Voyager 1 Jupiter Encounter Press Kit

8/8/2019 Voyager 1 Jupiter Encounter Press Kit

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ff~~tNewsNational Aeronautics andSpace Administration

Washington. D C 20546AC 202 755-8370

For Release THURSDAYFebruary 22, 1979

Press Kit Project voyager 1 u p i t e rRELEASE NO : 79-23 Encounter

ContentsGENERAL RELEASE ................................. l 1-11

VOYAGER CRUISE PHASE .............................. 12-15

VOYAGER 1 SCIENCE EXPERIMENTS ..................... 16-29

VOYAGER EXPERIMENTS ............................... 30-31

JUPITER AND ITS SATELLITES ........................ 32-36

THE SATELLITES OF JUPITER ......................... 37

PLANETCOMPARISON TABLE ........................... 38

SCIENCE TEAMS ...................................... 39-41

VOYAGER TEAM ...................................... 42

Mailed:February 22, 1979


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National Aeronautics andSpace AdministrationWashington. D C 20546AC 202 755-8370

Nicholas Panagakos For ReleaseHeadquarters, WaFhington, D.C. THURSDAY(Phone: 202/755-3680) February 22, 1979

Frank BristowJe t Propulsion Laboratory, Pasadena, Calif.(Phone: 213/354-5011)

RELEASE NO: 79-23

VOYAGER 1 EXAMINES JUPITER

NASA's Voyager 1 wil l conduct th e most detai led

scientif ic examination ever made of Jupiter, five of its

major sate l l i tes and the violent environment of the

Jovian domain during the first week of March.

After 18 months of f l ight , the instrument-laden

spacecraft will make i ts closest approach to th e

Solar System's largest planet on March 5, taking

pictures and making measurements.

The spacecraft crossed th e orbit of Sinope, th e

most distant of Jupi ter ' s 13 moons on Feb. 10 and will

spend more than six weeks swinging through th e Jovian

system en route to the ringed planet Saturn and its

retinue of moons.

Only four months behind is Voyager 2, an identicalspac-c ra f t bound fo r a close encounter w.,ith J iTupi t -r onJuly 9. Voyager 2 also will go to Saturn and maycontinue on to Uranus.

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The two-spacecraft expedition could span a decadeand is expected to produce a wealth of new information onas many as 15 major bodies of the outer Solar System.

Many scientists believe that the giant outer planets

may hold secrets to the origin of the Solar System that theyhave changed little since their formation, and that their

hydrogen-rich atmospheres ma y be similarto Earth's at-

mosphere in its geologic past. Voyager data thus may shed

new light or . the early history of the Solar System and onthe origin of our own planet.

Voyager 2 was launched first, (Aug. 20, 1977) fromCape Canaveral, Fla., aboard a Titan Centaur rocket. OnSept. 5, Voyager 1 was boosted onto a faster, shorter

trajectory, overtaking and passing its sister spacecraft

before the end of the year. At Saturn encounter, Voyager 1will be about nine months ahead.

Using Jupiter's enormous gravity, the trajectories

will be bent and the Voyagers accelerated for the Saturn

leg of the mission. Voyager 1 will arrive at Saturn in

November 1980 and Voyager 2 in August 1981. An optionexists to change the Voyager 2 trajectory at Saturn for aJanuary 1986 Uranus encounter.

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After completing their planetary missions, both space-

craft will search for the outer limit of the solar wind,

that boundary somewhere in our part of the Milky Way where

the influence of the Sun gives way to the other stars of

the galaxy.

Each Voyager uses 10 scientific instruments and its

spacecraft radio system to study the planets, their prin-

cipal satellites, Saturn's rings, the magnetic and radi-

ation regions surrounding the planets and the interplanetary

medium.

The Voyagers carry telescope-equipped, slow-scan TV

cameras; cosmic ray detectors; infrared spectrometer-

radiometers; low-energy, charged-particle detectors; mag-

netometers; photopolarimeters; planetary radio astronomy

receivers; plasma detectors; plasma wave instruments and

ultraviolet spectrometers.

More than 80 scientists make up the Voyager science

investigation teams.

The television investigation will result in as many as

50,000 pictures of Jupiter, Saturn, Saturn's rings, 11 of

their moons and open space near the planets in a search

for new Jovian and Saturnian satellites. Uranus, its newly-

disnovered ring system and one or more moons also may be-

come targets for the Voyager 2 cameras.

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The other Voyager instruments will study the planetaryand

satellite atmospheres and ionosphere; the magnetos-pheres of Jupiter and Saturn and the relationships be-tween these regions and the solar wind that streams fromthe Sun through interplanetary space; and radio signalsfrom Jupiter which, after the Sun, emit the strongestradio noise in our sky. Other objectives include all-skysurveys of interplanetary space and the measurement ofcosmic rays which invade the Solar System from other regionsof the galaxy.

Measurements of Voyager's radio communications waveswill provide information on the gravitational fields andatmospheres of the planets and their satellites, the ringsof Saturn, the solar corona and general relativity.

Until recently, our knowledge of Jupiter and Saturnwas only rudimentary. Ground-based studies by optical,infrared and radio astronomy have defined the most basicproperties of Jupiter and Saturn and their satellites andhinted at the unique scientific potential of these systems.Even less is known about Uranus and its environs.

In 1973 and1974, Pioneer 10 and Pioneer 11 made re-

conaissance flights to Jupiter. As the spacecraft spedpast Jupiter, their instruments began to reveal the com-plexity of its atmosphere and the extend and strength ofthe Jovian magnetosphere.

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(Both spacecraft are stilloperating. Pioneer 11 will

fly close by Saturn in September this year.) Building

on the Pioneer experience, Voyager is the next step in the

exploration of the Jovian and Saturnian systems.

Jupiter and Saturn are by iar the largest planets and

together account for more than two-thirds of all the moons

in the Solar System Jupiter's diameter is 11 times that

of Earth and theplanet contains more matter than all of

the other planets and moons combined. With 13 known moons,

four of them the size of small planets, Jupiter is a kind

of miniature Solar System. Saturn has 10 satellites and a

spectacular ring system which appears to be made up of tiny

pieces of ice and snow.

Both planets are giant gaseous and liquid balls, com-

posed mostly of hydrogen and helium, with no apparent solid

surfaces. They have their own internal energy sources,

radiating more energy than they receive from the Sun. Their

atmospheres are driven b- r the same forces that act on

Earth's atmosphere, but on a much larger scale. Major cy-

clones, like the Great Red Spot on Jupiter, are thousands

of miles across and persist for decades and even centuries.

Jupiter has a powerful magnetic field and is sur-

rounded by radiation belts similar to the Van Allen belts

around Earth, but thousands of times more intense.

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The vast, rapidly spinning magnetosphere makes Jupiter

a major source of several types of radio radiation.

The four largest satellites of Jupiter -- Io, Europa,Ganymede an d Callisto -- provide a varying and fascinatingmicrocosm of unique worlds. Some are believed composedlargely of rock, some largely ice and water. Their sur-faces may range from lunar-like cratered plains, to salt-covered beds

of extinct seas, to exotic landforms createdof ice and mud. Totally unfamiliar geologic forms areexpected.

Most of Saturn's satellites apparently are composedmainly of ice. Titan, (Saturn's moor,), has, in addition,a relatively extensive atmosphere. Scientists expect thatorganic molecules are being created today in Titan's

at-mosphere.

The spectacular rings that encircle Saturn are notwell understood; they may be the remains of a satellitethat came too close and was crushed by the strong tidalforces of the planet; or they may be remnants of materialfrom the birth of the Solar System that never coalesced to

form a moon.

Jupiter orbits the Sun at a distance of 778 millionkilometers (483 million miles). One Jupiter year equals'1 8G EarLh years. Jupiter's day is 9 hours, 55 minuteslong.

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Saturn orbits at 1.42 billion km (883 million mi.).

It compleates on e orbit every29.46 Earth years. A day on

Saturn is 10 hours, 14 minutes long. The widest visible

ring ha s a radius of about 137,000 km (84,500 mi.) at it s

outside an d width of about 26,000 km (16,000 mi.).

