Voyager Journey to the Outer Planets

8/8/2019 Voyager Journey to the Outer Planets

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A01; S O D H C Lab.) 18 p MFC S C L 2 2 AN77-332

OnclasG3/15J1979!

Voyager

fVJASANational Aeronautics andSpace AdministrationJet Propulsion LaboratoryPasadena, California

Journey to theOuter Planets


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,

I n t r o d u c t i o nW h e n Ga l i l e o Gali lei looke d th r ough h is f irs t p r i m i t i v e te lescope in 1610, the surpr i se that greeted h i m f o u r

h i t h e r t o u n k n o w n m o o n s o r b i t i n g J u p i t e r rocked th e world an d started a new r e v o lu t ion in scienti fic i n q u i r y t h a tcontinues today.

M e n had wondered for m i l l c n U about the stars that lit the n i g h t s k y . They pu/y.lcd over a few objects, pinpointsof l i g h t that didn't obey the rules, t h a t meandered among the fixed stars, r e f u s i n g to settle down in regular orbits. Mennamed them "planets" or "wanderers." Vi o l en t arguments even i n qu i s i t i o n s raged over the question, what werethese wandering stars? /

B ut each ne w c e n tur y f o u n d m e n t a k ing s m a l l , p a i n f u l s teps a long th e road cal led technology. N e w i n s t r u m e n t swere invented, f i rst to m e a s u r e the posi t ions of the wanderer s ; then , a t las t , to magni fy the ir im ages . Man real i/ed thewa nde r e r s we r e o th e r wor lds , p e r h a p s l ike h i s own.


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ID a w n of the Mission

As ear ly as Augus t 2 0 , 1977, soon a f t e r t h e S u nblazes out o f the At lant ic Ocean of f F lor ida ' s space coas t ,m a n - m a d e f i re an d thunder wi l l rattle th e pa l m e t t o s c r u b asth e f i rs t o f t w o Voyager spacecra f t r ides it s T i t a n - C e n t a u rl a u n c h ve h i c l e o n t h e f i r s t l e g o f a l o n g jo u r n e y t o Ju p i t e r ,S a t u r n , and b e y o n d , a vo ya g e to the outer plane ts .

T h e N a t i o n a l A e r o n a u t i c s a n d S pa c e A d m i n i s t r a t i o nwill send the two s pa c e c r a f t to advance our knowledge ofth e outer solar system Ju p i t e r , S a t u rn , t h e i r satellites,and pe r h a ps e ve n t h e f a r t h e r r e a c h e s o f space.

T h e s pa c e c r a f t w i l l b e l a u n c h e d i n th e l a te s u m m e r o f1977, and will f ly past J u p i t e r in 1979. Using the b igp l a n e t ' s i m m e n s e g r a v i t y to boost t h e m on t h e i r w a y , theV o ya g e r s will b e acce le ra ted toward Sa turn , reach ing th eringed plan et in 1980 an d 1981. Th ere is also the pos sibilitythat o n e o f t h e s pa c e c r a f t will visit Uranus in 1986.

A n d t h e n w h a t , b e yo n d S a t u r n ? B o t h s p a c e c r a f t w i llc o n t i n u e f a r t h e r f r o m th e S u n , p r o b i n g , s t u d y i n g an ds ea r c h i ng as they go . Far f r o m E a r t h they w i l l pe n e t r a t ein to ga lact ic space , beyond the in f lue nce o f the Sun, wh eret h e y will cruise f o r e t e r n i t y .

Th e Mission PlanTw el v e d a ys a f t e r th e f i rst l a u n c h , th e e a r l y m o r n i n g

sc ena r i o will be repea ted . This Voyager , however , wi l lsp r i ng af te r i t s ea r l ie r companion unt i l i t over takes andpasses it . This second- launched c ra f t , Voyager 1 , will arrivea t J u p i t e r f o u r m onth s be fore i t s m ate , and a t Sa turn wi l lb e n i n e m o n t h s a h e a d . T h e s lower , f i r s t - launched c ra f t ,V o y a g e r 2, may be ta rge ted on pas t Sa turn to exploreU r a n u s , i f i t s predecessor Voyager 1 has per f orm ed w el l.

Following in the f o o t s t e ps of the earlier recon-naissance of the P i o n e e r 10 and 11 miss ions , each Voyagersp a c ec r a f t will c o n d u c t 11 separa te sc ience inves t iga t ions ,p e r f o r m i n g f o r t h e i r h u m a n masters a t h o r o u g h ph ys i c a le x a m i n a t i o n of two plane ts , at least 11 satelli tes , the r ingsof Saturn , and in te rplane ta ry space .

The Voyagers wi l l begin the i r sc i en t i f i c studies soona f t e r l a u n c h . B o t h s pa c e c r a f t will a im t h e i r i n s t r u m e n t sb a c k t o w a r d th e r a p i d l y d i m i n i s h i n g E a r t h ; V o ya g e r 1 m a ypass over the Moon 's nor th pole and ge t a qu ick g l impse o fo u r o w n recent ly vis i ted sa te l l i te .

As mariners have done over the centur ies s ince theytook to the sea, these newes t wi l l sample the wind thist i m e t h e solar wind tha t s t reams a supersonic ga le ofcharged par t ic les f rom th e S u n . I n s t r u m e n t s w i l l s a m pl ethe s olar wind constantly and abou t every 50 m i l l i o nm iles, each Voy ager will spin slowly to scan the skyin all direc t ions .


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A n d , l ike sailors, th e Voyagers will thread their w a ypast interplanetary reefs and shoals, as they cross theasteroid belt, a wide band of rock and space debris betweenthe orbits of Marsand Jupiter.

e f f o r t s of centuries. Both Voyagers will f ly past th e equa-torial regions of Jupiter, and one will pass beneath thesouth pole of Saturn.

Each Voyager will pursue its own unique trajectory.Fl igh t planners ra n computer simulations f o r many possiblef l igh t paths to determine those that would return the maxi-m um possible knowledge a closer look than th e collective

An option exists: if everything has gone well atSaturn a nd t h e second spacecraft is still healthy, th e slowerVoyager 2 may fly on to Uranus, a planet only discoveredin 1781 and never visited by spacecraft.

