voyager at saturn, 1980

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he year is 1610. Shakespeare's plays are the glory of

England. The firs t newspapers have just been publishedin Europe. The Spanish are exploring the New Worldand feverishlyharvesting its riches. Scholars are compil

ing the King lames

versionof

the Biblethat will

be' - - - - - " ' ~ ~ . . . . . . . - - - I published next year.It is 67 years since a Polish priest named Nicolaus

Copernicus published his heretical description of the

universe-a description that is shunned by the Church,even in this enlightened year of1610.

On cold, clear winter evenings this year, an Italianscientist named Galileo Galilei is peering at the heavens with a new instrument

he has built. He learned about this device, which he calls a telescope, from aDutch spectacle maker. Am ong the celestial objects Gaiileo studies is Saturn, themost distant planet he know s about. Galileo turns his tiny, crude telescope toward

Saturn. He is ill-prepared for the surprise the instrument provides.Galileo 'has found other surprises in his surveys of the heavens, but Saturn

appears, believably, to have knobs protruding from its sides!Or are tlley knobs? Could they be-his telescope is primitive and smal l - Itcup

handles?" His next conclusion: Two or more satellites must lie so close to Saturnthat his telescope can't separate them. Later he decides even that conclusionis wrong.

Time passes andmore pressing things - includinghis own trial by the Inquisitionon charges ofheresy-occupy Galileo 's time and thoughts. Because he is themost respected scientist ofhis time, and by recanting his heresy that the Sun is

the center OJ the universe, Galileo survives the trial.Years later. old and isolated from the world, Galileo again turns his telescope to

Saturn. As he looks, his hands tremble, he shakes his head in disbelief, blinks his

eyes and stares again.The Itcup handles," or whatever they were,have disappeared!"God has tricked me," the deeply religious scientist writes this night in his journal.Today we know that every15 years Saturn 's rings, Galileo's Itcup handles," tum

edge-on to Earth and disappear from our view. But, because of its great distancefrom Earth, scientists still cannot be specific about Saturn's nature.

Cover: Voyager 1 photographed Satu rn andseveral of its satellites on September 17, 1980,from 75 million kilometers away. Shown areTitan (upper right) and Mimas and Enceladus(under the ring).

Facing page: Could Galileo have imagined thata spacecraft would travel to Saturn and , on itsway, photograph the 'Earth in tandem with theMoon? This unique photograph was taken onSeptember 18, 1977, when Voyager 1was 12 mil-lion kilometers from Earth ,



THE MISSION TO SATURN

The tw o Voyager spacecraft are

approaching Saturn, th e second

target of their multi-planet mission.Voyagers 1 and 2 were launched

from Cape Canaveral, Florida,toward the outer solar system in

late summer 1977. Voyager 1 flew

past Jupiter in March 1979 andVoyager 2 followed in July.

Each Voyager carries 11 instru

ments to examine Saturn, its satellites and rings. Voyager's radioits link with flight engineers and

scientists at the Jet PropulsionLaboratory-becomes a scienceinstrument as Voyager flies behind

Saturn, its huge satellite Titan,

and the rings themselves.

Voyager l's Saturn encounter

begins August 22, 1980, at a range

of 109 million kilometers from

A

the planet, and the instruments

begin a four-month-Iong task ofscrutinizing the Saturnian system .

Ultraviolet and infrared instru

ments scan the Saturnian system,

measuring the composition and

temperatures of Saturn's cloud

tops and the satellites' surfaces.Magnetometers and particle detec

tors analyze Saturn's magneticfield and how it reacts with the

solar wind. A radio-astronomy receiver and plasma wave sensor

will listen for the sounds of space

and of Saturn.By October 24, when Voyager 1

is 25,980,000 kilometers from

Saturn, the narrow-angle camera

can no longer capture all of Saturn's

disk in a single picture . Then both

the wide- and narrow-angle cam

eras will capture small segments

of the disk that will fi t together

B

in mosaics that should reveal ne w

details in the banded clouds.

By November 2, even four-pictu

mosaics of Saturn will not captu

the entire disk. The cameras wil

be directed, instead, to concentra

on atmospheric phenomena

that deserve special attention.

