Voyager the Grandest Tour. the Mission to the Outer Planets

8/8/2019 Voyager the Grandest Tour. the Mission to the Outer Planets

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Viiager

THE MISSIONTO THE

OUTER PLAN ETS


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The Voyager spacecraf t, originally meant to f ly by just two planets and operate forf ive years, accompl ished much more. In all , they explored four planets , tens ofmoons and the r ings and magnetic envi ronments of those planetary systems. Themission became one of those technological enterprises that actually deliveredmore than it promised. Together, the Voyagers and the men and women whobui lt the spacecraft and choreographed their fl ight accompl ished one of his tory 'smost ambit ious and rewarding expedit ions of discovery.

The Voyager mission, managed for the National Aeronautics and SpaceAdminist ration (NASA) bythe Cal ifornia Insti tute of Technology's Jet Propuls ionLaboratory (JPL) in Pasadena, Cal ifornia, was des igned to take advantage of a raregeometric arrangement of the outer planets one that occurs only once every176 years. This layout of Jupiter, Saturn, Uranus and Neptune allows a spacecraf tto swing from one planet to the next without the need for large onboard propul-s ion systems' the f lyby of each planet both accelerates the spacecraft and bends i tsfligh t pa th . Aimed properly, the spacecra ft can be sent on to the next planet.Without th is "gravi ty assist" techn ique , the flight time to Neptune is 30 years'with it, only 12.

Whi le a four-planet miss ion, promoted by planetary scientists asthe "GrandTour," was known to be possible, it was deemed too expensive to build a space-craft that could travel the dis tance, carry the instruments needed and last longenough to complete such an extended voyage. Thus, the project was funded toconduct intens ive flyby studies of only Jupiter and Saturn. More than I0,000 trajec-tories were studied before the two were chosen that would a llow close flybys ofJup ite r and its la rge moon Io, as we ll as o f Satu rn and its rings and large moonTitan. The chosen fligh t path for Voyager 2 a lso preserved the option to continueon to Uranus and Neptune.

O THE MARS ANTEN-NA IN CALIFORNI A'SMOJAVE DESERT

IS SHOWN IN ITS

2 1 O-FOOT-DIAMETER

CON FIG U RATION.O A HELIOCENTRICVIEW OF THE VOYAGERTRAJECTORIES. THE

SPACECRAFT ARE

NOW ON AN ETERNAL

JOURNEY TOWARDINTERSTELLAR SPACE.


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Aboard a Titan/Centaur expendable launch vehicle, Voyager 2 was launched f irst ,on August 20, 1977. Voyager I -- also on a Titan/Centaur- was launched on afaste r, shorter trajectory on September 5, 1977. Both launches were from theNASA Kennedy Space Center at Cape Canaveral in Florida.

All the unmanned spacecraft that came before them, whether they flew or fail ed,contr ibuted to the mechanical s tamina and elect ronic adaptabil it y bui lt into theVoyagers. The key to the Voyagers' success lay in this her itage, and in the acknowl-edgment that things could go wrong. By instal ling backups, or providing whatspacecraft engineers cal l " redundancy," to take over in the event of the failu re o fcritical components such as radio receivers and computers, Voyager engineerslaunched the spacecraft with the foresight of a driver who embarks on a cross-country t rip hav ing packed extra hoses, f il ters and a spare ti re in the trunk.

O VOYAG E RS 1AND 2 ARE IDEN-TICAL MARINER-

CL AS S S PA CE CR AF T,EACH WEI GHING

| ,820 POUNDS ANDCARRYING 1 1 SCIENCE

INSTRUMENTS.

O IN SUMMER 1977,A VOYAGER LEAVES

EARTH ATOP ITS

TITAN/CENTAUR EX-

PENDABLE LAUNCHVEHICLE.

O VOYAGER 1 T OOKTHE FIRST SINGLE-FRAME PHOTOGRAPH

OF EARTH AND THE

MOON.


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This prudence paid o ff early in the mission when Voyager 2's prime receiver failed.For the rest of i ts fl ight , telecommunications with Voyager 2, conducted throughthe Deep Space Network (DSN) operated for NASA by JPL, d epended on aflawed but funct ional backup receiver.

Demands placed on the Voyager spacecraf t grew with the dis tance they traveledfrom Earth. Commands from the DSN transmitters, trave ling at the speed of l ight,took longer and longer to reach the spacecra ft; co rrecting any mistakes or a t-tempting to remedy a problem on the spacecraft en ta iled uncomfortab ly long de-lays before the spacecraft received and executed the necessary commands andthen radioed back to Earth tha t the prob lem had been solved. At Jupiter, theVoyagers were less than an hour's li ght-time away, but by the time Voyager 2reached Neptune, it took more than four hours to make the rad io connectionfrom Earth. The spacecraft were programmed to orien t themse lves in a "sa fe"sta te --i nstruments po inted away from the Sun, radio d ish poin ted toward Earthready to receive commands whenever any spacecraft maneuver or normal op-era tions did not go as p lanned. The ability to detect flaws in the execution oftheir programs and to place themselves in a safe state whenever anything wentawry was of critical importance in keeping the Voyagers hea lthy over thei r longjourneys.

O THE MARSANTENNA WAS EN-LARGED FROM 210 TO230 FEET IN DIAME-TER FOR VOYAGER 2'S1 98 9 E NCO UN TE RW ITH NE PT UN E.

REMOTE DESERTLOCATION AND SUR-ROUNDING HILLSPROTECT THE SENSI-TIVE DSN RECEIVERSF RO M EARTH-BASEDRADIO "NOISE" THATW OUL D OV ER WHE LMSPACECRAFT SIGNALS.

