Pioneer Saturn Encounter

8/8/2019 Pioneer Saturn Encounter

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TU l O l R

MCD C NNELL D OUGL A S

RE R H & EN INEERI N LIBR R YS T . L O U S

M19-/799 )


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FOR WOR

Th e Pion ee r 10 and 11 spacecraft , launched in 1972 and 1973 ,respect iv ely , were well nam ed: th ey mad e th e first cros sing s of th eas t eroid b elt and were th e f irst to e ncount er Jupit e r and it s int ens erad iation b e lts . Pion ee r 11 s traje c to r y, bent into a hairpin c urv e byJupit e r s powerful gravitational fi eld , a ll owed it to rec ross th e solarsys tem to make th e first flyby of Saturn almo st a billion mil es fromEa rth wher e it ca me within 13 ,300 mil es of th e clo ud to p s.

Ass embl ed in thi s publication is a selection of the pi c tu r esre turn ed by Pion ee r 1 1 of Sa turn and it s larg es t moon Titan . Th eseima ges are of grea t b ea uty as we ll as of great sc ientific inter es t , servin g to wh e t our ap pe ti t e for th e mor e detail ed observations to be

mad e by Voy age r in 1980 and 1981. Tracking of both Pion ee rs willco ntinu e for man y mo r e y ears , providin g fundam ental data on th ena tu r e of in t e rplan e tary space in th e d e pths of th e solar system. Th eres u lt s o f th ese ou t e r-planet Pion ee r missions hav e far excee d ed ourhop s a nd ex pec tation s of a d eca d e ago wh e n th e program wa sinitiat ed.

Rob e rt A Fros ch , AdministratorNational A e ronauti cs and Spac e Administration

Sep t emb er 1979

NI\S \Na tion a l Ae ro na ut ics a ndS pace Admin is tra tion

m e s R esea rc h e nt e rMoffe tt Fiel d a liforni a 940 35


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Artist s drawing of Saturn and its rings showing the Pioneer Saturn spacecraftpassing under the rings and nearing closest approach to the planet . Actual pic-tures could not be obtained t this time because of the high speed of thespacecraft.

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I N TR UC TIO N

We have ent ered into a new era of space exp loration . Missionsund e rtak en during th e lunar ex p loration of the 1960 s typicallylast ed a matt er of days with co mmands iss ued and carried ou t in nearrea l tim . Now, a decade la t er , planetary voyages may last for manyyears as th e spiraling traj ec tori es of th e spacecraft mak e periodicin t ers ectio ns with the orbits of the planets . Communicating with usacross the vastness of space, th ese spacecraft rep o rt to us th ei r experiences as th ey trav erse th e outer reac h es of th e solar system.

Amon g th ese deep space trav elers, Pion ee rs 10 and are ap propriately nam ed , for th ey truly are pione ering the ex ploration of th eouter solar system. Launch ed in 1972 a nd in 1973 , respec tiv ely , th ey

were th e first spacecraft to fly by Jupit er in 1973 and 1974). AtJupit er, Pion ee r II s traj ec tory was ca ref ully targeted to swing itto w ard Sa turn for an encount er n September 1979 . We see some ofth e ea rly resu lts in this publication.

Ot h er spacecraft are following along the trail blazed by Pion ee rSaturn . Vo yage r 1 passed by Jupit e r in Mar c h 1979 a nd will reac hSaturn in November 1980 . Voya ge r 2 has also passed beyond Jupit era nd will e nco un t e r Saturn in August 1981 , with th e further possibility of traveling on to Uranus a 1986 e ncount er). Under developmentare th e Galileo orbiter and atmospheric entry probe, d es tin ed tojourn ey to Jupit er where the orbiter will re turn mo r e detailedinformation , including hi gh-r eso lution pictures of th e Gali lea nsate llit es, a nd the probe will penetrate deep be low the Jovi a n clouds.

n th e coming years, eac h of th ese follow-on missions will e nrichour und ers tandin g of the solar system, grea t ly sup p lem entin g th eobservations of Pione e rs 10 and 11 . But one thin g will n eve r change.The Pion ee rs were first.

Thomas A Mu tchAssociate Administrator for Spa ce Scie nc eNational Aeronautics and Spa ce Administration

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PIONEER S TURN

Pioneer Saturn spacecraft.

Pion ee r Saturn has give n us our first clos e view of th e spectac ul arring ed planet Saturn and its system of moon s The spacecraft bega nit s journ ey to th e g iant planets Jupit er and Saturn on April 5 , 1973 ,as Pion ee r l l t reached Jupit er on Decem b er 2, 1974 , passingwithin 42,760 km of the Jovian cloud tops and taking th e only ex isting pictures of Jupit e r 's polar regions. Jupit er's massiv e gravitationalfield was used to swing Pion ee r 11 back across th e solar systemtoward Saturn. Additional man euvers were exec ut ed in 1975 and1976 to place th e spacecraft on a suitable trajectory , with thefinal aim point selected in 1977 .