TITAN B O O S T P H A S E

C E N T A U R B O O S T P H A S E P R O P U L S I O NM O D U L E P H A S E

Each spacecraft will be boostedaloft aboard a Titan / Centaurlaunch vehicle (right). Approxi-mately 8 minutes after ignitionand 1450 kilometers (900 m i l e s )downrange, the first stage Titanwill separate from the Centaur,and tumble back toward Earth.One hour after l i f t o f f , the space-craft will separate from the Cen-taur stage. The final boost on itstrajectory toward its first objec-tive, Jupiter, will be provided bythe Propulsion Module.

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Science Instruments MassHigh-Gain Antenna DiameterRadioisotope Thermoelectric

Generator (RTG) Power(a t Saturn)

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Data Storage CapabilityX /S Band Data R a t e

at Jupiterat Saturn

Science Scan Platform Scan RangeAzimuthElevation

Received at 26-meter ground antennas


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Eight M o n t h s in the Jov i an S y s temMore than 16 months after launch about December

15, 1978 the speedier Voyager 1 wil l betransmitting photos of the brilliantlycolored disk of Jupiter which

of light for more than 40 minutes to reach scientJet Propulsion Laboratory in California.

Ultraviolet, infrared and polarimetric observJupiter's visible disk wil l shed

on the planet's u n u s u a l

will match the best resolutionnow available with Earth-based cameras.At this point, eighty m i l l i o n kilometers f r o m Jupiter,Voyager 1 will swivel its scan platform to allow the narrow-angle television camera to begin its "observatory phase,"80 days before closest approach.

The narrow-angle camera wil l take hundreds of colorpictures of Jupiter's visible disk, of the bright red andyellow bands, and of boiling storms like the Great RedSpot, as the planet revolves before the camera once eachten hours. The pictures will be recorded on Voyager 1'stape recorder and played back to Earth daily, across dis-

and composition. Tastronomy experiment wil l

presence of Jupiter and the radio bursts that emit as much energy as several nuclear bombs.particle sensors f r om the plasma detector at thof the energy spectrum to the cosmic ray counhigh-energy e nd should stil l be observing sphenomena although there wil l be occasional energetic particles f r o m Jupiter.

Voyager 1 wil l cruise on, drawing evergrowing target, until in early February 1979,30 m i ll i o n kilometers distant with 30 days to


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two and a half t imes larger in Voy a ge r 1 's eyes than itappe ared in De cem ber . All act iv i t ies wil l sh arp ly increase ;sc ient is ts will begin to select a tm os p h e r ic f e a tu r e s of s p e c ia linteres t , such as the Great R e d S p o t an d oth e r s to r m s f o rclose-up observat ion nearer th e p la ne t . J up i t e r ' s s a t e l l i t e swill begin to c om e wi th in re a c h of the s p a c e c r a f t ' s t e l e -v is ion eyes now. There are 13 or 14 know n J ov ia n s a t e l l it e s ;of th e s e , f our are c o m p a r a b l e in size to our Moon and twoof t he s e are larger than Mercury; a t leas t another four orbi tJ u p i t e r "in the wrong direct ion," in wh a t a s t r onom e r s c a l lretrograde orbits. On e o f t h e biggest satellites, lo, has ane f f e c t on t hos e great Jovian radio burs ts which Voyager wil llisten to and r e p o r t o n.

St il l t h e p la n e ta r y v oy a ge r will sai l on. Eight daysbe f or e closest a p p r oa c h , J up i t e r wi l l b e so ne a r t h a t th enar row-angle camera can no longe r e f f e c t iv e ly c ov e r th ee n t i r e d i s k ; th e wide-angle camera wil l assume that chorew h i l e th e o t h e r i n s t r u m e n t lo o k s a t s e l e c t e d p h e n o m e n a .Sp e c t r om e te r s will scan th e a t m o s p h e r e and the cloud t op sto h e lp de t e r m ine th e c o m p o s i t i o n and na tur e of thebr i l l i a n t c loud ba nds .

1979: The F i r s t E n c o u n t e rN e a r - e nc ounte r wi l l oc c ur dur ing th e f i r s t d a y s of

March 1979. By now, high-energy radiat ion in Jupi ter ' sm a g n e t o s p h e r e will be b o m b a r d i n g th e s p a c e c r a f t and itsi n t r i c a t e e l e c t r on ic s .

A c t i v i t y on board th e Voy a ge r and back at Ear th willr e a c h a p e a k . Sh or t ly be f o r e Voy a ge r 1 ' s c los e s t a p p r oa c hto J u p i t e r ear ly on M a r c h 5, 1979 the s p a c e c r a f t willp a s s t iny A m a l th e a , inne r m os t o f J up i t e r ' s s a t e l li t e s , a ndwill p h o togr a p h i t f r om a d is t a nc e of 4 1 5,0 0 0 k i l o m e t e r s .The p ic tures th at f lash toward Ear th wil l give sc ient is tstheir f i r s t -ever close look at A m a l t h e a .

Voy a ge r 1 will whip by J up i t e r a bout 280,000ki lom e te r s (a di s ta nc e e q ua l to 4 J u p i t e r r a d i i ) f r o m th evisible sur face of the p la ne t . No w t h e r a d ia t ion bom ba r d-m e n t will r each it s p e a k , but Voy a ge r 1 will bore ahead.T h e n . . . n o t h i n g . N o word f rom th e V o y a g e r f o r a l m o s ttwo hours as i t s l ips behind Jupi ter . I ts computer s wil ldirect th e f l igh t , th e data col lect ion by i ts in s t r um e nt s , an d

V o y a g e r will scrutinize t h e g i a n t p l a n e t Ju p i t e r a n d f ive o f i t s 1 3 o r 1 4 k n o w n s a t e l l i te s .


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December 1978Encounter - 80 d a y s

February 1979Encounter - 29 days

February 1979E n c o u n t e r - 18 days

March 1979Encounter - 7 h o u r s

In December 1978, eighty days before closest approach, Jupiter will be a mere speck in the eye of Voyager's wide-angle cameras. But with eachpass ing day , it will grow until it o v er f l o w s the cameras' f i elds of view as the s p a c ec ra f t n ea r s its closest a p p r o a c h point in early March 1979. Thewhite squares indicate the field of view of the narrow angle cameras.