Voyager scientific operations began shortly after

launch with observations of the Earth and Moon. Fields

an d particles measurements have been made almost constantly

during th e long cruise.

Th e Jupiter reconnaissance spans nearly eight months

during 1979 -- from early January until August.

Voyager 1 began it s "observatory" phase on Jan. 4, 1979,

studying large-scale atmospheric processes on Jupiter and

phenomena associated with the relationship between Jupiter's

magnetosphere an d th e large inner moons.

Voyager 1 makes its closest approach to Jupiter at

4:42 a.m., PST*on March 5 at a distance of about 278,000

km (172,750 mi.) from the visible cloud tops. Activity

will reach a peak during the "near encounter" phase, a

39-hour period surrounding closest approach.

*The spacecraft actually flies closest to Jupiter at 4:05a.m., PS T on March 5, 1979. Bu t if one were able to watchthe event from Earth, th e ey e would see th e close encounter37 minutes, 40 seconds, later -- the time it takes light totravel nearly 680 million kilometers (422 million miles).Th e same is true using the Voyager radio to wa'zh th e event.Hence, all times listed here are in Earth-received time.

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The spacecraft instruments will continue studying

the Jupiter system until early April.

Planetary operations for Voyager 2 will begin in lateApril and continue through early August. Closest approachto Jupiter -- this time at an altitude of about 644,000 km(400,000 mi.) -- will occur shortly after 4 p.m. PDT July 9.

The trajectory for each spacecraft is unique and was

designed for specific observations at both Jupiter and

Saturn. Voyager 1 will fly past Jupiter just south of theequator, while the second spacecraft makes its Jupiterpass deep in the southern hemisphere.

Although Voyager l's encounters with Jupiter's innermoons occur after the spacecraft swings past the planet,

Voyager 1 began photographing them two weeks earlier --Callisto Feb. 18 , Ganymede Feb. 25, Europa March 1, IoMarch 2 and tiny Analthea (the closest to Jupiter) March 4.

Voyager 1 flies to within 19,000 km (11,800 mi.) ofIo at 7:51 a.m. PST on March 5; Ganymede 112,000 km (69,600mi.) at 6:53 p.m., March 5; and Callisto, 124,000 km(77,000

mi.) at 9:47 a.m. on March 6. The closest Voyager 1will come to Europa will be 732,000 km (455,000 mi.) at9:56 a.m. March 5.

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The Voyager 1 close passage at Io was designed tosend the spacecraft through the so-called Io flux tube, a

magnetic link between 1o and Jupiter. The flux is a cylin-

drical zone where charged particles spiral along the Jovian

magnetic lines of force as they pass through Io. Voyager 1

will spend about five minutes in the flux tube under the

south pole of the satellite.

Another To phenomenon to be examined by Voyager I's

instruments is a cloud of sodium vapor surrounding the

satellite. Because Io's surface is believed to be a source

of sodium, the cloud may be generated by intense radiation

bombardment of the surface material.

The Voyager 1 trajectory also carries the spacecraft

behind Jupiter, relative to Earth and the Sun. As Earth

disappears behind Jupiter, then emerges after two hours

of occultation, the changes in the signal characteristics

of the spacecraft rad.o link with Earth will give informa-

tion about the vertica. structure of the atmosphere,

ionosphere and clouds. As the Sun is occulted by Jupiter,

the ultraviolet spectrometer tracks the limb of the planet

oDserving similar scattering effects on sunlight as it

penetrates the atmosphere. The Earth occultation experiment

begins at 8:14 a.m. PST March 5, and continues until

10:20 a.m.

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Su n occultation extends from 9:07 a.m. to 11:24 a.m.At Saturn, in late .1980, Voyager 1 will conduct Sun

and Earth occultation experiments with the planet, themoon Titan and Saturn's rings. If Voyager 2 goes on toUranus, occultations may be possible with the planet andit s rings.

Voyager 2 will fl y past Jupitezk's satell i tes on theinbound passage

as follows: Callisto on July 8 from 220,000 km(136,000 mi.) an d Ganymede an d Europa on July 9 from 60,000 kin(37,000 mi.) and 201,000 km (125,000 mi.), respectively.

It will no t repeat the close flyby of Io .

NASA's Office of Space Science has assigned managementof the Voyager Project to the Jet Propulsion Laboratory, (JPL) agovernment-owned facility in Pasadena, Calif., which

ismanaged for NASA by the California Insti tute of Technology.

JPL designed, assembled an d tested the Voyager spacecraft,an d conducts tracking, communications and mission operations.JPL operates the Deep Space Network fo r NASA's Office ofTracking and Data Systems.

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NASA program manager is Rodney A. Mills and the JPL

project manager is Robert J. Parks. Dr . Milton A. Mitz

is NASA program scientist. Dr. Edward C. Stone of Caltech

is project scientist.

Estimated cost of the Voyager Project, exclusive of

launch vehicles, tracking and data acquisition and flight

support activities, is $343 million.

(END OF GENERAL RELEASE. BACKGROUND INFORMATION FOLLOWS.)


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VOYAGER CRUISE PHASE

Voyager 2, first of two spacecraft launched duringhe 1977 opportunity, l if ted of f Complex 14 at Cape Cana-eral , Fla., aboard a Titan Centaur launch vehicle at0:29 a.m. ED T on Aug. 20. Liftoff occurred less than fiveinutes into th e window on the first day of the 30-day launcheriod.

Sixteen days later, an identical spacecraft , Voyager 1,as boosted into a faster, shorter trajectory which wouldarry it past Jupiter four months earlier than Voyager2nd Saturn nine months earlier than its twin. Th e secondaunch wa s delayed four days when it was no t immediatelyertain that Voyager 2's science boom ha d fully deployednd locked itself in place. Voyager 1 lifted of f at 8:46.m. ED T on Sept. 5, 1977.

On Dec. 15, 1977, Voyager 1 caught up with and passedoyager 2 at a distance of about 170 million km (105illion mi.) from Earth. Both spacecraft ha d begun passagehrough the asteroid belt a fe w days earlier -- or) Dec. 10oyager 1 exiting Sept. 8, 3978, and Voyager 2 on Oct. 21.he debris-strewn asteroid belt, which circles th e Sun be-ween the orbits of Mars and Jupiter, is about 360 millijonm (223 million mi.) wide and, at one time,was believed toresent a hazard to intruding spacecraft. The Voyagerser e the third and fourth spacecraft '-o mike such a crossing,ollowing the early Jupiter reconnaissance flights ofioneer 10 and 11 in 1973 and 1974.

First of a series of trajectory correction maneuverso refine flight paths and assure precise arrival times andncounter distances was executed by Voyager 1 on Sept. 11,977, an d by Voyaqer 2 on Oct. 11, 1977. Voyager l'snit ial maneuver wa s conducted in tdo parts and was completedn Sept. 13. A second maneuver was executed Oct. 29, 1977nd a third on Jan. 29,1979. Additional maneuvers werecheduled for Feb. 20 and March 16. A trajectory correctionaneuver fo r Voyager 2 wa s conducted onMay 3, 1978, withhe third and fourth maneuvers planned for May 26 an dune 27, 1979.

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Two major engineering problems -- one with eachspacecraft -- arose during th e long cruise phase of th emission.*

During a calibration of Voyager l' s scan platform onFeb. 23, 1978, th e platform's azimuth gears slowed andstalled to a standstill. During th e next three months,engineers determined that a small amount of soft, pliabledebris -- apparently retained in th e unit during it s as-sembly -- ha d found it s way into th e gears. By maneuveringthe platform through th e problem area, th e bits of debriswere crushed by th e gears, freeing th e platform.

Voyager's scan platform, upon which ar e mounted theplanet-tracking instruments, including th e tw o televisioncameras,can be rotated on two axes fo r precision pointing.