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T HE ANT ENNA'SSECONDARY MIRROR,SUPPORTED BY THETOW ER , RE FL ECT SRADIO SIGNALS DOWNTO THE RECEIVER INTHE CENTRAL CORE.

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Jupiter , Saturn, Uranus and Neptune are giant planets with deep atmospheresmade up primari ly of hydrogen and he lium. They and their moons and ringsfo rmed and have evolved in a much colder, darker region of space than the innerplanets o f the sola r system. None of these outer planets has a so lid surface.At their centers are spheres o f molten ice and rock.

Wi th a d iameter of 88,846 miles, Jup ite r is t he la rgest p lanet in the sola r system.Sulfur compounds and ammonia give i ts atmosphere i ts yel low and orange huesand whi te c louds. Saturn, second largest with a diameter of 74,900 miles, lacks thechemicals in the upper atmosphere that give Jupiter i ts colors . Saturn has less in-herent colo r, reflecting mainly the pale yellow of the Sun. Uranus is 31,764 mi lesin diameter, making it thethird largest. Neptune isclose behind with a diameterof 30,776 miles. Methane in thei r atmospheres gives these last two planets theirblue tones. Besides study ing them for thei r own sake,sc ient is ts consider otherplanets to be labora tories where different facets o f the early history of Earth andthe solar system as a who le can be exp lored and theor ies can be tested .

As d ifferent as the outer plane ts and their moons are from Earth and other bod-ies closer to the Sun, they all share certain character is tics and physical processes.The f ie ld of comparative planetology investigates the common threads l inking thepasts and fu tu res of the planets, thei r systems of moons and rings and their mag-net ic envi ronments . Processes observed on one body can be ext rapolated and ap-plied to another. For example , studying and comparing the meteorology of theouter planets can help scientists understand the basic forces that cont rol c limateand weather on Earth . Studies of the large moons of the outer sola r system helpdivulge the geophys ical processes common to sol id bodies. Studies of small moonsand p lanetary rings reveal in fo rmation about how the so lar system formed andhow i t has evolved.

EACH YEAR. VOYAGER EARTH TO THE SUN

O IMAGES OFURANUS WERE COM-

BINED AND PROCESSED

TO ENHANCE VARIA-TIONS IN THE PLANET S

ATMOSPHERE.

O CLOUDS INNEPTUNE'S ATMO-

SPHERE: THE GREAT

DARK SPOT AND ABRIGHT WHITE CLOUD

ARE JUST LEFT OF

CENTER.

O SATURN, WITHMOONS TETHYS

(ABOVE) AND DIONE,AS CAPTURED BY

VOYAGER 1.

O A MONTAGE OFJUPITER AND THE

FOUR GALl LEAN

SATELLITES. FROM

LEFT ARE IO,

GANYMEDE, EUROPAA ND C ALL IS TO.

O


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Voyager


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Rs the closest o f the four g ian t planets, Jupiter was bette r known than the othersbefore the Voyagers arr ived. Studied through telescopes s ince Gali leo, the broad,v is ib le character ist ics of the Jov ian system --the Great Red Spot and the lat itudi-nal bands - - were apparent, aswere the four Ga lilean moons, so-ca lled because oftheir discovery by the 17th-century I ta lian ast ronomer. E ight other moons alsocould be counted. Beyond the v is ib le features of the Jov ian system was the intenseradio noise emi tted from the planet. The noise , which is due to electrons spiralinginJupiter 's magnetic field, is loud enough to make Jupiter the second noisiest radioemitte r in the sola r system; after the Sun.

NASA's Pioneer 10 and I I spacecraft preceded the Voyagers to Jupiter in t heearly 1970s and sent back impor tant information about the powerful Jovian mag-net ic envi ronment, including measurements of the searing radiation bel ts aroundthe planet. The Pioneer exper ience at Jupiter led Voyager engineers to take ext rameasures to harden the spacecraft's electronic components against radiation.

Voyager I took 18 months to reach Jupiter, 400 million miles away. The space-cra ft made its closest approach on March 5 , 1979, while Voyager 2 followed onJuly 9 the same year. A torrent of new discoveries began as the spacecra ft nearedthe giant planet.

The ult raviolet spect rometer observed auroral activi ty so intense i t was detect-able on the day side o f Jupiter. As on Earth, the auroras are caused by chargedparticles plunging into the atmosphere.

O 1o (LEFT) ANDEUROPA, PASSINGBEFORE THE GREAT

RED SPOT, LARGESTOF JUPITER'S MANY

STORMS.

O RED-ORANGE IoAND WHITE EUROPA

ORBITING ABOVE

JUPITER'S MULTIHUED

CLOUDS.

O


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The complicated, swirling turbulence of the atmosphere and its beauty, reminis-cent of works by Vincent van Gogh, awed scientists and non-scientists alike. Theyellow, red and orange belts around Jupiter were shown to contain roiling, fluid-like storms. Huge cyclones moved through the upper atmosphere. A white jetstream of ammonia crystals clipped along at 350 miles per hour. The Great RedSpot, a hurricane-like storm three times the size of Earth, was seen surrounded byswirling currents that rotated around the spot and were sometimes consumed byit. The color of the spot, still unexplained more than a decade after the Voyagerflybys, may be due to traces of sulfur or phosphorus compounds.