Voyager 1's closest approach to Jupiter will be about 280,000kilometers f r o m the vis ib le surface, as it passes between the greatplanet and its nearest satellite, Amalthea ( h i d d en behind the planetin this s i m u l a t ed view). Closest a p p ro a c h to each satellite will occuras Voyager 1 passes out of the Jo v i a n system.

Voyager 2, on a more cautious trajectory to avoid the intenseplanetary radiation, will pass 645,000 kilometers f r o m J u p i t e r at itsclosest point. This f l i g h t path will cause Voyager 2 to encounter thesatellites before the p l a n e t i t s e l f , as seen in this simulated v i ewshowing closest a p p ro a c h to each satellite.

the r e c or d ing for l a t e r p l a y b a c k to E a r t h . As V o y a g e r 1d i s a ppe a r s be h ind J up i t e r , s c i e n t i s t s wi l l ob ta in v a lua b lem e a s u r e m e n t s o f t h e a t m o s p h e r e o f t h e p la ne t as s u n l i g h ta nd th e dua l - f r e q ue nc y ( S- ba nd a nd X - ba nd) r a d io l inkspass t h r o u g h and are a f f e c t e d b y t h e a t m o s p h e r e .

A s V o y a g e r 1 s oa r s a wa y f r om J up i t e r , boos t e d in toi t s new f l igh t p a t h t o w a r d S a t u r n , it will e x a m i n e al l f o u r oft h e b ig Ga l i l e a n s a t e l li t e s o f J up i t e r : lo f r om 22,000 kilo-m e te r s a t +3 h our s a f t e r c los e st a p p r oa c h , E ur op a f rom

7 33 , 000 k i lom e te r s a t +5 h our s , a nd Ga n y m e de a ndCall is to f rom 120,000 ki lom e te r s a t + 14 h our s a nd +2 9h our s , r e s p e c t iv e ly .

J u p i t e r , and p e r h a p s al l of the ou te r p la ne t s , h a s a ne x t e n s i v e m a g n e t o s p h e r e , a r e g io n s u r r o u n d i n g th e p l a n e t inw h i c h e ne r ge t i c p a r t i c l e s a re t r a p p e d by th e p la ne ta r y m a g-ne t ic f i e ld . The outer , leas t dense r egion of the Jovia n m ag-n e t o s p h e r e i s h i gh l y v a r ia b le in s iz e , p e r h a p s due to v a r y ingp r e s s ur e o f t h e im p ing ing s o la r wind .


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A s V o y a g e r 1 br ush es p a s t lo, i t will p a s s t h r ough th e"flux tube," a region o f ma gne t i c and p l a sma i n t e r a c t i onb e t w e e n J u p i t e r and lo c a used b y t h e i n t e r s ec t i on o fJ u p i t e r ' s m a g n e t i c f i e ld with lo . Th i s t ube , be l i ev ed to be ac i r c u l a r c y l i nde r o f about th e d i a m e t e r of lo , a f f e c t s th eEa r t h - r e c ep t i on o f J u p i t e r ' s r a d i o emi s s i ons , and i s a r eg i ono f ma gne t i c and p l a sma i n t e r a c t i on .

J up i t e r ' s g r a v i t y wil l s l i ngsh o t V oy a ger 1 t ow a r dS a t u r n , 80 0 m i l l i o n k i l o m e t e r s f a r t h e r f r o m th e S u n . A sJ up i t e r g r ow s sma l l e r a n d s m a l l e r in the i n s t r u m e n t s ' f i e ld s

o f v i ew , V oy a ger 1 will c o n t i n u e to e x a m i n e th e p l a ne t a n di t s s a te l l it e s u n t i l m i d- A p r i l , a bou t s i x w eeks a f t e r c lo se s ta p p r o a c h .

Th e s ec ond c r a f t , V oy a ger 2 , s a i l ing a l ong beh i nd i t ss p e e d i e r c o m p a n i o n , will now m o v e w i t h i n r a n g e o f J u p i t e r .It s "obse r v a t o r y phase," s i m i l a r to Voyager 1 ' s , wil l begina b o u t A p r i l 2 0 , 1979 a nd will l as t f o r a p p r o x i m a t e l y f ivew eeks . V oy a ger 2 wil l c o n t i n u e t h e l o n g - t e r m o b s e r v a ti o n s ,m a k i n g w h a t a m o u n t s to a m o t i o n p i c t u r e o f a t m o s p h e r i cm ov em ent o f th e l a rge s t p l a n e t in t h e so l a r sy s t em.

Between J u p i t e r and its satellite lo, Voyager 1 w i l l pass t h r o u g h thef l u x tube ( y e l l o w ) , a region o f h i g h m a g n e t i c an d p l a s m a i n t e r -action. Jupiter is encircled by a cloud of hydrogen, w h i l e c l o u d s ofhydrogen ( b l u e ) and sodium ( o r a n g e ) s u r r o u n d lo, and t ra i l thesatellite'sorbital path.

V oy a ger 2 will na v i ga t e a more c a u t i ous c our se : toa v o i d m uc h o f t h e i n t ense r a d i a t i on ne a r th e p l a ne t , i t w i l lf ly muc h fa r t h e r f r om Jupiter than i ts predecessor nocloser than 645,000 k i l o m e t e r s ( a b o u t 9 J u p i t e r r a d i i) . I twill n o t r e p e a t th e c l ose f l y by of lo , but b e f o r e J u p i t e re n c o u n t e r i t will su r v ey Ca l l i s t o f r om 220,000 k i l o m e t e r sa n d G a n y m e d e f r o m 55,000 ki lometers . I t wi l l f ly wi t hi n201,000 ki l ome t e r s o f E u r o p a and will take a f la sh inggl a nc e a t A ma l t h ea f r om 550,000 ki l ome t e r s . V oy a ger 2will m a k e it s closest approach to J u p i t e r o n J u l y 9, 1979.A s it h e a d s o n t ow a r ds Sa t u r n , th e c r a f t wi l l s p e n d th e restof J u l y a nd ea r ly A ug us t l ook i n g ba c k ov e r i t s sh ou l de r a tt h e r e c e d in g J u p i t e r .

The outer, least dense region of the Jovian magnetosphere is h i g h l y variable in s ize , perhaps due to v a r y i n g pressure of the i m p i n g i n g solar wind.