From th e many possible targe ting options for th e first Saturnflyby , two aimpoints were considered, both of which would result ina near-equatorial flyby that would give th e b es t mapping of the highenergy particl es and th e magnetic field near the planet. The differ

ence between these two aimpoints, which came to be known as theinside and outside options, was their relationship to Sa turn' suniq u e ring system first discovered by Galileo in 1610 . The out-side option was finally selected because it was considered to be ofless risk to the spacecraft and more valuable in planning the subsequent encounter of Saturn by Voyager 2, which will reach Saturn in


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1981. Final ta r ge tin g was completed durin g ea rly 1978 , wh en a seriesof tim ed rock e t thrusts locked Pion ee r into th e d es ired traj ec tory .

On S ep temb er 1 , 1979 , th e spacecraft, now d esigna t ed Pion ee rSa turn , reache d Saturn aft er 6 yea rs in flight. t pas sed throu gh th erin g pl an e outsid e th e ed ge of Saturn s A-rin g and th en swung inund er th e rin gs from 2 ,000 to 10 ,000 km b elow th em. At the pointof clos es t approach, it attained a speed of 1 14 ,100 km /h(71,900 mi /h) and ca me within 21,400 km of th e plan e t s cloudto p s While it was approaching, encount erin g, and l ea vin g Saturn , th espacecra ft took th e first clos eup pictures of th e plan et , showin g

J U PITER DE CEMBER 1974

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S AT U RN S EP T EMB ER 1, 1979

Pioneer Saturn voyage.

Encounter trajectory.

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20 to 30 tim es more detail than th e best pictures taken from Eart h ,and mad e th e first close meas ur e me nt s of it s rin gs and several of it smoons , in c ludin g th e largest moon , th e plan e t-siz ed Titan. Titan ,along with Mars , has b ee n co nsid e red by many scien tists to be th emost lik e ly p lace to find life in th e solar sys t e m.

Pione e r Saturn unrav e led many myst e ries. t determined thatSa turn has a magnetic field and trapped rad iation belts , measuredthe mass of Saturn and some of its moons , and stud ied the characterof Sa turn s interior. It co nfirm ed th e presence a nd determined thema gnitud e of an in t e rnal h ea t source for Saturn. Its instrumentsstudied the temperature di s tribution , composition, a nd other proper-ties of the cloud and a tmospheres of Saturn a nd Titan , and tookphotometric and po larization measur e me nts of Iapetus , Rhea , Dion e,and Tethys. Pion ee r may also have discov ered a previously unknownmoon of Saturn . The spacecraft m eas ured th e mass , structure , andother cha r ac te ristic s of Saturn s rings, a nd passed safely through th e

outer E-rin g which posed a potential h aza rd for Pion ee r.t

a lsodiscovered new rin gs. On e of th ese rings , ca lled th e F-ring by th ePion ee r team, lies just outside th e A-ring. The gap between th eF -rin g and the A-ring has been te ntativ e ly designated th e Pion ee rDivision. Th e other new ring has been ca lled the G-ring, which lieswell outside th e F-ring.

Pioneer carries a scientific pay load of operating instruments ;another instrum e nt , th e asteroid/meteoroid detector, was turned offin 1975 . Two other exper im e nts , celestial mec hani cs and S-bandoccu lt ation of Sa turn , use the spacecraft radio to obtain data.Pion ee r Saturn is a spinning spacecraft , which gives its instruments afull-circle scan 7 .8 time a minut e. t uses a nucl ea r source fore lec tric power beca use the sunli ght a t Jupit e r and bey o nd is tooweak for a solar-powered system.

Two booms project from th e spacecraft to deploy th e nucl ea rpow e r source about 3 me t e rs from th e se nsitiv e spacecraft instrum e n-t a tion. A third boom posi tions th e ma gnet o me te r se nsor abou t6 m ete rs from the spacecraft. Six thrust e rs provide velocity, attitude ,and spin-rate co ntrol. A dish ant e nna is lo cate d a lon g th e spin axisa nd looks back a t Earth throu ghout th e mission, ad justing it s view bycha n ges in spacecraft attitude as th e spacecraft and Earth mov e inth eir orbits around th e sun.

Tr ac kin g facilities of NASA s Deep Sp ace e twork , lo ca ted at

Goldstone, California, and in Spain and Australia , supported Pion ee rSaturn during in t erplan e tary f1ight and e nco un t er. Pioneer s radiosignals, trav elin g at th e sp ee d of light, took 85 minut es to reachEarth from Saturn , a round-trip tim e of almost 3 hours , somewhatco mpli cati ng gro und co ntrol of the spacec raft. Almost 10 ,000co mmands were e nt to th e spacecraft in th e 2-week period b efore


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clos est ap p ro ach . Continu ed co mmuni cations should b e possibl ethrou gh at leas t th e mid 1980 s.