Nothing in Jupiter's realm was as exotic or more unexpected than the discover/of active volcanoes on Io; it was the first time these had been spotted on any ob-ject in the solar system besides Earth. In March, Voyager I's cameras saw nine vol-canoes spraying sulfurous ejecta above the surface of the innermost Galileanmoon. When Voyager 2 arrived four months later, eight of those volcanoes werestill erupting. Io's volcanism explained its orange, red and black pepperoni-pizzaappearance. The spots were not some kind of strange impact crater, but probablylakes of molten sulfur .

Io occupies an orbit that makes it the perpetual object of a gravitational tug-offwar between Jupiter and the moon Europa. In the same way that the Moon raisestides in Earth's oceans, Jupiter's gravity raises a bulge in Io's waterless surface. Thetidal flexing of Io's crust heats the moon's interior and creates volcanic eruptionsof sulfur and sulf ur dioxide.

The next moon out is Europa. Its thin, icy crust is only weakly heated by tidalflexing, yet a tidally heated ocean may lie beneath the surface. The Voyagersfound Europa's face to be remarkably smooth and uncratered, yet striated withlines that are probably fractures in the ice surface.

O JUPITER'S ATMO-SPHERE OF VIOLENT

STORMS AND SWIRLING

EDDIES, INCLUDINGTHE GREAT RED SPOT

AT UPPER RIGHT.

O ONE O FOKI,

NINE VOLCANOES SEENERUPTING ON Io AS

VOYAGER 1 FLEW PAST

T HE S AT EL LI TE .

O ICY, CRUSTEDHE

SURFACE OF EUROPA

IS LACED WITH LINES

AND MAY HAVE MELTEDLONG AGO.


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_anymede and Cal isto, both about the size o f Mercury, a re each ha lf rock andha lf ice. When all the major moons in the solar system were finally measured bythe Voyagers, Ganymede turned out to be the la rgest, with a diameter of3,280 miles. Global tectonic processes appear to have been at work on this largebody, creat ing crisscrossing ridges. These rif ts, created by geophysical upheaval,surround other older , cratered terrain.

Call is to has escaped the geophys ical transformation the other Gali lean moonshave undergone, and instead retains all of i ts original sur face, s ti ll scarred bytherains of debris that l it tered the nascent solar system. This heavily cratered relicfrom the earliest days of the planets possesses one of the oldest surfaces n thesolar system.Much to the surprise of Voyager scientists , a r ing was found around Jupiter .Al though suggested by Pioneer II data, the finding was unexpected ; the theoryexplain ing the long-term stabi li ty of planetary r ings did not predict the existenceof a th in, dusty, Jovian ring. Two of the th ree smal l moons the Voyagers found or-biting between Io and Jupiter appear to shed the dust-size grains tha t form thetenuous r ing. Ult imately, new theories about r ings were developed asthe space-craft found unique ring systems at each of the four giant p lane ts.

CALLISTO, THEOUTERMOST GALILEAN

SATELLITE, FEATURES

A HUGE, MULTIRINGEDIMPACT S TRUCTURE

THAT RE SEMBLES

IMPACT BASINS ON

THE SURFACE OF THEMOON.

THE JOVIANRING SPARKLES AS

VOYAG ER 2 LOOKS AT

THE DARK SIDE OF THE

PLAN ET.

GANYMEDE'S SUR-FACE SHOWS CONTRAST-

ING, ANCIENT, DARKTERRAIN AND SOMEWHAT

YOUNGER, LIGHTERTERRAIN.

A M U LTI PLE EXPO-SURE OF JUPITER'S RING

AND TRAILS OF DISTANT

STARS APPEAR I N TH IS

RING-DISCOVERY PHOTO-GRAPH BY VOYAGER 1.


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The largest enti ty in the Jovian system is the inv is ib le force of the planet's mag-net ic f ie ld. Planetary magnetic f ie lds are created by the mot ion of f lu id inter iors .F if teen thousand miles deep wi thin Jupiter's interior , hydrogen undergoes adramatic change. At a pressure three mi ll ion times that at Earth 's sur face, and attemperatures exceeding 19,000 degrees Fahrenheit, the hydrogen changes frommolecular l iquid to a state called l iquid metall ic hydrogen, an excel lent elect ricalconductor . The l iquid metall ic hydrogen and the planet's rapid rotat ion(9 hours 55 minutes) generate electr ic currents that create Jupi ter's magneticfie ld, which is more than 10 times stronger than that o f Earth .

Jupiter's r ing and moons are embedded in an intense radiation bel t of elect ronsand ions trapped in the magneticfield. The Jovian magnetosphere,which compr ises these particles andfields, ball oons two or th ree million miles toward the Sun andtapers into a wind-sock-shaped ta il extending a t least 465 million miles behind Jup iter- as far asSaturn's orbit.

The re lationsh ip between the magnetic field and Io is un ique . As the magneto-sphere ro ta tes with Jupiter , it sweeps past Io, stripp ing away about a ton of mat-ter per second and forming a torus -- a doughnut-shaped ring around Jupiterpredominant ly composed of elect ri fied oxygen and sul fur glowing in the ult ravio-let. As these heavy ions migrate outward, their pressure inf lates the magneto-sphere to more than twice i ts expected size. Some of the more energetic ionsfal l into the atmosphere along the magnetic f ie ld to create Jupi ter's auroras.

As Io moves through Jupi ter's magnetic f ie ld, i t acts asan elect rical generator,developing 400,000 volts across i ts diameter and generating an elect ricalcur rent of three million amperes. The cur rent fl ows a long the magnetic fie ld toJupiter's ionosphere.