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Exploring SaturnBoth craft will fly on through space, making their

measurements and observations of a variety of phenomenaand radioing their findings back to Earth. About one yearafter the Jupiter mission has ended, Voyager 1 will begin

On its inbound journey, Voyager 1 wiexamine an object of intense interest to scientistsatellite Titan. Titan is 5,800 kilometers in diamslightly smaller than Mars. It is the only satellite have a dense atmosphere; the methane that surroumay be as dense as Earth's atmosphere. Titan'

studying Saturn, in August 1980. By now the speedierVoyager 1 will have gained nine months on its laggingsister-spacecraft.

As Voyager 1 draws closer to the ringed planet, theplanet's image wil l grow in the narrow-angle camera's f ie ldof view through the fa l l of 1980. By mid-October, the ringswill be too large to be captured in a single f r a m e andscientists back on Earth will combine several frames. Sinceno one knowshow extensive Saturn's magnetosphcre is, thef ields and particles instruments wil l begin continuous high-rate data acquisition a month before the closest approachnear November 13. Recent radio-astronomy observationsf rom Earth satellites appear to confirm the presenceof radio emissions f r o m Saturn, suggesting that a mag-netic f i e ld and an accompanying trapped radiation belt

about twice that of the Moon. Some scienspeculated that complex organic compounds cformed in the atmosphere of Titan and now ressurface, leading to f u r t h e r speculation that it mharbor some p r i m i t iv e f o r m of life. Voyager 1course only 4,000 kilometers f r o m the surfacethen will fly behind the satellite, cutting off the rato Earth and passing through Titan's solar shadothe purpose is to analyze the atmosphere, just asdid at Jupiter. Instruments aboard Voyayeracquiring unique data: scientists cannot see Titf r o m Earth.

Saturn has other satellites: ten have been dso Voyager 1 will not be satisfied by its brush witwill also survey Tethys during the approach to S


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Beginning in late 1980, Voyager will encounter the ringed planet Saturn and six of its satellites.

V o y a g e r 1 will p a s s a bou t 140,000 ki l ome t e r s be l owS a t u r n 's s o u t h p o l e , s n a p p i n g h u n d r e d s of p i c t u r e s as itp a s se s . W i de - a ng l e and na r r ow - a ng l e c a mer a s , p o l a r i me t r i c ,u l t r a v i o l e t an d i n f r a r e d i n s t r u m e n t s will scan th e p l a n e t a ndit s r ings . T h e m a g n e t o s p h e r e an d a c c o m p a n y i n g r e g i o n s o fc h a r ged p a r t i c l e s will b e c h a r t ed a l ong th e s p a c e c r a f t ' s p a t h .

S a t u r n ' s r i n g s are not sol id t h e y c o u l d n ' t bebec a use t i da l fo r c e s an d d i f f e r e n t r o t a t i ona l sp eeds a td i f f e r e n t d i s t a n c e s f r o m S a t u r n w o u l d b r e a k t h e m up soV o y a g e r 1 's radio s ignal s will b e able to t r i c k l e t h r ough th e

rings and tel l scientists a great deal about those u n i q u eobject s . A n d t h e scient i s t s will l e a r n m u c h m o r e a b o u tS a t u r n 's a t m o s p h e r e an d c o m p o s i t i o n as the Sun ' s r a y s an dth e spa cecraf t radio s ignal s fade an d r ea p p ea r a ga i n , as a tJ u p i t e r a n d Ti t a n .

A s V o y a g e r 1 f l ies a w a y f r o m S a t u r n , it s i n s t r u m e n tscan p la t fo rm wi l l look back a t the nor t h p o l a r r eg i on , th el i gh t ed c r e s c e n t and the t i l t ed r ings , col lect ing long-rangei n f o r m a t i o n f o r a n o t h e r m o n t h .


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A Sec ond L ook at Sa t urnS ix m o n t h s a f t e r th e f i r s t e n c o u n t e r e n d s , th e

fo l l ow i ng cra f t , V o y a g e r 2 , will cruise into range in J u n e1981. W h i l e th e c o u r s e of V o y a g e r 1 h a s been care fu l lyc h a r t e d , m i s s i o n p l a n n e r s h a ve l e f t the i r opt ions open forV o y a g e r 2 .

T h e r e are two choices . T h e second spacecra f t couldrepe a t the m iss ion of i t s predecessor , co m ple te wi th c loseT i t a n pa s s and f l i g h t b e n e a t h th e s o u t h po l e . B u t , i f al l hasgone wel l w i t h t h e f i r s t e n c o u n t e r w i t h S a t u r n , i ts ri n g s ,and T i t a n , and i f the t ra i l ing c ra f t is h e a l t h y , c o n t r o l l e r sa nd s c i e n ti s t s m i g h t p i c k a n e w pa t h , u s i n g S a t u r n ' s g r a v i t yto boost V o ya g e r 2 toward d is tant U ranus . This plan cal lsf or anoth er c lose pass by Sa turn , brush ing near the ou te redge o f the r ings . Voy ager 2 would wh ip pas t the r ing pla nea b o u t 38,000 k i l o m e t e r s b e y o n d th e outer edge v i s ib l ef r om E a r t h , f o r e g o i n g a r e pe a t o f t h e close Titan pass.I ns t ea d , V o ya g e r 2 w o u l d pa s s a b o u t 3 53 ,0 0 0 k i lo m e t e r s

f r om Titan . Rhea would come with in 254,000 k i l o m e t e r sof V o ya g e r ; Tethys, 159,000 ki lometers ; Ence ladus , 94,000k i l o m e t e r s ; Mi m a s , 33,000 k i l o m e t e r s ; a n d f i n a l l y D i o n e ,196,000 k i l o m e t e r s d i s t a n t .

V o ya g e r 2 wil l f lash pa s t S a t u r n on A u g u s t 27 , 1981,h e a d i n g e ve r o u t w a r d f r o m th e Sun, looking back towardS a t u r n u n t i l la te September .

B y that t i m e , t h e two Voyagers wi l l have com ple tedc lose-up examina t ions o f two plane ts and the i r sa te l l i te s .T h e qu a n t i t y o f p l a n e t a r y d a t a c o m pi l e d will e xc e e d t h a tamassed by M a r i n e r 9 in i t s 11-month survey o f Mars , tow h i c h m ay be added severa l years ' worth o f f ields an dpa r t i c l e s m e a s u r e m e n t s i n u n e xp l o r e d r e g i o n s o f t h e o u t e rso la r sys tem. T h e a t m o s p h e r e s an d m a g n e t o s p h e r e s o f t h ep l a n e t s will have been ana lyzed and char ted , and f ive Jo v i a nand s ix Sa turnian sa te l l i te s wi l l have been surveyed inc ons i de r a b l e deta i l .