Af t e r th e spac ec ra ft p asse d Saturn , it hea d ed out of th e solar sys-t e m , trav elin g in th e di r e ction th e solar sys t e m mov es with r espec t toth e lo ca l stars in our ga laxy and in approximat e ly an opposite direc-tion from it s s is te r spacec raft, Pi o nee r 10 . Bot h spacec raf t hav eplaques atta ch ed to th em which contain a m essage from Earth fora ny in t ellige nt spec ies th a t m ay in t ercep t th e spacecraft durin g th e ire ndl ess journ eys throu gh in t e rs t e llar spa ce.

Pi o nee r S a turn Sc ie ntifi c I n strum e nt s

Instrument Prin cipal Invest iga to r

Helium vector ma gneto met er Edward 1. SmithJet Pr opulsi on Lab o rat o ry

Flux ga te magne to mete r Mario Acun aG oddard Spa ce Fligh t Cen ter

Plasma anal yze r Jo hn H. Wolfe

Char ged particl e

Cosm ic ray te lesco pe

Geiger tube te lescope

Tr app ed radiation de tec tor

Ame s Re searc h Ce nter

J o hn A. Simps onUni vers it y of Chi cago

Frank B. McDona ldG oddard Spa ce Fli ght Cen ter

James A. Van AllenUn iversity of Iowa

R . Walker Filliu s

Unive rsity of Ca li forn ia , San DiegoAst eroid / mete oro id de te c to r Robert K. Sobe rman

Me teoro id de te c to r

Ra dio tr ansm it t er a nd DS

Ult raviolet ph o tom eter

Im aging ph o topo lari mete r

In fra red radi o me te r

Rad io transmitter and DS

o t currently o perat iona l.

General Electri c Co. a ndPhiladelphi a Drexe l University

William H. Kin ardLan gley Resea rch Center

Jo hn D. Anders o nJ et Pro pul sion Laborato ry

Darrell L. Ju dgeUni ve rsity of South e rn Ca l forniaLos An ge les

T o m Gehr els

Universit y of Ari zo na, Tu cso nAndrew In ge rso llCa li forn ia In stitut e of Tec hn ology

Arv y das J . K io reJe t Prop ulsio n Lab o ra tory

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Exper iment Obje ct ive

Ma gnet ic fie ld s

Magnetic fie ld s

Solar plasma

Charged particleco mposition

Cosmi c ray ene rgyspe ct ra

Charged pa rticl es

Trapped radiation

Aste ro id /met eo ro idas tr ono my

Meteo ro id dete c ti on

Ce les tial mec hani cs

Ultrav iolet photometry

Ph o to imag in g and

po lar imetryInfrar ed th ermalst ru ctur e

S-b and occ ult a ti o n


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S TURN IM GES

Sa turn , ca lled th e wond er of the h ea vens by ea rly astronomers h asbee n st udi ed from Ea rth for man y ce nturi es. h e n Galileo f irstfoc used hi s telesc op e o n S a turn in 1 6 10 , h e rea lized that th e appear -ance of th e plan e t was unu sual, but h e neve r kn e w it s rea l ch aracte rbeca use th e pow e r of hi s ho m em ade te lesc op e was far too low. H ethou ght he was loo k in g a t th r ee glob es, on e large a nd two small ,whi ch seem ed to chan ge slo w ly in app ea ranc e . In 1 655 , Hu y ge ns ,after years of obs e rvin g th e plan e t , fina lly rea liz ed th at th ese proj ec -tions w ere actua lly a fla t rin g slightly separated from th e main glob e .

In 1 675 Cassini found th e first br eac h in th e s uppos edl y so lid ,rigid, and opaque ring wh en h e discov ered th a t it w as divid ed into

First sketch of Saturn Calileo, 1610).

First sketch showing a division in the rings Cassini , 1675).


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First successful photograph of Saturn (Andrew Common, 1883) . (Discoveringthe Universe , Colin A Ronan , Copyright 97 by Colin A Ronan, Basic Books ,Inc. , Publisher s, New York.)

two parts by a dark line, now known as Cassin i s Divi sio n . In l ateryears he a lso detected some of Saturn s moonsThe ea rli es t successf ul photograp h of Saturn was tak en in 1883 by

Andrew Common. In 1895, Jam es Ke ele r suggested that th e rings arein fact a swarm of particles in n ea r-ind epe nd ent orbits . These rin gs,until the recen t discoveries of faint ring systems aro und Jupit e r andUranus, were considered uniqu e in th e so lar system.

