l gal-

ght infrared stars, and nebulae— are done e achthe turning Earth brings the south Atlan-

S, but also when the satel l ite passes over the

Twice each day IR A S radios scientif ic data down to Earth ason at R utherford Ap pleton

oratories in C hi l ton, England. Two onboa rd tape recorders

store the data while IRA S is surveying, then "dump" the pre-vious half day's data at a rate of one million bits per second atC hilton. Engineering information on satellite operations is alsosent down with the scientific data.

A t the sam e time, instructions for carrying out the ne xt halfday of the survey are radioed up to the IRA S comp uters, aftera team in England has determined that al l spacecraft opera-

tions and data are norm al. Only a prel iminary check-out of thedata is performed in England, whereas the ful l set of IRA S datais sent by com munications satel li te from there to the Jet Pro-pulsion Laboratory for extensive computer processing.

IRA S and Infrared Astronomy

A ccording to modern theory, a star is formed from a co olcloud of interstellar dust and gas that is somehow triggered tobegin condensing. The mutual gravitational pull of its particlescauses the cloud to contract further and heat up. E ventual ly itcol lapses into a hot, dense sp here, and internal pressures andtemperatures reach a point where nuclear fusion processesbegin. A star is born.

The unique contribution of infrared astronomy is that it can



detect these proto-stars long before they "turn on" in visiblel ight by sensing the heat they emit as they contract. Theprototype for this kind of object, the Bec klin-NeugebauerKleinmann-Low source in Orion. was discovered by infraredastronom ers in the mid-1960s The nebula is about ten timesthe mas s of the Sun, but has a tem perature of only 600°K(62.0°F) IRA S sho uld be able to locate proto-stars that aremuch sm aller—down to the size of the Sun—over much of thegalaxy.

Stellar "hatcheries" leave other infrared clues as well Veryyoung, hot stars, for ex ample, em it ultraviolet light that breaksup the atoms in nearby clouds of hydrogen and leaves the gasionized. Such clouds of ionized hydrogen are called H IIregions, and are believed to ma rk the sites of ongoing starformation. They also appear at all IRAS s urvey wavelengths.

IRAS will be particularly good at picking out these H 11regions throughout the galaxy. In addition, the IRAS spec-trometer should detect certain interstellar molecules, amongthem water and ammonia that appear widely throughout thegalaxy a s very cool clouds of dust and gas. These clouds ofmolecules also are thoug ht to be associated with the birth ofstars.

By detecting, identifying, and showing the distribution ofthese dusty proto-stars, H II regions, and clouds of moleculesthe IRAS survey will allow astronome rs to estimate the rate atwhich stars are forming in our galaxy, and by extension, othergalaxies in the universe.

At the waning e nd of a star's life cycle it runs low on its ownnuclear fuel and begins to redden and die. As it does so it maycough out clouds of material from the interior—heavy elements

formed from fusion processes deep inside the star. Dust canenvelop the fading star so thickly as to b lock its visible lightcompletely. Here infrared astronomy can contribute in twoways. First, since the dust is much m ore transparent at longinfrared wavelengths, astronomers are still able to see infraredlight from the fading star. Se condly, the surrounding dust isitself heated up by the star to re-radiate its own infrared light.Because of this, IRAS can take inventory of the stars dying inour galaxy.

The m aterial ejected from these fading stars in turn re-supplies the large clouds that will eventually become n ewstars. The dust is composed mainly of smoke-sized silicategrains, and these have c haracteristic signatures at specificinfrared wavelengths. By mapping the galaxy-wide distributionof this silicate dust, IRAS will be an important key to under-standing the ongoing na tural processes of stellar birth, death,and the recycling of matter.

Infrared also opens up an im portant region o r the sky nearlyinvisible to optical astronom ers---the center of our Milky Waygalaxy, located in the direction of the constellation Sagittarius.W ith its ability to see through the thick dust in the plane of thegalaxy IRAS can map previously unseen structure at its core,including further detail in the giant H II regions and m olecularclouds that have already been observed there at radio andinfrared wavelengths Infrared sources and structure in nearbygalaxies like the Mag ellanic Clouds will also be revealed inthe IRAS survey.