As Voyager 1 sails t h r o u g h the Saturnian system, it will pass140,000 k i l o m e t e r s f r o m the ringed planet's southern polar region,an d cruise past six of the ten known Saturnian satellites. This views h o w s the approximate position of each satellite at closest approachby the spacecraft.

Voyager 2 will gain an e v e n closer look at Saturn. For the Uranusoption trajectory, it will pass within 38,000 kilometers of the outeredge of the rings, and again will survey the same six satellites. Thissimulated view shows the approximate position of each satellite atclosest approach by Voyager 2.

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Voyager Science O p e r a t i o n sThe s c i e n t i f i c i nv e s t i ga t i ons of the Voyager mi ss i on

are m u l t i p u r p o s e : most are i n t e n d e d to obtain data in avariety of environments. For example, the ultravioletspectrometer is oriented toward planetary a n d sate l l i teatmospheres a n d toward studies o f i n t e r p l a n e t a r y a n d

i n t e r s t e l l a r h y d r o g e n a n d h e l i u m . T h e m a g n e t i c f ie ldsexperiments will examine the magnetospheres of theplanets a n d t h e search fo r t h e t r a ns i t i on between solar a n dgalact ic regions.

"intT h u s , it is d i f f i c u l t to separate "planetary" f r o m

:erplanetary" instruments and experiments. There is,

V O Y A G E R S C I E N T I F I C I N V E S T I G A T I O N SE x p e r i m e n t

C os mic Ray

Imaging S cience

Infrared Inter ferometerSpectrometer

Low-Energy ChargedPart ic les

Magnetometer

Photopolar imeter

Planetary Radio Astr onomy

Plasma

Plasma W a ve

Radio Science

U l t r a v i o l e t Spectr ometer

Principal InvestigatorR. E . Vogt,C al i forn ia Inst i tute ofTechnologyTeam Leader,Br adfor d Smi th,University of Ar izona,TucsonRudolf Hanel,Goddard SpaceFlight CenterS . M. Krimigis,Johns HopkinsUniversity,Applied Physics Lab.Nor man Ness,Godda r d SpaceFlight CenterCharles Lillie,Univer s i ty of Colorado

Jam es War wick,Univer s i ty of ColoradoHerbert Bridge,Massachusetts Instituteof TechnologyFreder ick L. Scar f ,T R W S yst em s Group

Team Leader,Von R. Eshleman,Stanfor d Univer s i tyA. Lyle Broadfoot,Kitt PeakNat iona l Obser vator y

Pr imary MeasurementsEnergy spectra an d isotopic composit ion of cosmic ra yp a r t i c l e s a n d energetic part icles in outer planetarymagnetospheres.Imaging of planets and satel l i tes at resolut ions and phaseangles not possible f r o m Ear th . A tmospher ic dynamics andsurface structures.Energy balance of planets. A tmospher ic composit ion an dt e m p e r a t u r e f i e l d s . Composition a n d p h y s i c a lcharacterist ics of satel l i te sur faces and Saturn r ings.En ergy spectra and isotopic com posit ion of low-energyc h a r g e d p a r t i c l e s i n p l a n et a ry m a g n e t os p h e re s a n dinterplanetary space.

Planetary and interplanetary magnetic f ie lds.

Methane, ammonia, molecular hydrogen, and aerosols inatmospheres. Comp osit ion and ph ysical charac ter ist ics ofsatel l i te sur faces an d Saturn r ings.P l a n e t a r y r a d i o emissions and plasma resonances inplanetary magnetospheres.En ergy spectra of solar-w ind electrons and ions, low-en ergycharged part icles in planetary environments, and ionizedinterstel lar hydrogen.Dynam ics of planetary magn etospheres and satel l i te per-turbation phenomena associated with charged part icle-p l a s m a w a v e interactions. Determination of electrondensi ty .Physical propert ies of atmosp heres and ionospheres. Planetan d sate l l i te masses, densit ies, an d gravity f ie lds. Structureof Saturn r ings.Atmospher ic composi t ion inc luding hydrogen to heliumratio. Thermal structu re of upper atmospheres. Hyd rogenan d hel ium in in ter p lanetar y and in ter s te l lar space.

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h owe v e r , a no th e r w ay o f g r o u p i n g : th e optical scanners ,m o u n t e d on the s p a c e c r a f t ' s s c a n p la t f o r m , h a v e na r r owfie lds of v ie w and m u s t be a c c ur a te ly p o in te d . T h e y c o l le c tradiant energy ( l ight , fo r exam ple) f rom the ir targets andcreate images o r s p e c t r a l in f o r m a t ion that permit sc ient is tsto unde r s ta nd th e p h y s ic a l f o r m or c h e m ic a l c om p os i t ionof th e planets and sate l l i tes . Exper iments in th is groupinc lude th e im a ging- s c ie nc e ins t r um e nt s ( t h e c a m e r a s ), th ei n f r a r ed i n t e r f e r o m e t e r - s p e c t r o m e t e r and r a d iom e te r , th eul t r a v io le t s p e c t r om e te r an d t h e p h o t o p o l a r i m e t e r .

A second family of e x p e r im e nt s s e ns e s m a gne t icfields and f luxes of charged par t ic les as the s p a c e c r a f tpasses through them. These in s t r um e nt s , f ix e d to the bodyof th e spacecraf t , have var ious f ie lds of view; their datata ke n toge th e r wi l l p r ov ide in f o r m a t ion o n planetarymagnet ic f ie lds an d t r a p p e d r a d ia t ion z one s ( a nd ind i r e c t ly ,

on inter ior s t ructures) , on S u n - p l a n e t an d planet-sate l l i tein t e r a c t ions , an d o n cosmic rays and the o u t e r r e a c h e sof the s o la r p la s m a . T h e s e e x p e r im e nt s are the p l a s m a ,low-energy charged par t ic les , cosmic ray and m a gne t ic -f ields inves t igat ions .