Sinc e Galileo fi rst used his hom e made t elescope to v iew Saturn,th ere have been many observers. Th ere have also be e n majoradva nces n te lescopes; a resulting modern view of Saturn is shownon the next page . Th e correspond ing sketch shows th e nom e ncl a tu r eof th e brightest rin gs

Now th e p lanet has been see n for th e first time not from Ea rt h ,but in much closer views by an instrument on a sp acec raft , th eim aging photopolarimet er on Pion eer Sa turn . Th e instrum ent separa t ely measures th e s tr e ngths of th e red and blu e co mpon e nt s of sunlight sca tt ered from the clouds o f Sa turn a nd converts thi s inform ation nto numb ers. Th e data are transmitt ed to Earth as part of th espacecraft te le m et ry. Th e signal s are th en converted by co mput erinto shades of gray on photographic film , and th e two compon ent splu s a sy nth esize d gree n ima ge ca n b e rec ombin ed into a color imag ethat app roxim ates th e planet s tru e co lor. Som e of th e res ultin gim ages a re shown on th e pa ges followin g th e Ea rth-bas ed view. Th esepictures were produced by a scientific tea m from th e Univ ersity ofArizon a .

Th ese ima ges, whil e h elpin g to unrav el some of th e myst ery surrounding th e plan et , hav e created eve n mo r e in t eres t r ega rding it . Weare rea lly just b eginnin g to know Saturn. t is up to futur e spacecraftto mor e c ompl e tely revea l her secrets a nd so lve h er myst eries.

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Hi gh-qu alit y c on t em po ra ry a rth-b ase d view o f Sa tu rn Ph oto a ta lin a Ob se r-vato ry , Uni ve rsity of Arizo na) .

O UT E R A) RI NG MIDDL E 8 ) RI NG

C R E P E C) RI G

Nomenclatur e of bri ght rin gs.

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Pioneer Saturn image of Saturn and its rings from a distance of 8 ,4 , kmAugust 22 , 1979 , 1 days before closest approach). Resolution has not yet

reached Earth-based quality. Faint banding is visible on the disk of the planet .The silhouette of the rings can be seen in front of the planet. Slightly abovethis silhouette is the shadow of the rings on the disk. Beyond the disk , structure can be seen in the rings . The rings have a distinctly different appearance inthis and subsequent images than in Earth-based pictures because they are illuminated from below rather than from above s we view them from Earth . TheA-ring and C-ring are bright , and between them the B-ring is dark. The CassiniDivision at the inner edge of the A -ring is also bright , but is blended with thering at this distance.

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Pioneer Saturn image of Saturn and it s ring s from a distance of 5,500,000 km(August 26 1979 , 6 days befo re clo ses t approach). Re so lution of fea tu r es isbeginnin g to be approximately eq ual to that of Earth-based pictur es. Belt s onthe planet are beco min g mor e distinct , and considerable s tructur e can be see n inthe rin gs Th e sma ll blu e spot at the bottom of the planet is du e to incompl etedata and will be co rrecte d b y further processing . A no t ch appea rs a t the top left

of the planet , which ma y be a data tra nsmi ss ion problem or the shado w of oneof Saturn s moon.

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Pione er Saturn ima ge of Saturn and it s rin gs from a di stanc e of 2 ,800 ,000 kmAu gust 29 , 19 79 , 72 ho ur s befor e clo ses t ap proach ). The a ss ini Di vision a t th e

inn er edge of th e A-rin g is clearl y res olved and i s bri ght Th e A-ring i s dark outsid e th e a ss ini Divi sion b ecau se it ha s mo r e parti culat e matt e r th ere . Pol ar b e lt sare b ec oming m o re v isibl e on th e fac e of th e plan e t. Irr egul a riti es that app ear inth e rin g s ilh oue tt e a nd shado w a re du e to steppin g anomali es in th e im ag ingph o t op o la rim e ter a nd will b e re mo ved in furth er proce ssin g. Th e sm all ro undim age t ha t app ear s abo ve th e plan e t is th e moon Titan .

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These pictures of Saturn and its rings were taken by Pioneer Saturn from a distance of 2 ,500 ,000 km (August 29 , 1979 , 58 hours before closest approach).The imaging photo polarimeter gathers data using the red and blue componentsof the light reflected from Saturn. These two views are from the two color components (upper, blue ; lower, red). The banded structure on the planet s particularly evident n the upper image . Because the spacecraft s nearing the planet , therings are partially outside the field of view of the instrument. The speck of lightbelow the planet s Saturn s moon Rhea , which is 1450 km n diameter , bout

one-half the size of Earth s moon.

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This image was produced by combining the two images on the facing page,adding a synthesized green component, and adjusting the intensity of each toobtain an approximation to Saturn s true color. (The same process was used n

ll of the Pioneer color images of Saturn shown here.) Although not evident onthis reproduction , scientists believe that, from detailed study of both the uncombined and the combined images , they can begin to see evidence of j t streams inSaturn s upper atmosphere. The slight blue edge is an artifact from thecomputer-enhancement procedure.

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Pioneer Saturn view showing the structure of Saturn s ring system n detailnever before seen . The image was taken from a di stance of 943 , km

(August 31 , 1979 , about 44 hours before encounter). The moon Teth ys, s een atthe top of the image , s 1200 km n diameter. There i s a very faint unidentifiedSaturnian moon at the lower right , just off the tip of the bright A-ring (ma y notbe visible n this print). The newly defined F-ring appears faintly just outside thebright edge of the A -ring. The region between the A-ring and the F -ring has beententatively named the Pioneer Division. This print has been processed to enhancede tail of the main rings.