Voyager 2's primary radio receiver failed on April 5,1978, an d the spacecraft's computer command subsystem auto-matically switched in th e backup receiver. Unlike theVoyager 1 scan platform problem which ha s been resolved,Voyager 2's radio emergency remains a concern. Only asingle receiver is available to th e spacecraft which maybe expected to operate through Uranus encounter -- January1986 -- an d it is functioning with a faulty tracking-loopcapacitor.

Th e existing receiver ca n no longer normally followa changing signal frequency. Telecommunication engineers,however, have developed a technique of determining thefrequency at which the receiver is listening, then com-puting the frequency at which th e Deep Space Network stationmust transmit commands. This procedure ha s worked suc-cessfully since mid-April.

Because of th e loss of th e redundant receiver capability,a backup mission sequence Wds transmitted to Voyager 2 onJune 23 an d stored in the on-board computer. Th e backupsequence assures a minimum science activity at both planetsin the event ground command capability is lost. The se -quence can be updated periodically.

*Minor problems with several science instruments on eachspacecraft have occurred during th e mission. Status ofthese problems ha s changed periodically during the past18 months an d is expected to change throughout th e mission.A deetiled status report on each instrument will beavailable prior to Jupiter encounters in March an d July.

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During the long cruise, numerous tests an d calibrationsav e been conducted to allow scientists to evaluate theirnstruments an d th e acquired data.All of the instruments on both spacecraft made mea-urementsz during the cruise -- interplanetary magneticields, th e solar wind and sk y scans by the spectral in-truments -- ultraviolet spectrometer, infrared inter-erometer an d spectrometer an d photopolarimeterAnalysis of cruise data acquired by the Plasma Sciencenstrument on both spacecraft is underway -- data fromoyager 1 to about 4.5 astronomical units (AU) an d fromoyager 2 to

about 4.2 AU had been processed by the en d of978. (An astronomical unit is the average distance be -ween th e Su n an d Earth, about 150 million km (93 mil-i: n mi.).

The Magnetometer investigators measured direction an dtrength of magnetic fields in interplanetary space fo rorrelation with th e plasma data.Th e Low Energy Charged Particles experiment has ob -ained good data on solar flare an d interplanetary particlesnd Jovian electrons.

Th e Planetary Radio Astronomy experiment has definedhe polarization characteristicsof Jupiter's radio emis-ions. It also measured fo r the first time Earth's po-arization in the frequency range of 100 to 300 kilohertz- information valuable for comparison with Jupiter data.Voyager l' s Plasma Wave instrument, which measuresaves of charged particles moving in space in several fre-uency ranges including the audio, demonstrated its sensi-ivity by recording the operating sounds of th e spacecrafttself. Motions of the scan platform and firings of thettitude control gas thrusters were detected by the in-trument.

Th e television camera system has been exercisedhroughout the flight. Tw o weeks afterlaunch, as part ofn optical navigation and video recording an d playbackest, Voyager I captured both the Earth an d th e Moon inhe same picture. The two crescent bodies, never beforeortrayed together, were more than 11 1/2 million km (7 mil-io n mi.) from the spacecraft camera.


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By Dec. 10, 1978, still 85 days and 85 million km(53 million mi.) from Jupiter, Voyager l' s narrow-angle

TV camera obtained a series of pictures of th e planet re-vealing more detail than the very best ground-based tele-scopic photographs. Only a fe w Jovian disk images obtainedby Pioneer 10 in December 1973 an d Pioneer 11 in December1974, during their final 24 hours of approach to th e planet,exceed the resolution in th e distant Voyager pictures.

Th e December pictures as well as others obtained duringperiodic camera calibrations have shown dramatic changes inthe visible appearance of Jupiter during the past four

years. These broad-scale differences in the huge featuresof th e planet have been observed also by Earth-based astro-nomers.

Voyager l's Jupiter observatory phase, that segmentof th e mission when al l science activity wa s 2irected to-ward the planet, it s satellite system an d the Jovian en -vironment within th e solar system, began on Jan. 4, 1979,with the spacecraft 61 million km (38 million mi.) fromJupiter an d continued through th e month.

Activities during the 26-day period were designed toprovide scientists with a long-distance, long-term look atthe entire Jovian system an d Jupiter's large-scale atmos-pheric processes in preparation fo r near

encounter opera-tions on March 4 an d 5.

Th e narrow-angle television camera recorded four images,each through a different color filter, every tw o hours --the time it takes Jupiter to rotate 72 degrees -- providinga zoom effect at five selected longitudes. Th e pictures ar e beingstudied to determine the most interesting an d most rapidlychanging planet features fo r possible retargeting of imagingsequences closer in.

During a four-day period beginning Jan. 30, the narrow-angle camera took a picture of Jupiter ever 96 secondsusing three color filters. Some 3,500 images are beingprocessed into a color record of th e planet's rapid ro-tation with a ne w color image each three degrees fo r 10full Jupiter days.

Voyager 2 operations have been at a low level sinceDecember 1978 an d will resume a more active pace at thebeginning of th e observatory phase in late April 1979.


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VOYAGER 1 SCIENCE EXPERIMENTS

Voyager l's science experiments at Jupiter fall intothree broad classifications:

1. Jupiter's atmosphere an d it s dynamics, studied by:

o Imaging;

o Infrared Interferometer Spectrometer an dRadiometer;

o Ultraviolet Spectrometer;

o Photopolarimeter;

o Radio Science.

2. Five of th e satellites of Jupiter -- Io, Europa,Ganymede, Callisto an d Amalthea, studied by:

o Imaging;

o Infrared Irnterferometer Spectrometer an dRadiometer;

o Ultraviolet Spectrometer;

o Photopolarimeter;

o Radio Science

3. Jupiter's magnetic field an d it s interaction withthe solar wind an d the satellites, studied by:

o Plasma Science;

o Low-energy Charged Particles;

o Cosmic Ray;

o Magnetometers;

o Planetary Radio Astronomy;

o Plasma Wave;

o Radio Science.

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Magnetic Fields Investigation

The magnetic field of a planet is an externallymeasurable indication of conditions deep within it s interior.Four magnetometers aboard Voyager 1 gather data on th eplanetary magnetic fields at Jupiter, th e satellites, solarwind an d satellite interactions with th e planetary fieldan d th e interplanetary (solar) magnetic field.

The magnetometers allow th e interplanetary medium tobe examined -- th e tenuous, ionized an d magnetized ga s thatforms th e solar wind.

Th e Sun constantly emits electricallycharged particles --mostly protons an d electrons -- from the ionization of hydrogen.

Those particles are in th e fourth state of matter, calledplasma. Th e other three states ar e solid, liquid an d neutralgas. The plasma is of extremely lo w density, less than 100particles pe r cubic centimeter; it fills al l interplanetaryspace and forms th e solar wind. Because it is ionized(contains either more or fewer electrons than in its neutralstate), th e solar wind is an electrically conducting medium.

Th e solar wind is deflected by planetary magnetic fields(such as th e Earth's an d Jupiter's), an d streams around an dpast th e obstacles, confining th e planet's magnetic field toa region called th e magnetosphere.

Th e shape of Jupiter's magnetic field is no t very Noellunderstood. Because Voyager 1 and Voyager 2 arrive fourmonths apart, scientists can make long-term, continuousmeasurements of th e solar wind near Jupiter an d of themagnetosphere itself as it changes size and shape underchanging pressure of the solar wind.

Jupiter's magnetic field is shaped much differentlyfrom Earth's. Th e planet's rapid rotation rate ma y be on eexplanation, fo r th e magnetic field rotates with the planet.At great distances from th e planet, th e magnetic fieldlines appear to form a spiral structure, which could beexplained by outward-flowing plasma, on e of the thingsVoyager 1 will search for.

Interactions between th e large satellites an d Jupiter'smagnetosphere depend on th e properties of the satellite an dits ionosphere, on th e characteristics of the field-and-particle environment an d on the properties of Jupiter'sionosphere.