A t h i r d f a mi l y of e x p e r im e nt s c ons i s ts of t h ep la ne ta r y r a d io a s t r onom y and the p la s m a - wa v e e x p e r i -m e nt s , wh os e long a n te nna s will l i s t e n f o r r a d io e m is s ionsf r om J up i t e r and wi l l m e a s ur e wa v e s in t h e p la s m as ur r ounding th e p la ne t s .

A r a d i o e x p e r i m e n t will use S-band and X-band radiol inks be twe e n th e s p a c e c r a f t an d E a r th to g a t h e r i n f o r m a -t ion o n p la ne ta r y an d sate l l i te ionospheres an d a t m o -s p h e r e s , and s p a c e c r a f t t r a c k ing data to c h a r t g r a v i ta t iona lfields t h a t a f f e c t Voy a ge r ' s course .

P.A

The giant antennae of the world-wide Deep Space Network such asthe 64-meter dish at Goldstone, C a l i f o r n i a , w i l l receive the spacecraftradio signals and route them on to Mssion Operations for analysis.

Mssion Operations at the Jet Propulsion Laboratory, Pasadena,California, will be the foca l point of Earth activities dur i ng the8-1/2-year mission.

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Our solar system may h a v e had its beginnings within an immense, rat i fied gaseous body such as the Orion Nebula.

W h a t We Hope to LearnA m a jo r o b je c ti ve o f t h e s pa c e p r o g r am i s t o a c qu i r e

a b a s i c u n d e r s t a n d i n g of t h e solar system and its origin andevolut ion . Most theore t ica l models state s i m pl y t h a t thesolar system started w i t h a g a s e o u s n e b u l a . T e m pe r a t u r e ,p r e s s u r e a nd d e n s i t y of t h e ga s d e c r e a s e w i t h d i s t a n c e f r o mthe S u n . F o r m a t i o n of the p l a n e t s is bel ieved to h a v er e s u l t e d f r o m a c c r e t i o n of t h e n e b u l a r m a t e r i a l . O b s e r ve dd i f f e r e n c e s in the p lane ts a re accounted for by var ia t ions inth e m a t e r i a l a n d condi t ions a t f o r m a t i o n . K n o w l e d g egained at each plane t or sa te l l i te can be re la ted to o t h e r sand c o n t r i b u t e s to o ve r a l l u n d e r s t a n d i n g .

Miss ions to Ma r s , V e n u s , Me r c u r y a nd th e Mo o n h a vec o n t r i b u t e d g r e a t l y to this body of k n o w l e d g e . E a c h oft h e s e p l a n e t s h a s i t s own pe r s o n a l i t y , s i g n i f i c a n t l y d i f f e r e n tf rom o t h e r s b e c a u s e of i t s u n i q u e c o m p o s i t io n a nd re la t ion-sh ip to the S u n . I nd i v i dua l as they are, they are genera l lyre la ted as bodies that orig ina ted near t h e Sun a nd a r ec o m p o s e d m a i n l y o f h e a v i e r e l e m e n t s . T h e y a r e c l as s i f ie d a s"terrestrial" p l a n e t s .

S c i e n t i s t s h a ve k n o w n for a l o n g t i m e that J u p i t e r ,S a t u r n a n d t h e o t h e r o u t e r p l a n e ts d i f f e r s igni f icant ly f r o mth e te rres t r ia l p lane ts . They have lo w average dens i t ie s ; onlyh y d r o g e n and h e l i u m a m o n g ai l the elements are l igh te n o u g h to c o m p r i s e th e b u l k o f t h e s e p l a n e t s . Ju p i t e r a n dS a t u r n are su f f i c i en t l y mass ive (318 and 95 t i m e s E a r t h ' smass , re spec t ive ly) to i n d i c a t e t h a t t h e y h a ve re t a i n e da l m o s t all of t h e i r o r i g i n a l m a t e r i al . T h e y a r e , h o w e v e r ,o n l y r e l a t i ve l y p r i s t i n e e xa m pl e s of the m a t e r i a l f r o m w h i c h

th e so la r sys tem form ed because , whi le l i tt le or nop l a n e t a r y m a t e r i a l h as been los t , th e p l a n e t s h a v e e vo l ve dd u r i n g a lmos t 5 bi l l ion years . If t h a t e vo l u t i o n can bet raced , s c ient i s t s could obta in an un de rs ta ndi ng of the ear lystate of tha t reg ion of t h e s o l a r s y s t e m .

G r o u n d - b a s e d o b s e r va t i o n s an d P i o n e e r s 10 and 11h a v e d e t e r m i n e d that Ju p i t e r r a d i a t e s m o r e e n e r g y t h a n itrece ives f r o m t h e S u n , a n d t h i s a ppe a r s t r u e a t S a t u r n , a l s o .A n exact m e a s u r e of t h i s i m b a l a n c e is i m p o s s i b l e f r o mEarth , s ince only a po r t ion of the plane ts ' sur faces th eful ly l igh ted day s ide can be seen . I t i s d i f f i c u l t t o s t u d ye a r ly m o r n i n g a n d l a te a f t e r n o o n r e g i o n s , a n d i m p o s s i b l e tosee the i r n igh t s ides .

The Inter iors :C l ues to the Univer seA ny discuss ion of i n t e r i o r t e m pe r a t u r e s of the o u t e r

p l a n e t s i s i m p o s s i b le w i t h o u t k n o w i n g h o w m u c h e n e r g yf l o w s f r o m those in te r iors . Discuss ion o f t h e i r e v o l u t i o nr e qu i r e s k n o w l e d g e o f w h e t h e r t h e e n e r g y i s c a u s e d b yp r i mor d i a l , h e a t , g r a v i t a t i o n a l c o n t r a c t i o n , o r p e r h a p s j u s tenergy f r o m a s h o r t - p e r i o d s t o r ag e m e c h a n i s m . A n u n d e r -s t a n d i n g o f a p l a n e t ' s a t m o s ph e r i c s t r u c t u r e a n d d yn a m i c sr e qu i r e s a c c u r a t e k n o w l e d g e o f t h e m a g n i t u d e a n d l o c a t i o no f s o u r c e s h e a t i n g t h a t a t m o s ph e r e .