O IO S VOLCANOESCONTINUALLY RESUR-

FACE IT, SO THAT ANYIMPACT CRATERS HAVE

DISAPPEARED.

O THE CENTRIFUGALEFFECTS OF IO STORUS INFLATE

JUPITER'S ALREADY

IMMENSE MAGNETIC

FIELD.


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For 2,600 years, sky watchers have observed the slow progress of Saturn in i tsorbit around the Sun.Through his telescope, Gali leo noted strange appendageson either side of the pale yellow planet that he took to be attendant bodies.Chr is tiaan Huygens in 1655 concluded that the appendages were not attendants ,but a r ing enc ircl ing the planet. Later , ast ronomers would discover gaps separat inga multiple-ring system. Scientists have long sought to determine what the r ingsare made of, how they got there and what keeps them in orbit around Saturn.Some of the questions began to be answered when Voyager I ar rived at Saturnon November 12, 1980, and Voyager 2 arrived on August 25, 1981. But theVoyager discoveries would soon present many more new puzzles than solutions.

Saturn iscomposed of the same consti tuents asJupi ter, but in different mix-tures. Saturn rad iates 80 percen t more energy than it absorbs from the Sun. TheVoyagers found tha t the amount of helium in the atmosphere is less than ha lf thata t Jup ite r, consistent with the suggestion tha t the precip ita tion o f he lium out ofthe mainly hydrogen atmosphere could be supplying the internal heat that Saturnradiates.

Even with its self-manufactured heat, Saturn's atmosphere, compared toJupiter's, ismuch less turbulent and somewhat placid-looking. Given Saturn's rela-t ively bland appearance, scientists were surprised atthe high-velocity equatorialjet stream that blows some I, I00 miles per hour.

Computer ized enhancement of some of the Voyager images made apparentSaturn's faint banding through the planet's hazy upper atmosphere, and severalhurricane-l ike storm systems could be discerned. Saturn's greatest beauty andmyste ry, however, were not in its atmosphere , but in its rings; by the time theVoyagers reached and photographed the r ings, they were acknowledged asthemost exquisi te s ight in the solar system.

O THREE OFSATURN'S MOONS

(TOP T O B OT TO M) ,TETHYS, DIONE AND

RHEA, ARE VISIBLE.

TETHYS' SHADOWAPPEARS ON SATURN'S

CLOUDS.

O A FALSE-COLORIMAGE TAKEN THROUGH

GREEN, VIOLET ANDULTRAVIOLET FILTERS

SHOWS THREE LARGE

STORMS IN SATURN'S

NORTHERN HEMISPHERE.

O RINGS SATURNF

CAST A SHADOW ONTHE PLANET S SMOOTH

S U R FAC E AS TWO I CY

SATELLITES, TETHYS

(ABOVE) AND DIONE,ORBIT.


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Rather than being broad, feature less sheets, the rings were found to have an in-tricate structure. Scientists were amazed by strange shadowy streaks crossing thebroad B-r ing and forming spoke-l ike pat terns around the planet. These spokes,which appear to form, dissipate and re fo rm in a matter of hours, possib ly aresmal l grains of dust elect rostat ical ly lev itated above the larger particles . Int ime-lapse movies of Voyager images, the motion of the spokes revealed the rotationof the rings around Saturn. "We never thought we would be able to see ther ings spinning l ike this, " said one scientist . "We thought we would see the planettu rning with the rings asa still li fe . It now appears to be more nearly the reverse."The mater ia l in the r ings ranges f rom dust to boulder- and house-s ized particles ;

the r ings themse lves are not much th icker than the la rgest particles and arethought to be the remnants of large moons that were shattered by impacts ofcomets and meteoroids. The irregular shapes of the smalles t of Saturn's18 known moons suggest that they too are fragments of larger bodies.

The Voyagers found several narrow gaps, some occupied by slender r ings un-seen from Earth. Kinks and wavy edges were discovered in some of the r ings andwere associated with "shepherding moons," small satel li tes orbit ing within ther ing plane. The elaborate and disordered structure of some of the rings is duelargely to the gravi tational effects of these moons, asdemonst rated by the rela-t ionship between the F-r ing and two small moons, Pandora and Prometheus, thatorbit near one another but on opposite sides of the ring. These were the first ofthe many shepherding moons found orbit ing the outer planets.

O TH FALSE-S

COLOR PHOTOGRA PH

OF SATURN'S B- AND

C-RINGS REVEALS FINEDETAILS AND SUBTLE

C OL OR D IF FE RE NC ES .

O THE INTRICATECOLOR VARIATIONS IN

SATURN'S THREE MAIN

RINGS ARE COMPUTER

ENHANCED IN THISVOYAG E R 2 I MAG E.

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P_HOT_

Moons embedded in the rings were found to sweep pathways clear of particles,creating the gaps between rings. In addition, the Voyagers found spiral waves inthe rings, generated by the gravitational wake of moons orbiting outside the rings.

Titan, the largest moon of Saturn, has a diameter of 3,200 miles. It is coveredwith an orange photochemical haze too thick to see through, but Voyager scien-tists hoped for some openings that would allow a look at the surface. AlthoUghthere were no such openings, instruments on the spacecraft provided intriguingglimpses of Titan that had scientists making early plans to send a probe to explorethe terrain of the planet-size moon in the future. Because of uncertainty aboutwhether Titan's surface is composed of oceans, lakes, dr/land or some combi-nation thereof, the probe to be parachuted into Titan's atmosphere early in thenext centur/is being designed to land or float safely on anything ranging from liquidseas to sol id ground.