Perhaps most intriguing will be the information IRAS returnson certain unusual objects outside the Milky Way that astron-omers know already emit large amount e of infrared energy.

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Data from the first day of the IRAS telescope's operation show a 25 degree-long scan across the plane of our Milky Way galaxyat two infrared wavelengths—Band 4, centered at 101 microns (top), and Band 1, centered at 11 microns. Clearly visible in bothgraphs is the galaxy's central bulge. The lower band 1 readings reveal the radiation of hotter point sources that show up asspikes, whereas the top graph shows the more uniformly distr ibuted (and colder) dust and gas.



IRAS MINI-SURVEY

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Before IRAS began its survey of the whole sky, it first ran a "mini-survey" of ten orbitalscans (covering 5 degrees of the sky)

sothat data processing techniques could be refined. The shaded track shows the satellite's north-south path superimposed on anecliptic sky map, where 0 0 latitude is the plane of Earth's orbit around the Sun. Th e track is "stretched" at the poles in this flatprojection. Also visible are the plane of our Milky Way galaxy (lightly shaded) and a projection of Earth's equator.

Quasars and Seyfert galaxies, for example, pour out unex-pectedly large amou nts of energy for their size at all wave-lengths. Some galaxies are known to have particularly largeexcesses in the infrared, and the reasons why are not clear.The IRAS s urvey is able to catalog such unusual sources sothat their energy output in the infrared can be co mpared w iththat at other wavelengths.

Closer to hom e, the IRAS survey can observe many o bjectswithin our own solar system. Asteroids, especially, shouldappear in the survey by the thousands, and infrared data can

help to determine the ir reflectivity, their surface compo sition,and even their diameters . Also visible to the sensitive detec-tors is the zodiacal light caused by sunlight reflecting fromdust within the plane of the solar system. To map the distribu-t ion of this dust is to gain insight into ho w our planetary sys-tern was formed. The IRAS telescope avoids the bright planetJupiter, however, because brightness is heat, and heat boilsoff the telescope's cryogenic helium faster than the desiredrate.

Approximately one year after IRAS ends its survey in space,the Jet Propulsion laboratory plans to release the mission'sfinal products—maps of the infrared sky and a catalog of allmajor infrared sources, along with crit ical information on eachone. This inform ation will be available to the entire scientificcomm unity worldwide.

The IRAS m aps, when com pleted, wil l show a m uch dif ferentsky than o ur familiar array of constellations, a different skyeven than the one astronom ers know from previous infraredstudy. We can only speculate on the total number of newsources that will be discovered. It is even quite pos sible thatIRAS wil l discover whole new classes of astronomical objectswho se only signature is in the faint infrared radiation they em it.As we learn to open o ur electronic eyes to the many "colors"of the electromagnetic spectrum, we co me closer to putt ingtogether all the clues, to understand ing the great and deepcomplications of the universe.

Classroom Activit ies

1. Construct a three-dimensional model of the Earth, Moon,and Sun in spa ce. Calculate the angular size of the Earthas seen from the 900-ki lom eter-high IRAS o rbit. Then plana strategy for surveying the entire sky in the fewestpossible orbits, keeping in mind that the telescope m ustpoint away from the Sun, Mo on, and Earth, and remember-ing the restr ict ions against observing wh en the satell ite isover the south Atlantic.

2. Using the sources l isted in the bibliography below as wellas other articles published in scientific journals, make amap comparing the f i f ty brightest known infrared sourceswith the fifty brightest visible objects in the sky.

3. Perform the experiment by which Herschel discoveredinfrared radiation.

4. Write a report based on preliminary IRAS f indings pub-lished in such magazines as Science, Astronomy andAstrophysics, and Nature.

Suggested Reading

G. Neugebauer and R.E. Leighton. "The Infrared Sky." Scien-tific American, August, 1968. pp. 50-60

G. Neugebauer and E.E. Becklin. "The Brightest Infrared

Sources." Scientific A merican, April, 1973. pp 28-40C.H. Annett. "The Infrared Universe " Astronomy, October,1981. pp. 74-79

Allen, David A. I r ;!rared, the New Astronomy. New York,K. Reid. 1975

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Documents

NASA Facts IRAS - Mapping the Infrared Sky