J u p i t e r a n d S a t u r n , u n l i k e t h e d e n s e , r o c k y p l a n e t sM e r c u r y , V e n u s , E a r t h a n d Mars, a r e c o m po s e d m o s t l y o fh y d r o g e n and h e l i u m b u t h o w m u c h of e a c h ? Th e a n s w e rwill revea l th e g e n e r a l s t r u c t u r e of t h e i n t e r i o r of the

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planets . I f the ratio of h y dr og e n to h e l ium i s t h e s a m e a sf o r t h e S u n , d i f f e r e n c e s in obs e rv e d p r op e r t i e s o f J up i t e ran d Sa tur n c a n be e x p la ine d by d i f f e r e n c e s in t h e i r m a s sand in th e a m ount o f c onde ns e d r oc k- f o r m ing m a te r ia l inth e planetary cores v e r y s m a l l f o r J u p i t e r an d p e r h a p ss l igh t ly m or e f o r Sa tu r n . I f t h e r a t io o f h y d r oge n to h e l iumvar ies f r om s o la r c om p os i t ion , a nd i s lowe r a t Sa tu r n th a na t J up i t e r , t h e n p la ne ta r y in t e r io r s wi th ou t r oc ky c or e s c a nbe i n f e r r e d .

Th e ratio of h y d r o g e n to h e l ium , th e r e f o r e , is off u n d a m e n t a l im p or ta nc e . Sp e c t r os c op y f r om E a r th of f e r sno a ns we rs be c a us e c o ld h e l ium c a nnot be d e t e c t e d .A t t e m p t s to s t u d y h e l i u m by i t s e f f e c t s on oth e r a tom s o rm ole c u le s h a v e be e n uns uc c e s s f u l be c a us e o f t h e c om p le xa tm os p h e r ic s t r uc tu r e o f t h e ou te r p la ne t s , wh ic h h a v emixed clouds , hazes , and gases that vary over the sur faces .

M e a s u r e m e n t s by th e P ione e r s p a c e c ra f t h a v e de t e r -m i n e d t h i s h y d r o g e n to h e l ium r a t io to be ne a r th e solarva l u e , but a m o r e p r e c i s e d e t e r m i n a t i o n to be m a d e b y t h eVoy a ge r s p a c e c r a f t at s e v e r a l de p th s o f t h e a t m o s p h e r e s o fJ u p i t e r a n d S a t u r n will p r o v i d e f u n d a m e n t a l b o u n d s o n t h ein t e r io r m ode l s o f both p la ne t s .

The m agnet ic f ie ld of a p lanet i s an external lym e a s u r a b l e indicat ion of condit ions deep in the inter ior .J up i t e r h a s a m a gne t ic f i e ld m or e th a n t e n t im e s s t r onge rth a n E a r th ' s . T h e planet ' s radio burs ts are re la ted to radia-t ion belts t h a t are in turn r e la t e d to the e x i s t e nc e o f t h es t r o n g m a g n e t i c f i e ld . A v a r i e t y o f r e l a t e d p h e n o m e n a c a nb e di r e c t ly m e a s ur e d b y in s t r um e nt s ne a r th e p la ne t . T h ede ta i le d s tudy o f t h e s e p h e no m e na and th e i r in t e r a c tionwith the solar f ield s ur r oun ding th e p la ne t w i ll r e v e a l m uc hof what l ies at v a r ious de p th s be low th e clouds .

D i r e c t e v ide nc e of a m a g n e t i c field an d re la tedp h e no m e na i s no t a v a i la b le f o r Saturn. O nly r e c e n t ly h a v ei n d i r e c t observat ions , f rom Ear th-o rbi t ing sate l li tes , indi-c a te d th e p r e s e nc e o f a m a gne t ic f i e ld . A c los e - up c om p a r i-son of S a t u r n w i t h J u p i t e r will be of great interes t .

The v is ible sur faces of Jupi ter and Saturn ares p e c ta c u la r . They are not solid surfaces at all, bute x t r e m e ly c om p le x op a q ue c loud s t r uc tu r e s wi th m a s s iv et u r b u l e n c e s y s t e m s suggest ing some of the m or e s e v e r eh ur r ic a ne s o n E a r th hat m ov e r a p id ly a c r os s th e p l a n e t .

Planetary turbulence systems ( l e f t ) maybe s imi lar in nature to this hurricane onEarth (above), photographed by theA p o l l o astronauts.

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O f t e n t h e s e s to rm s y s t e m s a p p e a r , m a tur e a nd v a n i s h in af ew days . O ne th e f a m o u s G r e a t R e d S p o t s e e m s to ben e a r l y p e r m a n e n t . I t di s a p p e a r s r a r e ly a n d t h e n f o r only as h or t t im e .

A s y s t e m a t i c s t u d y o f t h e d y n a m i c s an d c o m p o s i t i o no f t h e c louds o f J up i t e r a nd S a tur n wi ll p r ov ide in f o r m a -t ion on th e c loud la y e r s a nd p h y s ic a l m e c h a n i s m s c a us ingm o v e m e n t . T h e i n f o r m a t i o n , w h e n c o m p a r e d t o p la n e t sl ike E a r t h , M a r s a n d V e n u s , will p e r m i t s c i e n t is t s to testg e n e r a l t h e o r i e s o f c l im a to logy ov e r wide ly v a r y ingc ondi t ions . T h e s tud ie s will be m a de ov e r s e v e r a l m onth s a te a c h p la ne t .

Evolut ion of the Satelli tesT h e m a ny s a t e l li t e s o f J up i t e r a nd Sa tur n a nd

Sa tur n ' s s p e c ta c u la r rings p r e s e n t op p or tun i t i e s to unde r -s ta nd c onde ns e d m a te r ia l in the outer solar sys tem. Th es tudies will t e s t t h e or i e s t h a t p r e d ic t c h e m ic a l c om p os i t ionof c onde ns e d m a te r ia l a c c or d ing to d i s t a nce f r om th e Sun ,given a s s um e d c ondi t ions in the s o la r ne bu la . T h e c h e m i c a lh i s to r i e s a nd s u r f a c e e v o lu t ion ( inc lu d ing r e c or ds o fm e te or i t e im p a c t s ) o f t h e sate l l i tes will be e n t i r e ly d i f f e r e n t

f r o m th e p la ne t s o f t h e inne r s o la r s y s t e m ; they are , there-f o r e , o f p r im a r y in te r e s t f o r de ta i l e d s tudy .