Mosaics and single photographs w

cover progressively smallersections of the planet. Now the

infrared instrument can begin to

map the temperature of Saturn

and the satellites.Finally, Titan looms large in th

camera eye and, on November 11

Voyager 1 flies behind the bigsatellite. As it disappears, enginee

and scientists at JPL record and

analyze data as Titan's methane

atmosphere alters the radio signa

that passes through it. Those me

surements will tell scientists how



dense the atmosphere is at the

and how it decreases with

how far it extends abovee surface, what its composition

and how it s structure and mo-vary with altitude. Th e data

also yield accurate measure-of Titan's size.

Titan is an important target for

1. I t is the largest satellitesolar system and is the only

to have a substantial

Astronomers believeTitan, with an estimated di-

of 5,832 kilometers (3,624could be a ball of ice larger

the planet Mercury. The

that Voyager's cameras

see Titan's surface are con-to be low because it will

hide behind opaqueclouds .

As Voyager 1 rushes toward

c

*

D

Saturn, it will photograph and mea-sure other Saturnian satellites

-Rhea, Dione, Mimas, Iapetus,Tethys, Hyperion, and Enceladus.Then Voyager speeds past the

planet itself, at 3:45 p.m. (PST)November 12 , from only 124,200kilometers away. Peering back-ward as it streaks away, Voyager 1

will continue to monitor theSaturnian system until the eve-ning of December 15 . Then the

exploratory effort turns toward the

second spacecraft, as Voyager 1begins its journey ou t of the solarsystem to interstellar space in the

Milky Way galaxy.Nine months later, on August

25, 1981, Voyager 2 will soar past

Saturn. Its photographic targetsand other scientific goals will bebased on new knowledge acquired

in the first encounter. Voyager 2

*.w-.** . if.

*-if

*

"**

E

if -

*-

A. Two Voyager spacecraft were launch ed frCape Canaveral, Florida , in th e summer of1977, beginning their journey to Jupiter, Satuand beyond .

B. Voyager 1 took this picture of Jupiter, 10

(left) , and Europa (right), in Februaryfrom 20 million kilom eters.

C. November 12,1980 Voyager 1's closestapproach to Saturn will be 125 ,000 kilom etefrom th e cloud tops. Six of th e satellites that

will be photograph ed are shown in their appimate position s at closes t approach by th espacecraft .

D. August 25, 1981 Voyager 2 will fly closeto Saturn than Voyager 1, passing about 101,kilom eters from th e cloud tops. Again , th esatellites are shown at their closest approachposition s.

E. Artist's conception of Saturn as seen fromth e satellite Rh ea.

* .-..

*f

"**



be targeted to continue on too other planets, Uranus and

. It will arrive at UranusJanuary 1986. The target point

would carry Voyager on to

it would arrive in

eptember 1989. Then it, too, will

epart the solar system and cone to study space as it ventures

t among the st ars.

HIGH-GAIN ANTENNA

(3.7 m DI A)

HIGH-FIELD

MAGNETOMETER

LOW-FIELD

MAGNETOMETER (2)

PLANETARY RADIO

ASTRONOMY AND

PLASMA WAVE

ANTENNA (2)

RADIOISOTOPE

THERMOELECTRIC

GENERATOR (3)

VOYAGER SPACECRAFT FEATURES

Spacecraft Weigh t 808 kg 117

Science In strument s We ight 105 kg 1

High-Gain Ant enna D iamete r 3.7 m

Radioisotope T herm oelectr ic Generato r -

IRTGj Powe r la t Sa turn )

Data Storage Capability 538 mill io

X-Band Da ta Rate

at Jupiter 115,200 bits per s

at Sa turn

ULTRAVIOLET

S

PLASMA

COSMIC RAY

INFRARED

SPECTROMETER

AN D RADIOMETER

PHOTO POLARIMETER

LOW-ENERGY

C HARGED PARTICLE

OPTICAL CALIBRATION

TARGET

44,800 bits per s



GOALS

370 years of observing and

Saturn, scientists saystill have much to learn aboutmost beautiful of planets.

know it has a radius ofkilometers; it has at least

satellites and probably more;t requires almost 30 years to com

one turn around the Sun;s day lasts but 10 hours, 39.4

and it is mostly hydrogend helium . But they still want

know more about Saturn itself,its rings are made of, how

the ring particles are, and

are formed . They want

understand more about the

surrounding Saturn,it s magnetic field.

It was less than a year ago that

learnedSaturn

has a mag-

A

netic field. When NASA's Pioneer

11 flew past the ringed planet, it

made the first certain measurements

of a magnetic field, apparently

caused by the dynamo effect of alarge, rapidly rotating body.