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A strong factor in the choice of Voyager's flight pathas th e desire to observe the region of interaction betweenJupiter an d th e satellite Io, called the flux tube. Th eflux tube is defined by the magnetic lines of force fromJupiter that pass through Io. An electric current up to1 million amperes is believed to flow through the fluxtube, producing dramatic effects.

Voyager 1 will pass through th e Io flux tube about20,500 km (12,750 mi.) from th e satellite. Th e spacecraftis scheduled to spend about 4 minutes in th e tube an d makethefirst direct observat ions of the puzzling phenomenon.

Jupiter emits decametric radio bursts (from 10 to 40megahertz) that ar e probably connected with plasma instabilitieswithin Jupiter's ionosphere. Io appears to have some influenceon those radio emissions by way of th e flux tube.

Cosmic Ra y Investigation

Cosmic rays ar e the most energetic particles in naturean d ar e atomic nuclei, primarily protons an d electrons.They comprise al l natural elements known to man. Overcertain energy ranges and at certain periods of time, thecontent ofcosmic rays is similar in proportion to that ofll the matter in the Solar System. Generally, however,their composition varies significantly with energy, indicatingto scientists that a variety of astrophysical sources androcesses contributes to their numbers.

Cosmic rays may, as we search for their origins, telluc h about the Solar System an d it s origins and processes.Cosmic rays ar e material samples of the galaxy, an d mayreveal ho w stars synthesize the vcaious elements in theirinteriors.

Voyager's Cosmic Ra y investigation will address theenergy content, origin an d accelerationprocess, life historyan d dynamics of cosmic rays.

Planetary Radio Astronomy

On e goal of the planetary radio astronomy (PRA) investi-gation is to search fo r lightning in Jupiter's atmosphere,since lightning ha s been postulated as a catalyst fo r theformation of life. Together with the Plasma Wave experimentnd several optical instru-ents, PR A may be able to demonstratethe existence of lightning on Jupiter, if it does exist.

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It is thought that lightning in an atmosphere ofhydrogen, methane, ammonia and water could set of f reactionsthat eventually form complex organic molecules.

The PRA will measure radio emissions from Jupiter inthe low-frequency range from 20 kHz to 40.5 mHz. (AM radiostations broadcast at frequencies between 550 and 1,600 kHz.)Scientists say emissions ranging in wavelength from less thanone centimeter to thousands of meters can result from wave-particle-plasma interaction in the magnetosphere and ionosphereof Jupiter.

While scientists are sure Io plays an important role inthe pattern of Jupiter's radio emissions, the big satelliteappears not to have anything at all to do with Jupiter's 1 mHzemissions, at least in the low-frequency ranges. Preliminaryresults from early Voyager data show no correlation between1 mHz bursts and Io in the low frequencies.

Infrared Interferometer Spectrometer and Radiometer

Jupiter, with its colorful and distinctive bands ofclouds, has puzzled scientists for centuries: Why are thebands -- light zones an d dark belts -- so well-defined?What gives them their color? How deep is the cloud cover?What lies beneath it?

Voyager's Infrared Interferometer Spectrometer andRadiometer (IRIS) will probe the atmosphere for answers tothose questions. Jupiter's satellites also will be explored.

Each chemical compound has a unique spectrum. Bymeasuring the infrared and visible radiation given off andreflected by an object, scientists can learn a great dealabout atmospheric gas composition, bundaoce, clouds, haze,temperatures, dynamics and heat balance.

Hydrogen, deuterium (heavy hydrogen), helium, methane,ammonia, ethane and acetylene have been identified in Jupiter'satmosphere above the

upper clouds. Deeper measurements --through holes in the clouds -- indicate the presence of water,deuterated methane, germane and phosphene.

Once the composition of an atmosphere is determined,knowledge of its absorption properties can be used to measurethe temperature at various depths as it changes with pressure.

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Jupiter's clouds appear to form well-defined layersin

he atmosphere;above the clouds is a tenuous haze. Th ease with which those structures absorb or emit infraredadiation an d light permits determination of cloud depthnd state (i.e., ice or aerosol).

Satellite composition and temperature maps will beonstructed using the distinctive spectral signatures ofce s an d minei-ls found on the surfaces. Together withictures of the satell i tes, the maps can ne used to studyhe geology an d evolution of the bodies, an d how they differith distance from Jupiter.

Photopolarimeter

By studying the way sunlight is scattered by thetmosphere of Jupiter and the surfaces of it s satellites,oyager's photopolarimeter can answer many questions abouthose bodies.

Eight wavelengths in the ultraviolet an d visible regionsf the spectrum from 2,35U to 7,500 Angstroms, are measuredn intensity to determine the physical properties of thetmosphere of Jupiter (perhaps even seeing evidence ofightning and auroral activity), the satellite surfacesnd the sodium cloud around Io.

Th e photopolarimeter will examineboth the large-scalend micro-scale structure an d properties of the clouds ofupiter. It will measure the vertical distribution of cloudarticles, an d the particle size and shape, an d providenferences on atmospheric composition.

Similar studies will define t'.e structures of majorlanetary features such as the Great Re d Spot, zones an d belts.he photopolarimeter will search for evidence of crystallinearticles in those features an d will gather data on the effectsf scattering an d absorption of sunlight by the particles.Jupiter's atmosphere will be compared with others thatre already fairly well known -- those of Earth

an d Venus.The photopolarimeter will study the density of atmos-heres at the satell i tes, if atmospheres exist there, theexture an d composition of the surfaces, the bulk reflectivitynd the sodium cloud around Io.

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The spectral reflectivity of a body can help determineits surface composition, whether it is rock, dust, frost, iceor the remains of meteors.

In 1973 scientists first suggested that gases escapingfrom a satellite atmosphere might not be able to escape thegravity field of the main planet, and would form doughnut-shaped clouds around the planet. That kind of cloud hasbeen found in the vicinity of Io; it is composed primarilyof sodium and hydrogen. It may extend as far as the orbitof Europa.

Io appears to be covered with evaporate salts, includeinatomic hydrogen, sodium, potassium and sulfur and perhapsmagnesium, calcium and silicon. They appear to be sputteredoff Io's salt-covered surface into its atmosphere by chargedatomic particles trapped in Jupiter's scrong magnetic field.

Radio Science

The radio that provides tracking and communication withVoyager also explores the planets and space.

Measurements of Voyager' s radio signals provide informationon gravity fields and atmospheres of Jupiter and its satellites,the solar corona and general relativity.

Changes in frequency, phase, delay, intensity andpolarization of radio signals between spacecraft and Earthprovide information about the space butween the two and forcesthat affect the spacecraft and alter its path.

When the spacecraft moves behind a body as viewed fromEarth (called occultation), radio waves coming from the space-craft pass through the ionosphere and atmosphere on their wayearthward. Changes in signal characteristics during thoseevents give information about the vertical structure of theionosphere, atmosphere, clouds and turbulence.

Imaging

Astronomers have photographed Jupiter since at leastthe late 1800s, starting first at Lick Observatory in Californiaand continuing later at Lowell Observatory in Arizona. Untilabout 1960, photography of Jupiter was conducted in a more-or-less random way: if the night was clear and some time wasavailable at the telescope and someone was inclined, he mighttake a picture of Jupiter. The next opportunity might notcome for weeks.

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That will work for an object like the Moon or Mars, butJupiter is al l weather -- every observation ever made ofthe planet is of weather and weather patterns -- there isnothing else to see. Random photos of Jupiter are of limitedvalue: like taking an occasional picture of the cloudshrouded Earth and then trying to forecast the weather. Itdoesn't give you much good information.

In the early 1960s, astronomers changed their observationalapproach to one in which they took pictures of Jupiter everyhour all night long, on every night that was good for observing.Many of those pictures were of poor quality, far from :netextbook examples.

Bu t they contain a wealth of information. In 10 yearsastronomers learned more about Jupiter than they had learnedin al l the time that preceded the new program.

They discovered: There is a periodic oscillation in themovement of Jupiter's Great Red Spot. The spot moves slowlyaround Jupiter, it isn't anchored at one longitude. Butthe spot does not wander smoothly; it moves, then stops,then moves again. These oscillations occur almost preciselyevery 90 days.