Titan has been described as "Earth in a deep freeze." The chemistr/in Titan'satmosphere may strongly resemble that on Earth before life evolved. Titan's sur-face temperature and pressure are-292 degrees Fahrenheit and 1.5 atmo-spheres, respectively. Photochemist r/conver ts methane in the Titan atmosphereto other organic molecules, such as ethane, that are thought to accumulate inlakes or oceans. Other, more complex hydrocarbons form the haze particles thateventually fall to the surface, coating it with a thick layer of organic matter.Scientists believe this might be the kind of environment where, if it were warmer,primitive life might evolve.

The satellite with a history of the most surface activity is Enceladus. The manyvalleys and faults of this moon attest to its tectonic vigor. Likewise, Tethys hasundergone internal stresses, as indicated by a valley extending more than half themoon's circumference. Another moon, IVlimas, showed a crater so huge that theimpact that caused it nearly brokethe satel li te apar t.

-: , ..._.: , .. . _ :_ i : ""

.. ,,,,4 . .

ENCELADUS'SURFACE MAY BE AN

EXAMPLE OF CRYOVOL-

CANISM, FEATURING"LAVAS" OF WATER ICE

AND OTHER SLUSHES.

HAZES, SHOWN I NFALSE-COLOR BLUE

AND ORANGE LAYERS,ARE FORMED BY OR-

GANI C PHOTOCHEM-

ISTRY OCCURRING IN

MIMAS, ONE OF

SATURN'S ICY SATEL-

LITES, SHOWS A HUGEIMPACT CRATER ABOUT

81 MILES IN DIAMETER.

Q A DARK RING ATABOUT 70 DEGREES

NORTH LATITUDE IN

THIS VOYAGER 2 VIEWOF TITAN MAY BE AN

EFFECT OF THE SPRING

SEASON IN PROGRESS.

TITAN'S ATMOSPHERE.

ETHANE tS CREATED IN " ITAN'S ATMOSPHERE AND RAINS DOWN ONTO THE MOON'S SURFACE


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Saturn 's magnetic field is only 5 percent that of Jup ite r. Of all the magnetic fieldsstud ied by the Voyagers, Saturn 's was the most orderl y; the axis of the field is al-most perfectly a ligned wi th the rotational axis of the planet.

Titan is often embedded in Saturn's magnetosphere and sheds photochemicallyp roduced hydrogen to fo rm a toms, somewhat simila r to tha t produced by mated-al swept off Io in Jupiter 's magnetosphere. Ions from the surfaces of Saturn's icymoons form a sheet o f ion ized oxygen in the p lanet's equator ial plane .

Bursts of radio emission from Saturn were heard at a frequency of 10 hours39 minutes, giv ing the length of a Saturnian day.

When Voyager I l eft Sa tu rn, it le ft the plane ts forever, heading on a trajectoryIthat took it northward, above the ecliptic plane in which the planets orbit the Sun.As it departed , Voyager I sent back stunning views of the dark side of Satu rn, tak-en from a vantage point only a robot could reach. When Voyager 2 made its clos-est approach to Saturn, the spacecraft 's path was gravi tational ly altered by ther inged planet and directed toward Uranus, another bil lion miles away.

Now le ft a lone to continue in to uncharted ter ritory, Voyager 2 made a some-what shaky start toward Uranus. After the closest approach to Saturn, the steer-able platform holding many of i ts inst ruments ( including the cameras) becamestuck. Engineers diagnosed the problem asa loss of lubricant, which soon migratedback, a llowing the ba lky platfo rm to move again . Bu t new ru les were created to re-strict the frequency and speed of movement, p reserving the ab ility to poin t the in-s truments dur ing the Uranus and Neptune encounters.

O

SATURN'S SHADOWCUTS THROUGH THEPLANET S RINGS AS

VOYAGER | LOOKS

BACK ON ITS WAY OUT

OF THE SOLAR SYSTEM.

A MONTAGE OFSATURN AND ITS

M AJ OR S AT EL LI TE S.CLOCKWISE FROM

UPPER RIGHT ARE

TITAN, MIMAS, TETHYS_DIONE, ENCELADUSAND RHEA.

_ATURN'S INNER RING, THE D-RING, EXTENDS TO WITHIN 4,000 MILES OF THE PLANET S CLOUD TOPS


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Uranus, the third largest planet, is known as the one tha t was "knocked on itss ide." Sc ientists postulate that Uranus was permanent ly thrown off i ts normal axiswhen it was struck by a body at least the size of Earth. As a result, the planet'spoles receive more sunlight than i ts equator ia l region. As at other planets , r ingsand moons orb it around the equator, g iving the entire Uranian system its odd,bull's-eye appearance.

At a distance of two b illion miles from Earth , Uranus is hardly an easy ta rge t ofstudy, even th rough powerfu l telescopes. Prior to the arrival o f Voyager 2 ,five moons had been counted, and dark rings had been detected. Only broad char -acteri stics o f the p lanet could be discerned . With its polar reg ions exposed to sun-light or darkness for long periods, scien tists were not sure what to expect.

No specia l equipment was placed aboard Voyager 2 for its Uranus and Neptuneencounters , so the spacecraf t had to be taught new ways of gather ing, process ingand send ing data to obtain the information needed at these dark and d istant plan-ets . As a spacecraf t f lies farther away, i ts s ignal received on Earth becomes increas-ingly weaker. By lower ing the rate at which Voyager 2 t ransmit ted data, engineersincreased the time devoted to the transmission of each b it o f da ta . Th is i sakin tospeaking slowly to be better understood ; less is sa id, but what is spoken has agreater chance of being heard c lear ly . S ince this normally would result in less datareceived, a new method was devised for compressing the data gathered.