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A color print of a version of the same ima ge s shown on th e facin g page. Th eblu e dot t the outer edge of the C-rin g is an artifact. Thi s ima ge h s not bee ncomputer processed to th e sa me exte nt s th e f cin g im age. Teth ys for exa mple is only f intl y visible a nd th e F -ring c nnot be see n.

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Close-up image from Pioneer Saturn of Saturn from a distance of 400 ,000 Ian(September 1 , 1979 a few hours before closest approach). Inset shows the locationof the image on the planet. The part of the disk shown is about 25 ,000 by70,000 km. The sawtooth pattern is an artifact. The vertical stripe on the disk isdue to a gain change in the instrument. The image has not had its final correctionsfor shape. The rings and their shadows cut diagonal swaths across the image with theupper swath being the shadow . The rings , seen from the unlit side , are visible inthe foreground. The ring shadow shows clearly that Saturn s rings have two major divisions : the CassiniDivision dividing the outer A-ring from the middle B-ring, and a second division (previouslycontroversial) dividing the B-ring from the inner Coring These divisions show as parallel pinstripes in thebroad black band of the rings shadow , with the upper stripe being the Cassini Division. Some shearing inthe bands and belts on Saturn s disk is beginning to appear , although the low contrast on the planet(due to it s high-altitude haze) doe s not make this highly evident.

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Post-encounter image of Saturn from a distance of 850,000 km (September 2 ,1979 , 15 hours after encounter) . As planned, during the encounter Saturn sgravitational field turned the spacecraft s trajectory behind the planet and outtoward the edge of the solar system at approximately a right angle to theinbound trajectory. Thus, the planet is now illuminated from the side, and theview is of a crescent Saturn with the terminator on the right of the pir.ture. Therings appear dim when compared with the previous inbound pictures because ofthe different angle between the sun, the rings , and the spacecraft. When this picture was taken , the spacecraft , s seen from Earth , was about to pass behind thesun, and solar activity was interfering with spacecraft communications. Theimage quality, therefore , is not as good as for pictures taken inbound. With thisfarewell image from Pioneer , Saturn waits for the Voyager spacecraft in 1980and 1981.

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This image of cloud-


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S IEN E HIGHLIGHTS

Pio nee r S a turn h as a lr ea d y g rea tl y e xp a nd ed our kn o wl e dge o fSa turn , it s rin gs a nd m oo ns. We no w kn o w th a t S a turn , in m a nyways, rep rese nt s a n in t e nn edi a t e case be tw ee n Jupit e r , th e larges tp la ne t in th e so la r sys te m , a nd Ea rth . Th e co mp os iti o n o f Sa turn sin t e rior i s esse nti a lly th e sa me as Jupit e r s, di f ferin g o nl y in th e sizeand ex te nt of th e vari o us in t ern a l l aye rs. Meas u re m e nt s fo r S a turna re co n sis te nt with a ce nt r al co re of molt e n h ea vy e le m e nt s (p ro babl y m os tl y ir o n) whi ch is th e appro x imat e size of th e e ntir e Ea rt h ,but ab o ut thr ee tim es mo r e m ass ive. Surr o undin g th e ce nt r al co re isa n o ut er co re of hi ghl y co mp r esse d ho t , liqu ef ied vo latil es su ch asm e than e, ammoni a, a nd wa t e r. Th i s o ut er cor e is equiv ale nt to

ap pr ox im a t ely nin e Ea rth m asses . Th ese c or e reg ion s, ho w eve r ,rep rese nt a very s m a ll fr ac tion of th e p lan e t , whi ch i s compo sed prim aril y o f th e very light es t gases, hydro ge n a nd h elium , and is almo st100 tim es th e m ass o f th e Ea rt h . Beca use of th e hi gh pr ess ur e inSa turn s in t e rio r , th e h y dr o ge n is tr an sfo nn ed t o it s liquid m e tallicst at e. Ab o ve thi s m e t alli c hydro ge n sh e ll ar e liquid mol ecular h y droge n a nd S a turn s gase ous a tm o sph ere a nd cloud s whi ch m a ke up th eres t o f th e pl an e t.

El ec tri ca l cur r ent s se t up within th e m e talli c hydro ge n sh e ll pro-du ce Sa turn s m ag ne ti c fie ld , whi ch w as m eas ur ed b y Pion ee r. Insp it e o f S a turn s la rge size, th e m ag ne ti c f ie ld a t th e cl oud t o p s isonly slight l y wea k e r than th e fi e ld a t th e Ea rt h s surfa ce. Sa turn isuniq ue in th at it s m ag n e tic axis is nearly ali gned with its rotationaxi s, unlik e E arth and Jupit er.