Another discovery: The Great Red Spot is no t a smoothblemish, bu t is a giant vortex. It has been compared

with ahurricane on Earth. Observers do no t know if the vortexphenomenon exists al l the time or only occasionally.

A third example: There appears to be a semipersistenthigh-velocity jet stream at a constant latitude in Jupiter'snorthern hemisphere. The current flows in the same directionas Jupiter rotates. Planetary observers have measured thevelocity of the winds at 170 meters a second (380 miles an hour).

But for its velocity, that jet stream resembles the samephenomenon on Earth. Earth's stream meanders north and south,carrying storms from the tropics to temperate latitudes andfrom the Artic southward. But Jupiter's rapid rotation, aday is less than 10 hours long, nails the northern streamr toone constant latitude.

There may be many more features like these on Jupiter.But even though astronomers try to photograph the planetevery hour, every night, they are at the mercy of Earth'smoist, turbulent atmosphere. Often observers have seensomething there, bu t have been unable to identify it.Ground-based pictures cannot answer the flood of qIuestionsscientists ask about the planet: The Voyager observationsar e expected to explore this in detail.

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Whilo the two Pioneer spacecraft saw Jupiter at high-

resolution for only a few days, the two Voyagers will takehigh-resolution pictures of Jupiter for almost eight months.

That is enough to provide a significant advance in ourknowledge of the planet.

The satellites, meanwhile, are another story. They arebodies the size of the moon and the planet Mercury thatcannot be clearly seen because they are too far away. OneEarth-based photo of lo suggests an orange hue e.t the polesand a whitish appearance near the equator. A picture ofGanymede taken by one of the Pioneers suggests mottling onthe surface.

But the Galilean satellites' discs are only 1/25th theapparent diameter of the planet Mars and 1/12th the size ofthe planet Merciry in the eyepiece of a telescope. They are,one scientist says, too small to show anything that can betaken seriously.

So no one knows what those t-iq, Moon-sized objects looklike. Scientists admit they will be surprised by the satellitepictures from Voyager no matter what they look like.

Io, for example, probably has a surface covered withfresh-looking craters. Europa's surface is probably covered

with ice, but no one knows the depth. If the ice is a fewcentimeters thick, Europa may look like Earth's Moon coveredwith snow; if the ice is tens of meters or kilometers thick,then all bets are off.

Ganymede and Callisto are probably almost entirely ice.But they must contain some silicates and they surely containsome radioactive material. Therefore, there is some internalheat so that at depth the moon is probably mostly liquidwater. How does that heat reach the surface? Uniformly?Or in convection cells as happens on Earth? If it is uniform,then Ganymede and Callisto may be bland, whitish, round ice-covered objects that show little. If the heat radiation is

no t uniform, then the satellites may look like almost anything.

Impact craters on the icy satellites may be spectacularfeatures, if the body penetrated deep enough to reach liquidwater. But ice flows and in a relatively short time thescars should disappear.

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Finally, there is an Earth application fo r th e ne wknowledge from th e Voyager images of Jupiter:

Th e inner planets -- Venus, Earth and Mars -- haveatmospheres dominated by solar heat that arrives at theequator and flows by a variety of methods toward th e poles.Th e equator, therefore, is hotter than th e poles.

But Jupiter is not that way. Jupiter's atmospherictemperature does no t appear to be dependent on latitude.There's a lo t of heat moving from within Jupiter to thesurface without any regard fo r where th e equator is , orth e poles or even where th e Su n is . It wells up uniformly

everywhere.

On Earth one region is different from al l th e rest. Itis a convective region called th e Intertropical ConvergenceZone (ITCZ). In that region near the equator, warm, moistair wells up to high altitudes. Th e moisture is condensedou t of th e air which then becomes very dr y an d cold. Thatdry, cold ai r moves outward and descends again, warming as itdoes. On either side of the zone, where the dr y ai r reachesthe surface, there is an arid, nearly useless desert. NorthAfrica is th e classic example.

There is a similarity, then, between Earthand Jupiter,on e that may allow scientists studying th e two planets to

assist each other.

Low-Enerqy Charged Particles

Th e Low-Energy Charged Particle instrument (LECP) is astrong coupling factor in Voyager's complement of fields an dparticles investigations, contributing to many areas ofinterest, including th e so'ar wind, solar flares, particleaccelerations, magnetic fields, cosmic rays an d satellitesurface structure.

Tw o detectors allow measurements during the longinterplanetary cruise and the encounters. Th e wide dynamicrange, combined with wide coverage in energy an d species,allows characterization of almost al l energetic particleenvironments that Voyager traverses.

The LECP measures particles traveling 2,400 to morethan 150,000 km (1,500 to more than 90,000 mi.) a second.High-energy particles travel at or near the speed of light --299,792 km s (186,282 mi./s).

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Observations of particle accelerations aid in betterunderstanding of solar flare processes, cosmic ray accelera-tions and processes in Earth's magnetosphere.

Next to th e Sun, Jupiter is th e Solar System's mostpowerful radio source. The reasons fo r that ar e not under-stood completely, but ma y come from th e interaction betweenJupiter an d Io. Th e Io-Jupiter interaction could be ofimportance in understanding other astrophysical radio sources.

Scientists also believe fusion research might benefitfrom Voyager's studies of trapped radiation around Jupiter.Particles in confined lasmas, forced to fuse in th e laboratory,release enormous amounts of energy. Scientists are trying tocontrol that energy source to solve some terrestrial problems.Jupiter is easily able to confine charged particles in it smagnetosphere.

Plasma Experiment

Traveling at supersonic speed, averaging 400 km s (250 mi./s),plasma streams in al l directions from th e Sun, forming th e solarwind. When th e solar wind reacts with Earth's magnetic field,many phenomena result, such as th e northern lights an dgeomagnetic storms. Similar events have been observed atother planets.

Voyager's plasma instrument measures plasma properties

including velocity, density an d temperature fo r a wide rangeof flow directions in the solar wind and planetary magnetospheres.

At Jupiter, the plasma team will study th e interaction ofth e solar wind with Jupiter; the sources, properties, forms,an d structure of Jupiter's magnetospheric plasma; and theinteraction of th e magnetospheric plasma with th e Galileansatellites.

Io is known to have a tenuous atmosphere made up ofatoms of sodium, sulfur and perhaps other species. The

atmosphere extends around part of the Io orbit.

Although the plasma investigation cannot observe theneutral atoms of these clouds directly, the neutral ga s iseventually ionized an d becomes part of th e Jovian magneto-sphere plasma. The instrument ha s been designed to detectionized sodium an d sulfur close to the orbit of Io.

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It is possible, too, that Ganymaede, fourth satelliteut from Jupiter (Europa orbits between Io and Ganymede),as a ring of neutral particles that serve as a source forons in the Jovian magnetosphere. If that is the case, thelasma instrument should detect some of those ions whenoyager is near Ganymede's orbit.

Jupiter's magnetosphere extends into space at least00 times the planetary radius. Since Jupiter's radius isbout 71,400 km (44,000 mi.), that places the leading edgef the magnetosphere about 7 million k (4.2 million mi.) orarther from the planet. That distanceappears typical for quiet magnetosphere. On at least two occasions theagnetopause -- edge of the magnetic field -- wa s found at0 planetary radii, half the other distance. It is probablehat the magnetosphere is compressed when the solar wind'sressure increases.