New software was radioed to the spacecraft. The programming enabled onecomputer, a backup, to be devoted to compressing and formatting all imaging datapr ior to transmission . Far fewer bits were needed to rebu ild a picture receivedf rom Voyager 2.The process allowed the spacecraft to return thousands, ins teadof hundreds, of pic tures from Uranus and Neptune.

O

O

O TITANIA, THELARGEST OF URANUS'

MOONS, SHOWS LONG,DEEP FAULT VALLEYS.

O A DISTANCEROM

OF 600,000 MILES,VOYAGER 2 CAPTURED

THE SUNLIT CRESCENTO F U RA NUS .

O EVIDENCE OFOST

EARLY CRATERING ONARIEL HAS BEEN

ERASED BY SURFACE

MELTING AND OTHER

ACTIVITY.

O MIRANDA MAY HAVEBEEN FROZEN IN THEMIDST OF A GEOLOGIC

UPHEAVAL DURING

WHICH ITS BODY WAS

ALMOST LITERALLYTURNED INSIDE OUT.

O IS THEMBRIEL

DARKEST O F U RA NU S'

LARGER MOONS, RE-FLECTING ONLY21 PERCENT OF THE

SUNLIGHT IT RECEIVES.

O OBERON'S ICYSURFACE DISPLAYS

SEVERAL LARGE

IMPACT CRATERS.A |3-MILE-HIGH PEAK

PROTRUDES AT LOWER

LEFT.


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At DSN complexes around the wor ld, mul tiple antennas were elect ronicallyl inked together to improve the recept ion of Voyager 2's faint transmissions.

High noon on Uranus is not as br ight asdusk on Earth, and Voyager 2's cam-eras were not designed to opera te at such low ligh t levels, so new techn iqueswere deve loped to allow the cameras to track their targets while making longexposures. The ent ire spacecraft was moved with the camera's shutter open forexposures as long as 15seconds. This engineering feat would y ield sharply fo-cused c lose-ups throughout the remainder of Voyager 2's journey.

Due to the orientation of the Uranian system and Voyager 2's pa th th rough it,the spacecraf t' s discoveries there nearly came all at once.

A high layer of smog-l ike haze was found around the sun lit po le of the planet.Sc ient is ts cal led ultrav iolet l ight emanating f rom this region "day glow." Thebe lted latitudina l pa ttern seen in the atmospheres o f the other gian t planets ismaintained on Uranus despi te i ts orientation. A few methane clouds seen in theatmosphere revea led the presence of winds of nearl y 374 mi les per hour at themiddle lat itudes. Otherwise, the shallow hydrogen-helium atmosphere wasfound to be relatively inact ive, with no v is ib le hurricane-l ike storm systems.Voyager scientists had thought the poles would be warmer than the equator ia lregion , bu t were surprised to find that most o f the plane t shows nearly thesame temperature at the c loud tops.

O TIME-LAPSE IM-AGES SHOW CLOUDSNEAR THE LIMB MOV-

ING C OU NTER CL OC K-

WISE AND FASTERTHAN URANUS RO-

TATES m BEHAVIOR

EXACTLY CONTRARY

TO EXPECTATION S.

O COMPUTER EN-HANCEMENT EMPHA-

SIZES THE HIGH-LEVELHAZE IN URANUS'

UPPER ATMOSPHERE.

CLOUDS ARE OB-

SCURED BY THE OVER-L YI NG A TM OS PH ERE .

O A LATITUDE--LONGITUDE GRIDSUPERIMPOSED ON A

COMBINATION IMAGE

SHOWS THAT URANUS'ATMOSPHERE CIRCU-

LATES I N TH E SAM E

DIRECTION AS THE

PLAN ET ROTATES.


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Doyager 2's c losest approach to Uranus on January 24, 1986, also brought thespacecraf t near a l it tle moon cal led Miranda. I twould be the c losest satel li te f lybyof the encounter with Uranus, but the Voyager team had no way of knowing inadvance if Miranda wou ld be of particular interest or pale in compar ison to o ther,less accessible Uranian moons. Nevertheless, a series of close-ups of the moonwas planned. Fortunately for planetary scientists, Miranda was the scientific equiv-alent of pay dir t.

The smallest of the five Uran ian moons known before Voyager 2, Miranda"seems too diminutive and co ld to possess the diverse geologic features it has.Giant faul ts have s liced canyons asdeep as 12miles into the surface. Rol ling plainsthat are heavi ly cratered abut large patches of grooved and r idged terrain. Thesecharacter is tics suggest Miranda may have undergone strong tidal heating in the

past, possibly during a t ime when the small moon occup ied a chaotic orb it a roundUranus. Whatever forces shaped Miranda, they seem to have stopped. Planetaryscientists have theorized that Miranda was frozen in the midst of a geo log ic pro-cessthat most terrest rial (solid, Earth-l ike) bodies are thought to have undergoneat an early age , a process in which the body almost litera lly tu rns inside out. Stillother theories pu t forth that Miranda is the reaggregated sum of the parts o f oneor more moons tha t shattered in a co llision with a comet.

The other large Uran ian moons display the same variety found in Jup ite r's andSaturn's collec tions. Ar ie l and Titania both have val leys formed by faul ts . T itaniahas fault lines that may be the resu lt of the expansion of subsurface water as themoon's inter ior froze. Oberon and Umbriel have old surfaces l itt le changed s incetheir formation . Umbriers surface is darker than those of its compan ions and mayhave been b lanke ted by a layer of dark debr is. Any methane in its icy crust wou ldhave been darkened from the continua l bombardment by ions trapped in theUranian magnetic field.