Sa turn s ma gne ti c field i s a lso mu ch mo r e regul a r in shap e th an th efi e ld s of th e ot h er pl a ne ts . t large di sta n ces from S a turn , th e m agne ti c f ie ld i s de form ed b y th e inw a rd pr essur e of th e sol a r wind .Nea r th e noon m e ridi an (clos e to th e inbound Pion ee r tr aj ec to r y ) ,th e sol a r wind ca uses a c ompr ess ion of th e field ; in th e dawn m eridia n (cl ose to th e o utbound tr aj ec to r y ) , th e f ie ld is swept b ac k andp resu m abl y fo nn s a lo n g m ag n e ti c ta il. In both ca ses, Pion ee r S a turncro sse d th e o ut er bound ary of th e ma gne ti c fie ld se veral tim es as th efield m o ved in a nd o ut , resp ondin g to ch a ng in g sol a r wind pr ess ur e .

Pi o neer also o bse rve d inw ard and out wa rd b o un d aries .Th e m ag n e ti c e nv e lop e s urroundin g Sa turn is in t e nn ediat e in size

a nd en erge ti c pa rti cle population be tw ee n th ose o f th e E a rth andJupit er, t he o nl y tw o o th er p lan e ts kn o w n t o b e str o ngly mag neti ze d . Th e th r ee ot h er pl a ne ts inv es ti ga t ed th u s far (V e nu s, Mars, a ndMercur y ) a nd Ea rt h s moon hav e littl e or no m ag n eti sm . Virtu ally all

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ou r knowl e dge of Sa turn s ma gne ti c e nviro nm e nt h as b ee n obtainedby Pion ee r. Th e spacec raft found rin gs of particu late ma t e ri a l andsevera l sm a ll moon s nea r th e rin gs, whi ch s tr o ngly a ff ec t S a turn strapp ed radiation. Th ese feat u res prov id e im porta nt dia gno sti c capabiliti es . A uniqu e finding is th e nea rly total abse nce of radi at ion be ltpa rti cles a t d istan ces c loser to th e p la ne t th a n th e outer ed ge ofth e visibl e rin gs.

Th e in n e r r eg ion of th e thi c k ma gne ti c e nv e lop e of Sa turn , ca lledth e m ag n e to sp h ere, co ntain s trapp ed hi gh -e ne rgy e lec tron s and proto n s with so m e ev id e nce fo r h ea vie r nu cle i. Th e ove ra ll form of th ema gne tosph e re is simple and co mp a c t , mo r e s imil ar to that of Ea rththan of Jupit e r. Th e uniqu e m eas urin g capabi li t ies of th e Pi onee rSaturn radiation d e tec to r s le d di r ec tl y to th e discov ery of a diffusene w rin g of particulate matt er in th e reg ion from about 10 to15 planeta ry radii I Saturn rad iu s = 6 0 ,000 km from Saturn . Thisrin g ha s b ee n t e ntativ ely des ignated a th e G -rin g . Th e G-rin g c lea rl y

causes pa rticl e a bsorption n ea r th e e quatorial reg ion. Mo r eo ver ,Pi oneer dis cov ered ano th e r reg ion insid e 7 or 8 plan e tary radii inwhi ch th e radia tion b e lt pa rticl es we re subj ec ted to a strong loss o rabsorpt io n , p res umably caused by th e presence of an ex te nsiv e cloudof plasma corotati n g with th e plan e t.

In sid e abo ut 10 plan e tary radii , th e tr apped radiation sho w s a highd eg ree of ax ia l symmetry aro und Saturn and is co n sistent with th ece nt ered dipo le m ag n e ti c field observed by Pion ee r. Saturn s ringsan nihilat e all trapp ed radiation at th e out er ed ge of th e A-rin g, lea vin g a sh ield e d r eg ion clos e to th e plan e t in whi ch th e radiation inten-sity is th e lowest so far en co un t ered in this mi ssion. This shie ldin gp reve nts th e furth er buildup of e lec tr on in t ensiti es a t low e r altit ud es,which ot h erwis e would h ave b ee n present a nd would h ave m adeSaturn a strong radio sou rce observab le from Earth.

Pione er fou nd that several of Saturn s moons absorb trapp ed p a rticl es from th e radiation b elt s , prod ucin g promin e nt dips in th ein t e nsity. Th e effec tiv e ness of absorption a t th e moon s T e th ys andEnceladus is parti c ul a rl y as tonishin g, a nd suppo rts th e id ea thatradia tion belt particl es are drifting inw ard slowl y across th e moon s o rbits.