During Voyager l's encounter with Jupiter, the pressuref the solar wind at Jupiter and the size of Jupiter'sagnetosphere can be predicted using data from Voyager 2 --arther from Jupiter and closer to the Sun. By comparingata from both spacecraft during the Voyager 1 encounter,cientists will try to show how the Jovian magnetosphereeacts to changes in the incoming solar wind.Voyager's first encounter with Jupiter's magnetosphereil l be detected when the spacecraft crosses the bow shockave, a region of demarcation between the solar wind and theupiter environment. Voyager 1 is expected to cross theow shock about Feb. 26, a week before closest approach.Immediately behind the bow shock is a transition regionalled the magnetosheath that separates the solar wind fromhe magnetosphere. The inner boundary of the .. agnetosheath,he magnetopause, separates the modified solar wind plasma inhe Jovian magnetosheath from the plasma in the magnetosphereroper. Plasma in the magnetosheath slows down and is heatedy passage through

the bow shock. Plasma in the magnetosphereomes from several sources -- Jupiter's ionosphere, ions fromatellite surfaces and atmospheres, and the solar wind.In the inner magnetosphere, plasma trapped by theagnetic field is forced to rotate with the planet. Thisegion of corotation may extend as far as the magnetopausehe farther from the planet, the more the centrifugal forceauses stretching of the magnetic fie1ld l , mare or lessarallel to Jupiter's equator.

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The stretched field lines form a thin disk that confinesthe particles within an intense, thin sheet of current flowingaround th e planet.

Plasma Wave Experiment

Th e Solar System is filled with a low-density, ionizedgas called plasma. That plasma composed entirely of atomsthat are broken apart into electrons and charged positiveions, is a good electrical conductor with properties thatar e strongly affected by magnetic fields.

Plasma sources include the Sun, th e planets and perhapssome of their satellites. Low-density plasmas are unusual;ordinary collisions between ions ar e unimportant, an dindividual ions and electrons interact with th e rest ofthe plasma by means of emission and absorption of wag'es.

Localized interactions between waves and particlesstrongly control th e dynamics of the entire plasma medium,and Voyager's plasma wave instrument will provide the firstmeasurements of these phenomena at the outer planets.

Plasma waves are low-frequency oscillations that havetheir origins in instabilities within the plasma. They ar eof two types -- electrostatic oscillations or electromagneticwaves of very low frequency.

Th e plasma wave instrument measures Ch e electric fieldcomponent between 10 and 56,000 Hertz. By way of comparison,Voyager's magnetometers measure the magnetic vectors ofelectromagnetic plasma waves below 10 Hz, while theplanetary radio astronomy instrument measures waves withfrequencies aihove 20 kHz.

Plasma ions an d electrons emit and absorb plasma waves.While the resulting particle-wave interactions affect themagnetospheric dynamics of the outer planets and theproperties of the distant interplanetary medium, they have

never been directly observed in those regions, since plasmawaves cannot generally be observed fa r from their source andsince there have been no previous wave studies at th e outerplanets.

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Voyager is therefore returning the first directobservations of wave-particle interactions at great distancesfrom the Sun. Some effects to be studied include heating ofsolar wind particles at the outer planet bow shocks,acceleration of solar wind particles that produce high-energytrapped radiation, and the maintenance of boundaries betweenthe rotating inner magnetospheres and the solar wind streamingaround the planets.

Another objective of the plasma wave investigation is tostudy the influence of wave-particle effects on the interactionsbetween the inner satellites and the planet's rapidly rotatingmagnetosphere

Control of Jupiter's decametric radio bursts throughcoupling of Io's ionosphere with Jupiter's magnetic field isan example.

Special intensive plasma wave measurements will be madeas Voyager 1 passes through Io's flux tube, where strongcurrent systems are driven by Io's motion through theJupiter magnetosphere.

Io is thought to have salt deposits on its surface thatare weak conductors.

As Io moves through Jupiter's magnetic field, it producescurrent flow along the magnetic field lines connecting Io toJupiter, the flux tube.

Detection of lightning bolts in the atmosphere of Jupiterwould also be significant, as described earlier. The plasmawave instrument searches for "whistler" signals that escapeinto the magnetosphere from lightning discharges.

The descending-scale signal that is characteristic oflightning is caused by scattering of similar velocities whenthe direction of travel is along magnetic lines of force:higher frequencies arrive at the receiver sooner than lowerfrequencies. Using the high-rate telemetry usually reservedfor transmission of imaging data, the Plasma Wave instrumentwill play to Earth the entire audio signal of space -- plasmawaves, spacecraft power, thruster firing and other instruments.

Ultraviolet Spectrometcr

Voyager's ultraviolet spectrometer will study thecomposition and structure of Jupiter 's atmosphere and thet e n u o u s aLUimospilere surrounding at leas t one sa te l l i t e .

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Two ultraviolet techniques have been developed to probea planet's atmosphere without entering that atmosphere:

o Airglow observations of the weak emissions high inthe atmosphere -- where collisions between atoms and moleculesare rare.

o Measurements that look through the atmosphere at theSun, reading its ultraviolet radiation to measure absorptionand scattering by the planet's atmosphere as the spacecraftmoves into shadow.

Airglow observations measure atomic hydrogen and heliumin the upper atmosphere by recording the resonance scatteringof sunlight. Resonance scattering is what happens when atomsand molecules absorb solar UV at specific wavelengths andreradiate at the same wavelengths. That differs fromfluorescence, in which the activating wavelength is absorbedand energy is reemitted at different wavelengths. It is alsopossible that auroral-type emissions will be observed atJupiter.

As the spacecraft moves behind Jupiter, the planet'satmosphere passes between the Sun an d the UV spectrometer.Since the gases that make up an atmosphere have identifiableabsorption characteristics at short wavelengths, the spectrometercan measure how much of each gas is present at what temperature.

The important point is not how much sunlight enters theatmosphere, but what happens to it after it enters -- how itis abosrbed and scattered.


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JUPITER AND ITS SATELLITES

Everything about Jupiter is enormous: when the Solar Systernformed, 71 per cent of the material that di& not end up inthe Sun went to make Jupiter.

Jupiter is the fifth planet from the Sun. It completes oneorbit every 11.86 Earth years.

A day on Jupiter is complete in 9 hours, 55 minutes and30 seconds. The extremely rapid rotation causes the planetto be flattened at the poles: equatorial radius is 71,600 km(44,490 mi.), and the polar radius is 67,000 km (41,632 mi.).

Jupiter has 13 known satellites; a 14th may have been seenby Charles T. Kowal of Caltech, who also found the 13th in1974. The four largest satellites were discovered by thefirst ma n to aim a telescope at Jupiter - Galileo Galilei in1609-10. Galileo's discovery that Jupiter has satellitesprovided evidence that the Copernican theory of the SolarSystem was correct and that Earth is not the center. The foursatellites discovered by Galileo (grouped together and calle-the Galilean satellites) are Io, Europa, Ganymede andCallisto. They range in comparable size from the planet.Mercury to the Moon. All will be studied by the Voyageis.

Jupiter is comprised primarily of hydrogen. Indeed, it isso massive that very little of its original material couldhave escaped in the 4.6 billion years since it formed. Thesecond most abundant element in Jupiter is helium. The ratioof hydrogen to helium on Jupiter is about the same as in theSun. The solar ratio is roughly one atom of helium for 10molecules of hydrogen.

Three other substances have been identified spectroscopically:ammonia, methane and water. The presence of hydrogen sulfidehas been inferred.

The currently popular model of Jupiter's structure beginswith a small iron-silicatecore only a few thousand kilometersin diameter. The core is inferred because cosmic abundancesof the elements include small amounts of iron and silicates.The temperature there is thought to be about 30,000 degreesKelvin (53,500 degrees Fahrenheit).


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Surrounding the suspected core in this model is a thick layerin which hydrogen is the most abundant element. The hydrogen

is separated into two layers. :I n both it is liquid, but indifferent states: the inner layer, about 46,000 km (28,500 mi.)radius, is liquid metallic hydrogen, which means that thehydrogen is electrically conductive like ordinary metals. Thatform of hydrogen has not been observed in laboratories sinceit requires immense heat and pressure. On Jupiter it is thoughtto exist at temperatures around 11,000 degrees K (19,300 degreesF.) and at pressures about 3 million times Earth's sealevelatmosphere.

Th e next layer -- liquid hydrogen in it s molecular form --extends to about 70,000 km (43,500 mi.). Above that layer,reaching to the cloud tops for another 1,000 km (620 mi.) is

the atmosphere.