O

A 1 40 -M IL E-WIDE PATCH OFM IRANDA'S SURFACE

SHOWS EVIDENCE OF A

VARIETY OF GEOLOGIC

PROCESSES.

T HI S NI NE- IMAG EMOSAIC OF MIRANDA

SHOWS OLD, HEAVILYCRATERED AREAS AND

YOUNG, COMPLEX RE-GIONS WITH SCARPSAND RIDGES.

tt

A FALSE-COLORVERSION OF PHOTO-

GRAPH "A" HIGH-LIGHTS THE TORTUOUS

CHANGES M [RANDA

HAS UNDERGONE .


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Ten new moons were found orbiting near the rings, bringing the total to 15.Based on the knowledge gained at Saturn, scientists predicted and found shep-herding moons usher ing the outermost of Uranus' r ings; the moons herding otherrings may have been too dark or tiny for Voyager 2 's cameras to record .

Voyager 2 counted two new Uranian rings in addition to the nine that were al-ready known. From the Uranian r ings, scientists have determined that planetaryrings may be short-lived phenomena that come and go throughout a p lanet's life-time. The particles in orbit around Uranus are thought to be pieces of a moontha t broke up; the rings contain rela tive ly li ttle dust, in contrast to Saturn's E- andF-r ings and Jupi ter's r ing. I t is thought that atmospher ic drag, due to the hydrogencorona that Voyager 2 observed around Uranus, causes dust particles to spi ral in-to the planet. In time, the rings may vanish. New rings would form only with thebreakup of another moon or a captured meteoroid or comet.

Perhaps the wei rdest aspect o f the Uranian system is its magnetic field . On oth-er planets , inc luding Earth, the axis of the magnetic field isnearly paral le l to theaxis of rotat ion, placing the magnetic poles near the geographic poles. But Uranus'magnetic axis i s t ilted about 59 degrees and offset from the cente r of the planetby 30 percent of its rad ius (4,500 miles), placing the magnetic po les nearer theequator. As it rotates with the planet, the field is f orced back by the wind ofcharged particles from the Sun and wound into a twisting magnetotail . Sc ientistsbe lieve the fie ld is genera ted at an in termed iate depth in the planet's in te rior --where the pressure ishigh enough for water to become elect rically conduct ive.

Radiation be lts a t Uranus were found to be of an in tensity similar to those atSaturn; i rradiation would quickly darken (wi th in 100,000 years) any methanetrapped in the icy sur faces of the inner moons and ring particles. Th is may ac-count fo r the darkened surfaces o f the moons and ring parti cles.

1t

O SMEARED IMAGESOF BACKGROUNDSTARS ARE VISIBLE

THROUGH THE RINGS

OF URANUS IN THIS

96-SECON D, WI DE-ANGLE EXPOSURE.

O URANUS' FAR-FLUNG RINGS,

PHOTOGRAPHED BYVOYAGER 2 AS IT

APPROACHED THE

PLANE OF THE URA-N[AN RING SYSTEM.

O VOYAGER 2'SPHOTOPOLARIM ETER

RECORDED THE DATAFOR THIS FALSE-COLOR PICTURE AS

THE STAR SIGMA

SAGITARI I PASSEDBEHIND URANUS'

DELTA RING.

O THE URANIANMAGNETIC FIELD'S

TILTED AXIS CAUSES

THE MAGNETIC TAIL TOTWIST BEHIND THEPLAN ET.


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R billion and a half miles beyond Uranus was Neptune -- and Voyager 2's last en-counter with a planet. At the time of Voyager 2's flyby, Neptune was the mostd istant member of the sola r system. (Pluto once again wil l become most d istant in1999.) The astonishing discoveries made by the spacecraf t at Neptune and i tsmoon Triton would make the fina l flyby the piece de r@sistanceof the Voyagermission.

Orbiting a t the edge of the sola r system, Neptune is t he smallest of the sola rsystem's giant planets. Very l it tle was known about i t before Voyager 2's arr ival onAugust 25, 1989, so nearly every thing the spacecraf t observed was new.

Because Neptune receives so l it tle sunlight (only 3 percent of Jupiter's total) , i twas expected to be somewhat less active than the o ther giant p lane ts. YetNeptune's atmosphere showed i tsel f to be surpr isingly dynamic, with windsb lowing westward, opposite the d irection of rotation , at more than 1,200 milesan hour -- faster than winds on any other planet. Neptune's winds are not veryturbulent , so lessenergy is needed to maintain their high speeds.

Early inV oyager 2's approach, a g iant sto rm the size of Earth peered out fromNeptune's atmosphere l ike a giant eye. Cal led the Great Dark Spot , i t resemblesJupiter 's Great Red Spot , with winds blowing counterclockwise and moving west -ward at almost 745 miles per hour. A small , i rregular ly shaped, eastward-movingcloud was seen darting around Neptune every 16 hours or so . This "scoo te r," asVoyager scientists cal led it, may be the top o f a cloud plume rising above a deeperc loud deck. Long, bright c louds simi lar to ci rrus c louds on Earth were seen castingtheir shadows on lower level c louds in the atmosphere.

O REGIONS HIGHLYREFLECTIVE IN

T HE U LT RA VI OL ET

APPEAR BLUE IN

THIS PICTURE OFNEPTUNE'S MOON

TRITON TAKEN

THROUGH GREEN,VIOLET AND ULTRA-

VIOLET FILTERS.