A precipitous dec rease in pa rt i cle in t e nsit y, las tin g only for about12 sec onds , w as obs e rved over a wid e rang e of e nergies for both pro-tons and e lec trons at a dist an ce n ea r 2.53 Saturn radii , 23 minut es

a ft er Pioneer cros sed th e Sa turn rin g plan e inbound . At a bout th esame tim e, an an om a ly wa s also observed in th e m ag n e tic fi eld measu re m ent s. Th ese phenomena h ave been te ntativ ely in t e rp r e t ed asindi ca tin g th e presence of a nea rb y ma ss ive body absorbing th etr app ed radiation and p e rturbing Saturn s ma gn e ti c fi eld . Th e es tim a ted radiu s of th i s object lies in th e ra nge of 100 to 300 km ,

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based o n th e e f fec tiv e ness with whi c h it ab so rb ed th e hi gh-e ne rgyradi a ti o n. Th e to tal rad iat io n dose rece ive d a t S a turn was eq ui va le ntto o nl y 2 m inu t es in th e J ov ia n rad ia ti o n be lt s beca use Sa turn sradi a ti o n b e lt s we re so mu ch wea k e r .

In ad diti o n t o im ages of Sa turn , th e bri ght n ess, co lo r , a nd po la riza ti o n of th e re fl ec t ed light we re a lso m eas ur e d b y th e im ag in gph o t o polarim e te r o n Pi o nee r S a turn . Th ese m eas ur e m e nt s a re use dto s tu dy th e cloud lay e rs of Sa turn a nd Ti t a n a nd to m od e l th eve rti ca l s tr u c tu r e of th e a tm osp h e res o f th ese two bo di es. In th esca n s th a t m a d e th e im ages , th e ba nd ed s tru c tu r es o f S a turn and ofth e rin gs we re o bt a in ed in f in e de ta il. Th ese a re esse nti al in stu dy in gth e a tm osp h e re, rin gs, a nd m o on s. A n ew S a turni a n rin g , whi ch h asbee n t ent a tiv e ly d es ig na ted t h e F-rin g was di sco vered in th e im ages.t is na rr ow e r th an 5 00 km in w id th , but is imp o rt a nt b eca use it

fo nn s a n o ut sid e ba rri e r to th e b right A- and B-rin gs. Th e ga pb e tw ee n th e F - an d A-rin gs h as b ee n d es igna ted th e Pi o nee r Di visio n

b y th e Pi o nee r t ea m . A sm a ll mo o n , whi ch e ith er w as p rev io uslyun k now n o r h ad bee n p rev ious ly di scove red fro m Ea rth b ut los taga in , was fo und in th e Pi o nee r S a turn im ages. Aft e r it s initi a ldi sco very, thi s new m oo n co ntinu ed o n it s 17-h o ur o rbit a ro und th ep la net a nd passed nea r Pi o nee r as th e spacec ra ft e nt e red th e rin gsys t e m . t is q ui t e co nce iva bl e th a t thi s m oo n is th e sa m e o ne th a tpe rturb ed th e radi a ti o n b elt p a rticl es and pr o du ce d th e an o m aly inth e m ag ne ti c f ie ld m eas ur e m e nt s.

In f ra red obse rva ti o n s o bt ain ed durin g th e Sa turn fl y by revea ledth e tem pe ra tu r es in th e a tm os ph eres o f S a turn and th e rin gs and inth e at m osp h e re of Ti t an . t was fo un d t h a t S at u rn h as a t em pe ra tu r eof ab o ut 100 K (a bo ut 28 0 F be low zero a nd , acco rd in g t o th eseo b se rva ti o ns , h as a n in t ern al h ea t so ur ce o f e no ugh s tr e ngth th a t th ep la ne t emi t s a pp r ox im ate ly 2. 5 tim es as mu ch en ergy as it abso rb sfro m th e su n . Th e eq ua to ri al ye llow ish b a nd o bse rva bl e in m a ny ofth e im ages w as fo und t o b e seve ra l d eg rees co ld e r th a n th e pl a ne t a to th e r la titud es a nd i s p ro ba bl y a zone of hi gh clo uds rese mblin gsi mil ar zo nes o n Ju p it e r. A s expec ted, th e rin gs we re ex tr e m ely co ld,6 5 t o 7 5 (a b o ut 3 30 F b elo w ze ro ) , at th e tim e o f e nco un t e r. Th ete m pe ra t u re d iffe re nces be twee n th e illumin a ted a nd u nillumin at edsid es of th e rin gs, a nd th e ra te of co o lin g as t h e rin g pa rti cles go in t oSa turn s s had o w , sugges t th a t th e rin g pa rti c les a re a t leas t seve ralce n t im e te rs in d ia m e te r an d th e rin gs th e m se lves a re m an y pa rt ic le

dia m et e rs t hi ck. Th e ve ry min o r pe rturb ati o n to Pi o nee r s tr ajec to ry,as it p assed un d e r th e vi sibl e rin gs, indi cates th a t th e rin gs p ro babl yco nsist of ices.