If the model is correct, Jupiter has no solid surface, butexists as a rapidly spinning ball of gas and liquid almost779 million km (484 million mi.) from the Sun.

One of the puzzles about Jupiter is the fact that it radiatesabout two and a half times the amount of heat that it receivesfrom the Sun. Early models postulated nuclear reactionsinside the planet, or heat from gravitational contraction.These ideas are no longer believed likely.

Because Jupiter is too small and too cold to generate nuclearreactions, scientists no w believe the excess heat beingradiated by the planet is stored heat left over from theprimordial heat generated when the planet coalesced out of thesolar nebula. As NASA's Dr. John Wolfe writes: "Jupiter can-not be radiating heat because it is contracting; on thecontrary, it is contracting because it is slowly cooling."

The visible surface of Jupiter consists of bands of clouds,alternating dark and light, from the equator to about 50 degreeslatitude. These bands appear to be convection cells that arestretched by Coriolis forces created by the planet's rapidrotation. By convention, the light features are called zonesand the dark ones belts. The light zones appear to be regionsof greater altitude and cooler temperatures than the dark belts.

Gas warmed by the planet's internal heat rises and cools inthe upper atmosphere and forms clouds of ammonia crystalssuspended in gaseous hydrogen. At the top of the zones, thecooler material moves toward the equator or the poles, isdeflected in an east-west direction by Coriolis forces, andthen sinks back to lower altitudes. A similar mechanism onEarth causes the Trade Winds.


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One of the most prominent features on Jupiter is the GreatRed Spot. It has been observed almost constantly since itsdiscovery 300 years ago by Giovanni Domenico Cassini. Itswidth is almost always about 14,000 km (8,700 mi.), but itslength varies between 30,000 km (18,600 mi.) and 40,000 km(24,800 mi.).

The Great Red Spot appears to resemble an immense hurricaneon Earth -- although it is much larger and ha s lasted muchlonger than any terrestrial storms. At one time scientistsbelieved it might be a phenomenon known as a Taylor column --a standing wave above a mountain or depression on the surface.But the current model of Jupiter has no solid surface, andthe Great Red Spot has wandered in longitude several timesaround the planet.

Other spots have been observed in the Jovian atmosphere thatare similar to but much smaller than the Great Red Spot.They, too, appear in the equatorial regions, but have relativelyshort lifetimes; the one most recently observed lasted justunder two years.

Above about 50 degrees latitude the bands disappear, and theJovian atmosphere becomes turbulent and disorganized. Itappears to contain many small convection cells such as thosethat create the belts and zones of the lower latitudes.

Radio astronomers found evidence for a magnetic field aroundJupiter during observations in the 1950s, when they discovered

radio-frequency emissions coming from the planet. Theemissions are confined to two regicis of the spectrum -- withwavelengths measured in tens of meters (decametric) and intenths of meters (decimetric). Another radio-noise contributioncomes from non-thermal mechanisms that depend on the planet'smagnetic field. This synchrotron radiation comes fromelectrons that move near the speed of light.

The satellite Io appears to have some link with the decametricradiation, since bursts of radio emissions seem to occur whenIo crosses the face of Jupiter. All radio emissions fromJupiter are associated with rotation of the planet's magneticfield.

While the Jovian magnetic field is essentially dipolar (northand south, like Earth's), its direction is opposite Earth's(the nee!die of a compass on Jupiter would point south). The

axis of the field is offset about 10.8 degrees from therotational axis, and the center of the axis is offset from thecenter of the planet by about one-tenth of a Jupiter radius.At the planet's cloud tops the field ranges between three and14 gauss. Earth's magnetic field at the surface averagesa ou on c-ha f g u ss.


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The shape of Jupiter's magnetic field is about the same asEarth's with some significant differences. The movement ofenergetic particles near the equator is intense, but at higherlatitudes falls off dramatically, there

is apparently anelectric-current sheet along the magnetic equator that trapsand holds particles there.

The five inner satellites of Jupiter affect the distributionof charged particles: as the satellites orbit Jupiter theysweep particles out of their watt and at the same time acquireintense radioactivity.

Jupiter's outer magnetosphere is highly variable in size,possibly due to changes in the solar-wind pressure; bothPioneer spacecraft flew in and out of the magnetosphereseveral times on their inbound legs. They first crossed themagnetopause -- outer edge of th e region -- at about 100 Rjand crossed again as close as 50 Rj. Earth's magnetospherewould shrink that much only in the event of the largest solarmagnetic storms.

High-energy electrons have also been observed in anotherunexpected place: ahead of the bow shock wave in interplanetaryspace. Scientists believe high-energy particles in Jupiter'smagnetosphere reach such velocity that they can escape.Reexamination of records from Earth satellites turned up thefact that these electrons had been observed for many years.They were believed, however, to be of cosmic origin. Nowscientists think they spin down the solar magnetic-field linesand intersect Earth, since their peaks occur every 13 monthswhen Earth and Jupiter are connected by the spiral lines ofthe interplanetary magnetic field.

Jupiter's satellites fall into three groups -- the largeinner bodies, then a group of four that are small, and afinal group, also four in number, that are far distant andhave retrograde orbits.

The five inner satellites are Amalthea (the smallest; about240 km (150 mi.) in diameter ; Io, larger than Mercury;Europa, Ganymede and Callisto.

Io is about 3,636 km (2,260 mi.) in diameter. It displayssome

of the most bizarre phenomena in the Solar System. Iohas red polar caps. For about 15 minutes after it emergesfrom behind Jupiter it is several per cent brighter than usual.The brightening occurs only about half the time. Scientistsbelieve it is caused by alteration of the colored material onthe surface while Io is behind Jupiter. Io has a layer ofionized particles about 100 km (60 mi.) above the surface; thesatellite has a tenuous atmosphere. Measurements indicate thethe atmosphere is about one-billionth as dense as Earth's.Io is surrounded by a yellow glow -- th e emission lines ofsodium.


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Europa, smaller than Io, has a diameter of 3,066 km (1,905 mi.),uropa appears to be a rocky body like Io, and

scientists says probably because it heated early in its history, to aemperature high enough to drive off the volatiles.

Ganymede is one of the largest satellites in the Solar System.ts diameter is 5,216 km (3,241 mi.). Ganymede may be mn'colyiquid water -- a planet-sized drop of water with a mud corend a crust of ice.

Callisto has a diameter of 4,890 km (3,038 mi.). It is darkerhan the other Galilean satellites apparently because moreocky material is exposed. It may have suffered the leasthange of all the big satellites since formation 4.6 billionears ago. Scientists believe it is probablyhalf water, and

ay containso little rock that radioactive heating has notelted or differentiated it.

All the outer satellites appear to be very different from thenner group. They are probably asteroids captured by Jupiter'sravity. Or they ma y be the remains of broken up satellites.heir orbits are fairly highly inclined (25 to 28 degreesrom the equatorial plane), and the outermost four pursueetrograde paths.


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THE SATELLITES OF JUPITER

NAME DIAMETER (km) DISTANCE (kmn) PERIOD

Amalthea (V) 24u 181,500 lh57m227sIo (I) 3636 422,000 d l 8 h 2 7 m3 3 .5sEuropa (II) 3066 671,400 3 13hl3m425Ganymede (III) 5216 1,071,000

7 d 3 h 4 2 m3 3 s) Callisto (IV) 4890 1,884,000 1 6 d 1 6 h 3 2 ml l 1 2 s

Leda (XIII) 7 (est.) 11,094,000 238.7dHimalia (VI) 170 11,487,000

2 5 0 . 5 7 dElara (VII) 80 11,747,000 259.65dLysithea (X) 14 (est.) 11,861,000 263.55dAnanke (XII) 14 (est.) 21,250,000

631dCarme (XI) 14 (est.) 22,540,000

6 9 2 dPasiphae (VIII) 16 (est.) 23,510,000 739dSinope (IX) 14 (est.) 23,670,000 758d


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COMPARISON TABLE

EARTH JUPITER SATURN

Radius 6,378 km (3,963 mi) 71,400 km (44,366 mi) 59,800 km (37,158 mi)(Equatorial)

Satellites 1 13 10 (?)