Saturn 's sa te l l i te Ti tan, as a speci f ic case , has ana tm os p h e r e p e r h a p s a s de ns e a s t h a t o f E a r th . Giv e n th ea s s um e d c onc e n t r a t ions o f m e t h a n e , and a sol id sur face onwh ich com plex m olecules can ac cum ulate , i t i s a l ikely s iteof s im p le o r ga n ic c om p ounds a nd , t h e r e f o r e , a n e x t r e m e lyi m p o r t a n t b o d y f o r d e t ai le d e x p l o r a ti o n .

The sm all sa te l l i tes of Jup i te r are s im ilar to as teroidsin s ize . P r e s um a bly , t h e y h a v e be e n p u l l e d f r om th ea s t e r o i d be l t a nd c a p tur e d by th e p la ne t . B e c a us e th e y a r es o small , they rap idly diss ipate any h e a t p r oduc e d byradioact ive decay a nd s h ould no t h a v e c h a nge d s ign i f i c a n t lys inc e f o r m a t ion .

Sa tur n ' s r ings a r e o f t e n e x p la ine d a s r e m na nt s o f aga s e ous d i sk th a t onc e s u r r ounde d S a tur n a nd f r om wh ic hi t s m a j or s a t e l li t e s f o r m e d . S inc e th a t de s c r ip t ion loos e lyc om p a r e s wi th t h e f o r m a t ion o f t h e p la ne t s t h e m s e lv e s ,de ta i le d s tudy o f t h e r ings m a y p r ov ide clue s to t h ebe h a v ior o f t h e ga s e ous d i s ks t h a t m igh t h a v e e v o lv e d in toth e sate l l i tes and, indeed, th e ent ir e solar sys tem.

C o m p a r i s o n of Earth, Jupiter, Saturn, U ra n u s and Their Major Satellites

E A R T HR = 6,370 kmDS = 150,000,000 km

(1 A UIP = 1 year

J U P I T E RR 7 1 , 6 0 0 km

DS= 5.2AUP = 11.9 year

S A T U R N

M O O NR = 1 , 7 4 0 kmDp 385,000 knP = 27 days

Y M E D ER = 2,600 kmDp = 1.071,000 kmP = 7.2 days

C A L L I S T OR = 2,450 kmDp 1,884.000 kmP = 16.7 days

R H E AR = 800 kmDp = 527,600 kmP = 4.5 days

TITANR = 2,920 kmDp= 1,222.600kmP= 15.9 days

U R A N U SR = 26.200 km

DS= 19.2 AUP = 84.0 years

M I M A SR = 200 kmDp= 185,800 km

P = 0.9 daysE N C E L A D U S= 2 2 5 k mDp = 238,300 km

P = 1.4 days

AMALTHEAR = 120 km Dp = 181,500 kmP = 0.5 days

T E T H Y SR- 525 kmDp = 294,900 k1.9 days

IOR = 1,820kmDp = 422,000kP = 1.8 days

E U R O P AR = 1.530 kmDp 671,400km

R - 425 kmDp = 377.900 kmP = 2. 7 days

M I R A N D AR = 100km*Dp = 129,800P = 1 . 4 days

A R I E LR = 300 kmDp = 190,900P = 2.5 days

U M B R I E LR = 200 km *

Dp = 266,000 = 4.1 daysTITANIA

R = 500 km Dp = 436,000P = 8 .7 daysO B E R O N

R = 400 km *Dp = 583,400 P = 13.5 day

K E Y :

R = Radius ( k m )D = M e a n O r b i t a l RadiusP = Orbi tal Period

R = Equatorial radius of bodyDS - Distance f r o m S un (mean orbital rad ius)Dp = Distance from planet (mean orbital radius)P = Orbi tal period (revolu t ion t ime)

Not an actual measurement, but based on apparent brightness o


19/20

B e y o n d SaturnIf th e op t i on to j o u r n e y on to U r a n u s is ex e r c i s ed ,

Voy ager 2 wi l l sa i l wi th in range of Ura nu s in Jan uar y 1986,m or e t h a n fou r y ea r s a f t e r l ea v i ng Sa t u r n .

U r a n u s is m a r k e d ly d i f f e r e n t f r o m J u p i t e r andSa t u r n . It is tilted so far on its axis that th e p o l e s li e a l m o s tin th e plane o f t h e ecl ip t ic . Thus, in c on t r a s t to the o t h e rplanets, U r a nus lies on its side. It circles the Sun once in 84y ea r s . O nc e in each orbi t t h e S u n sh i ne s d i r ec t l y dow n o nth e north p o l e ; 4 2 years la ter th e sou t h p o l e is l ighted. In1986, the or ien tat ion of Ura nu s wi l l a l low Voyage r 2 to f lya l m o s t p e r p e n d i c u l a r to the eq ua t o r i a l a n d sate l l i t e p lane.V o y a g e r 2 wil l get a good look at any m a g n e t o s p h e r e and

p l a s m a c l oud t h a t m a y b e p r e s e n t , a n d c o u l d p h o t o g r a p ht h e sun l i t h em i sp h e r e o f U r a nus a nd a l l o f i t s s a t e l li te s . Th espacecraf t wi l l a l so provide a f i r s t close observat ion of then e w l y di sc ov e r ed r i ngs o f U r a nu s . V oy a ger 2 w ou l d t h e nsail o u t t h r o u g h th e p l a ne t ' s w a ke , l ook i ng ba c k a t the d a r ks o u t h e r n h e m i s p h e r e .

Y e a r s a f t e r l a u n c h , p e r h a p s 3 0 t im e s f a r t h e r f r o m t h eSun t h a n Ea r t h i s , t h e i r a t t i t ud e c on t r o l ga s sp e n t , t h e t w oVoyagers will be unable to r e sp ond to attitude correctionc o m m a n d s f r o m t h e i r E a r t h m a s t e r s , a n d c o m m u n i c a t i o n swill f a d e a nd d i sa p p ea r a s t h ey d r i f t ou t o f r a nge .

Th e i r mi s s i on o f d i sc ov e r y and e x p l o r a t i o n c o m p l e t e ,the two craf t wi l l sa il on fo r ev e r .


20/20

JPL SP 43-39NASA - JPL - COML., L.A., CA.

Voyager Journey to the Outer Planets

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