As Pi o neer passed th ro ugh Sa t urn s rin g sys t e m , ve ry se nsiti vem e teoro id d e tec t o rs o b serve d th e im pa c t of five pa rti cles on th espacecra ft , pa rticl es th a t we re ab o u t 10 mi c ro m e te rs ( 0 .00 0 5 in ch )

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in d iam e te r. Two imp acts occurred while th e sp acec raft was aboveth e rin gs and th r ee whil e th e s pa ce c raft was b elow th e rings. N oimpa ct s we re d e t ec ted go in g through th e rin g pl an e, but th e Pion ee rin strument ca nnot de t ec t individu al impa c ts that occur le ss than77 minut es apart. This chara c te ristic would h ave pr eve nt ed d e tec ti o n

of rin g p a rticl es because of th e im pacts d e tec t ed ju st b efo r e both rin gp lane cros sin gs. t is un ce rtain wh e th er th e mi crometeo roids d e tec t edby Pion ee r S a turn were s tr ay rin g pa rti c les d e fl ec ted out of Sa turn slin g p lan e or wh e th e r th ey were p arti c les from in t e rplan e ta ry s p acedrawn inw a rd to w a rd S a turn by it s s tron g g ravit a tion al fie ld .

Clos e to th e point of clos es t approach to Saturn , th e spacecraft sradio transmissions w e re affec ted by Saturn s ionosph ere . Th em a nn e r in whi ch th e radio signals w e re abso rb ed indicat es thatSaturn has an ex te nsive ionosphere compos ed of ioni ze d atomich ydr o ge n with a t e mp e ratur e of about 1250 K in it s up p er r eg ions .This hi gh t e m pe ra tu r e req ui r es an ex te nsiv e e n ergy so ur ce other than

th e s un. Th i s phenomenon was also observed a t Ju p it e rPion ee r m eas ur ed ultraviol e t glow throu ghout the Saturni a n sys-

te m. This ultr av iol e t glow is due to th e sca t t e rin g of th e light fromth e s un b y a tomi c hydro ge n. Th e observations of ultr av iol e t e missionfrom an ex t e n sive c loud of h ydr o ge n gas s urroundin g Saturn s visib lerin gs are es pec ially in t e res ti n g . Th e rin gs th ems e lves ar e pr es um ab lyth e so urc e of thi s hydro ge n On th e pla ne t s disk, th e ultraviol e tobservations show significant latitud e variations , suggesting th e possi-bility of aurora n ea r S a turn po lar r eg io n s A similar ex t e nsive c loudof h y dr o ge n wa also seen partia lly surrounding Titan s orbit.

Th ese very preliminary f indin gs by Pi o nee r S a turn represent on lya s m a ll fraction of wh at w ill ultimat ely be lea rn ed ab o ut Sa turn andit s e nvironm e nt as th e spacec raft data are a nal yze d in grea ter d e ta ilover th e weeks and months ahead.

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MAGNETOSHEATH

SOLAR WIND

BOW SHOCK

Schematic of the solar wind interaction with Saturn s magnetosphere. The solarwind arrives from the direction of the sun, s deflected t Saturn s bow shock,and flows around Saturn in the magnetosheath (orange region), as indicated bythe arrows. The sizes of the magnetosheath and radiation belt s change inresponse to the external solar -wind pressure, becoming smaller when the externalpressure s larger and vice versa.

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Diagram of Saturn s inner trapped radiation belts. The energetic particle fluxesgenerally become more intense closer to Saturn. Decreases n particle flux at thelocations of Saturn s moons are due to the sweeping up of the energetic particlescaused by particles striking the moons and being absorbed by them. Also , thereare decreases n particle flux at the outer edge of the rings, where the energeticparticles are also absorbed, so th t a region free of trapped radiation s createdfrom the oute r edge of the rings to Saturn.

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Infrared radiometer image of Saturn and its rings. Brightness in the image isrelated to temperature , with the brightest areas at about 100 K (about -280 F) .The left version shows contrast in the colder regions (the rings). The rightversion shows contrast in the warmer regions the planet) . The image containsmany separate scans from top to bottom. Each scan is displaced to the rightfrom the one before by the motion of the spacecraft. The spacecraft was belowthe ring plane during most of the 3-hour observation period and was much closerto the planet at the end of the period (right of image) than at the start (left ofimage). Thus , the images are quite distorted. The small-scale pattern is instrument noi se. In the left image , the warm infrared radiation from the planet i s seenthrough Cassini s Division between the A and B-rings . The brightness level in theright image implies that Saturn emits heat at a rate that is 2.5 times the rate thatit absorbs energy from the sun.

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U TW R U N

oAfter in creas ing our knowl edg e of th e Saturni an sys t em manyfold

th e spacecraft is now ou tward bound t ca rri es a m essage e ngrav edon a plaqu e from Ea rth to any inh ab it ants of anoth er star sys t emwho mi ght discov er th e spacecraft. With its journ ey far from over Pion ee r Saturn tr ave ls on - to th e outer reac h es of space.

Th e Pion ee r Proj ec t is manag ed by th e Am es Resea rch Centerfor th e National A eronautics and Spac e Administration . Th e spacecraft was built by TRW Syst ems R edondo Beac h Ca li fornia.

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Pioneer Saturn Encounter