O

O NEPTUNE'S HAZEAPPEARS IN FALSE

COLOR AS A RED RIM.WHITE CLOUDS AND A

CIRCULAR STORM

FORM AS HYDROCAR-

BON GASES CONDENSEINTO BRIGHTER AREAS.

O IMAGES TAKENDURING SUCCE SSIV EROTATIONS OF THE

PLANET SHOW

CHANGES IN THE

CLOUDS AROUNDNEPTUNE'S GREAT

DARK SPOT.


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Before Neptune's magnetic f ie ld was measured, sc ient ists bel ieved Uranus' to beun ique in its huge offset from the p lane t's axis of rotation . But Neptune's fieldturned out to be highly tilted too -- 47 degrees from the rotational axis and off-set at l east 0.55 rad ius (about 8,500 miles) from the physica l cen te r of the p lanet.Comparing the two magnetic f ie lds, scientists think that the unusual orientationsmay be characteristic of flows in the interior of both Uranus and Neptune- andnot the resu lt o f magnetic fie ld reversals, o r, in Uranus' case, o f the planet beingtipped on its side.

L ike its sibl ing giant p lane ts, Neptune was found to be comple te ly encircled byrings rather than surrounded by the partial ri ngs or "ring arcs" that had been de-tected from Earth. Some of i ts r ings are dif fuse, dusty sheets; others have unevenlyspaced clumps of dense mater ia l. These concentrations of r ing mater ia l are puz-z ling; the mater ia l should have spread uni formly throughout a particular r ing in justa few years.

Two moons, Nereid and Triton, were known to orbit Neptune, but Voyager 2found six more. The new moons are all very dark and small, ranging from 30 to250 miles in diameter. Like the r ings, the smalles t of these are probably f ragmentsof larger, shattered moons.

Voyager 2's close look a t Triton did no t di sappo int a science team that hadlearned to expect surprises. This la rge moon, which hasa thin a tmosphere of ni-t rogen and methane, is the coldest object in the solar system, with atemperatureof-391 degrees Fahrenhei t. St il l, i t possesses a remarkable range of act iv ity, in-cluding geyser-like plumes. These erupt ions carry dark hydrocarbons upwards sev-eral miles, where they are b lown away in high winds.

O NEPTUNE ANDITS LARGEST SATEL-

LITE, TRITON (LOWER

RIGHT), SEEN

THROUGH VIOLET,CLEAR AND ORANGE

FI L TERS.

I N N EP TU NE 'SOUTERMOST RING,

39,000 M I LES OUT,MATERIAL MYSTERI-

OUSLY CLUMPS INTO

THREE ARCS.

MOVING EASTWARDAT DIFFERENT VELOCI-

TIES, NEPTUNE' SGREAT DARK SPOT

(TOP), "SCOOTER"(MIDDLE) AND DARK

SPOT 2 (BOTTOM)ARE RARELY SEEN

TOG ETH E R.


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@

Tr iton's relat ively high densi ty and i ts retrograde orbit (opposi te to the directiono f Neptune's rotation) indica te that the satel lite is not an original member of theNeptune system, but is a captured object. If tha t is the case, tida l heating couldhave melted Triton during the time it had a more eccentric orbit. The moonmight have been molten fo r as long asa billion years.

Fault valleys crisscross Triton's frozen, water-ice surface near the equator. Thefaults a re filled by r idges of i ce extruded from the in te rior . The volcan ic cra te rshave been repeatedly f looded and ref rozen, forming bright, lake- like features.

Neptune's radiation bel ts encompass Tri ton, possibly producing the pink andreddish hues on the moon's surface by i rradiation. F ine-grained f rost on Tri ton'ssurface appears blue.

Some of the charged particles travel ing along Neptune's magnetic f ie ld l inesst rike Tri ton's atmosphere, exc iting atoms there and producing an auroral glowdetected by Voyager 2's ult raviolet spect rometer. Tr iton and Saturn's Titan areamong the few satellites in the sola r system to have atmospheres, and the onlytwo moons known to have auroras.

Plu to may remain unexplo red , bu t Tri ton gave scientists a g limpse of what toexpect from that planet. Tri ton and Pluto share many characteristics : they areabout the same size and havethin atmospheres. Both are about three-quartersrock and one-quarte r ice; each has a layer o f methane ice at its surface . Giventhese similarit ies, planetary scientists recognize that Voyager 2's f lyby of Tritonprovided the best look we may have of Pluto for a 10ng time.

With the p lane ta ry phase of its long mission accomplished, Voyager 2 took asharp dive past Neptune, heading southward intoa course that eventually wil ltake the spacecra ft ou t o f the sola r system.

OO LONG, NARROWFAULTS ABOUT

20 MILES ACROSS

ARE VISIBLE IN THISIMAGE OF TRITON.

THIS OFIEW

AN AREA ON TRITONABOUT 300 MILES

ACROSS SHOWS A

LARGE, ICY BASIN.

DOZEN IMAGESWERE COMBINED TO

PRODUCE THIS COM-PREHENSIVE VIEW OF

T IlE NEP TUNE -FAC ING

HEM]SPHERE OFTRITON.

VOYAGER POST-'sENCOUNTER VIEW OFNEPTUNE'S SOUTH

POLE AS THE SPACE-

CRAFT SPED AWAY ONA SOUTHWARD

TRAJECTORY.

O


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Voyager the Grandest Tour. the Mission to the Outer Planets

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Transcript of Voyager the Grandest Tour. the Mission to the Outer Planets