Year 1 11.86 29.46

Day 24 h gh 5 5 m3 3 s 10 h 26m (?)

Mass 1 317.9 95

Gravity 1 2.61 0.9

Mean Distance1 AU 5.203 AU 9.523 AUFrom Su n


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SCIENCE TEAMS

Imaging Science

Bradford A. Smith, Universitv of Arizona, team leaderGeoffrey A. Briggs, National Aeronautics an d Space AdministrationAllan F. Cook II, Smithsonian InstitutionG. Edward Danielson,Jr., California Institute of TechnologyMerton E. Davies, Rand Corp.Garry E. Hunt, University College, LondonTobias Owen, State University of New York at Stony BrookCarl Sagan, Cornell UniversityLaurence A. Soderblom, U.S. Geological SurveyVerner E. Suomi, University of Wisconsin

Harold Masursky, U.S. Geological. SurveyTorrence V. Johnson, Jet Propulsion Laboratory

Radio Science

Von R. Eshleman, Stanford University, team leaderJohn D. Anderson, Jet Propulsion LaboratoryTom A. Croft, SR I InternationalGunnar Lindal, Je t Propulsion LaboratoryGerald S. Levy, Jet Propulsion LaboratoryG. Le n Tyler, Stanford UniversityGordon E. Wood, Jet Propulsion Laboratory

Plasma Wave

Frederick L. Scarf, TRW Systems, principal investigatorDonald A. Gurnett, University of Iowa

Infrared Radiometry and Spectroscopy

Rudolf A. Hanel, Goddard Space Flight Center, principal investigatorBarney J. Conrath, Goddard Space Flight CenterDa n Gautier, Meudon, FrancePeter J. Gierasch, Cornell UniversitvShailendra Kumar, Je t Propulsion LaboratoryVirgil G. Kunde, Goddard Space Flight CenterP. D. Lowman, Goddard Space Flight CenterWilliam C. Maguire, Goddard Space Flight CenterJohn C. Pearl, Goddard Space Fl igh t CenterJoseph A. Pir rag l ia , Goddard Space Fl igh t CenterCyr i l Ponnamperuma, Univers i ty of MarylandRobert F. Samuelson, Goddard Space Flight Center


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Ultraviolet Spectroscopy

A. Lyle Broadfoot, Kitt Peak National Observatory,principal investigatorJean L. Bertaux, Service d'Aeronomie du CNRSJacques Blamont, Service d'Aeronomie du CNRSThomas M. Donahue, University of MichiganRichard M. Goody, Harvard UniversityAlexander Dalgarno, Center for AstrophysicsMichael B. McElroy, Harvard UniversityJohn C. McConnell, York UniversityH. Warren Moos, Johns Hopkins UniversityMichael J. S. Belton, Kitt Peak National ObservatoryDarrell F. Strobel, Naval Research LaboratorySushil K. Atreya, University of MichiganBill Sandel, University of ArizonaDonald E. Shemanksy, University of Arizona

Pholtoplarimetry

Charles F. Lillie, University of Colorado,principal investigator (cruise)Charles W. Hord, University of Colorado,principal investigator (encounter)David L. Coffeen, Goddard Institute fo r Space StudiesJames E. Hansen, Goddard Institute fo r Space StudiesKevin Pang, Science Applications Inc.

Planetary Radio Astronomy

James W. Warwick, Univ. of Colorado and ScienceApplications Inc., principal investigatorJoseph K. Alexander, Goddard Space Flight CenterAndre Boischot, Observatoire de ParisWalter E. Brown,Jr. , Jet Propulsion LaboratoryThomas D. Carr, University of FloridaSamuel Gulkis, Jet Propulsion LaboratoryFred T. Haddock, University of MichiganChristopher C. Harvey, Observatoire de ParisYo'andc LeBlanc, Observatoire de ParisRobert G. Peltzer, University of ColoradoRoger J. Phillips, Je t Propulsion LaboratoryDavid H. Staelin, Massachusetts Institute of TechnologyAnthony Riddle, University of ColoradoJeffrey Pearce, University of Colorado


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Magnetic Fields

Norman F. Ness, Goddard Space Flight Center, principalinvestigatorMario H. Acuna, Goddard Space Flight CenterLen F. Burlaga, Goddard Space Flight CenterRon P. Lepping, Goddard Space Fl igh t CenterFr i tz M. Neubauer, de r Tech:- chen UniversitatBraunschweigKen Behannon, Goddard Space Fl igh t Center

Low-Energy Charged Par t ic les

S. M. Krimigis, Johns Hopkins University, principali nves t iga to rThomas P. Armstrong, University of KansasW. Ian Axford, Ma x Planck Institut fu r AeronomieChang-yun Fan, University of ArizonaGeorge Gloeckler, University of MarylandLouis J. Lanzerotti, Bell Telephone LaboratoriesCarl 0. Bostrom, Johns Hopkins University

Plasma Science

Herbert S. Bridge, Massachusetts Institute of Technology,principal investigatorJohn W. Belcher, Massachusetts Institute of TechnologyJoseph H. Binsack, Massachusetts Institute of TechnoloayAlan J. Lazarus, Massachusetts Institute of TechnologyStanislaw Olbert, Massachusetts Institute of TechnologyVytenis M. Vasyliunas, Max Planck Institut fur AeronomieLe n F. Burlaga, Goddard Space Flight CenterRichard E. Hartle, Goddard Space Flight CenterKeith W. Ogilvie, Goddard Space Flight CenterGeorge L. Siscoe. University of California, Los AngelesArt J. Hundhausen, Iligh-Altitude ObservatoryJim Sullivan, Massachusetts Institute of TechnologyClayne M. Yeats, Jet Propulsion Laboratory

Cosmic Ra y

Rochus E. Vogt, California Institute of Technology,pr inc ipa l inves t iga to rJ. Randy Jokipii, University of ArizonaEdward C. Stone, California Institute of TechnologyFrank B. McDonald, Goddard Space Flight CenterBonnard J. Teegarden, Goddard Space Flight CenterJames H. Trainor, Goddard Space Flight CenterWilliam R. Webber, University of New Hampshire


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VOYAGER TEAM

NASA Office of Space Science

Dr. Noel W. Hinners Associate Administratorfo r Space Science

Andrew J. Stofan Deputy Associate AdministratorDr. Adrienne F. Timothy Assistant Associate

drdministratorDr. Geoffrey A. Briggs Acting Director,

Planetary DivisionRodney A. Mills Program ManagerDr. Milton A. Mitz Program Scientist

NASA Office of Tracking an d Data Systems

Dr. William C. Schneider Associate Administratorfor Tracking an d Data Systems

Charles A. Taylor Director, Network Operationsand Communication Programs

Arnold C. Belcher Program Manager fo r DS NOperations

Frederick B. Bryant Director, Network SystemDevelopment Programs

Ma~urice E. Binkley Director, DS N Systems

Jet Propulsion Laboratory, Pasadena, Calif.

Dr. Bruce C. Murray Laboratory DirectorGen. Charles H. Terhune,Jr. Deputy Laboratory DirectorRobert J. Parks Project ManagerRaymond L. leacock Deputy Project ManagerDr. Peter T. Lyman Deputy Project ManagerRichard P. Laeser Mission DirectorGeorge P. Toxtor Denuty Mission D4 rectorCharles E. Kohlhase Manager, Mission Planning

OfficeEsker K. Davis Tracking an d Data System

ManagerMichael R.. Plesset Mission Computing System

ManagerMichael J. Sander

Director, DevelopmentIntegration an d TestJames E. Long Director, ScienceFrancis M. Sturms Director, Sequence Design

an d IntegrationMichael Devirian Director, Space Flight

Operations

California Institute of Technology, Pasadena, Calif.

Voyager 1 Jupiter Encounter Press Kit

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Transcript of Voyager 1 Jupiter Encounter Press Kit