Saturn is one of the solar sys tem's four giants; the others areJupiter, Uranus, and Neptune .While all are similar on a gross

scale- huge accumulations ofhydrogen and helium with small,rocky cores-each exhibits its

own significant and unique properties . In Earth-based telescopesSaturn appears to be a smaller

version of Jupiter, a likeness

that increased after Voyager discovered a ring around Jupiter.But there appear to be important

differences between the two

planets, such as size (a 60,000-kilometer radius

for Saturnversus

c

A

OD

P E

F'

B

N

B

a 7l,600-kilometer radius for Juter), and their distances from th

Sun. Although Saturn is twice afar from the Sun as Jupiter, it receives only one-fourth the amo

of sunlight and one-hundred.th t

amount that strikes Earth.One key difference between J

iter and Saturn that Voyager 1 w

pay special attention to is their

mechanisms for generating inte

nal heat. Both Jupiter and Satur

A. False-color processing of this Voyager 1photograph of Saturn show s ultravioletfeatures of the atmosphere. The blue, brightultraviolet areas represent a high-altitude hThis type of data helps determine th e composition and variation in haze altitude. Thephotograph was taken on August 24, 1980, 107 million kilometers.

B. The clearest Earth-based picture of Saturwas taken by the 61-inch reflector telescope th e Catalina Observatonj in Tucson, Arizon



radiate about twice as much energy as they receive from the Sun.

hat heat, theoreticians believe,was generated 4.6 billion yearsago, as the solar system formed.Jupiter is so large that it s primordial heat has been escaping eversince. But some scientists claim

that Saturn should have cooled off

to equilibrium long ago. Therefore,Saturn's current surplus heat must

be caused by some other mecha

nism, perhaps the separation ofhydrogen and helium deep within

Saturn . The heavier helium probably sinks toward the center.

The subjects of Voyager l's reat Saturn are:

The planet

The ringsThe satellites, with emphasis on

Titan

* The magnetosphere

The Saturnian system is too farfrom Earth to be studied as thoroughly as the Jovian system, and

the satellites are only point-light

sources in a telescope. In contrast,astronomers are able to photograph

the large Jovian satellites-Io,Europa, Ganymede, and Callisto.

Saturn

Since Saturn is so far from the

Sun, it is colder than Jupiter. Material in it s atmosphere freezes at

greater depths than on Jupiter.Ammonia, for example, freezesand forms clouds on Saturn at adepth of two to three Earth atmospheres, instead of one atmo

sphere as on Jupiter. (One "Earth

atmosphere" is 1,000 millibars, the

pressure at Earth's surface.) Saturn

also displays fewer and more

subtle atmospheric features than

Jupiter, probably because a highaltitude atmospheric haze obscureSaturn's clouds.

Astronomers have determined

that the wind at Saturn's cloud

tops blows about twice as fast asthe wind on Jupiter, approximatel

1,400 kilometers (900 miles) an

hour. These calculations are basedon Earth-based observations ofspots in Saturn's clouds, combine

with Voyager's radio measurement

of the rotation of the in terior

of Saturn.Voyager scientists want to unde

stand the atmosphere in terms ofl . Its overall structure and compo

sition-what gases and other

A. Th e giant 64-m eter anten nas of th e wor ld-wide Deep Space Network receive Voyagers'radio signals and route th em to Mi ssion

Operations at JPL for analysis.



chemicals are present, in what

ratios, and the fine-scale (orvery small) differences fromplace to place;

from the planet. It has been photographed from Earth.

F-ring; identified in images takenby Pioneer 11. It is a very narrow

ring, distinct and separate fromboth the E-ring and the A-ring.

bu t some scientists doubt its

existence.Observations made in 1966

yielded ring thicknesses of 1 to 3kilometers (0 .62 to 2 miles), although recent analysis suggeststhat the rings, particularly the

E-ring, may be thicker.

2. Differences in temperature and

composition between bright

zones and dark belts; A-ring; outermost ring visible in

small Earth-based telescopes .. How material changes with

depth in the atmosphere; B-ring; the brightest ring lies in

side the A-ring, separated from itby the Cassini Division. The division is not clear of material, bu t

contains a minute amount of dust

or ice. From Earth, the B-ring appears comple tely filled; a Pioneer

11 image, however, shows light

leaking through, which indicates

that there may be holes in the ring.

The rings appear to be entirely

ice or ice-covered material a fewcentimeters to a few meters in diameter. Scientists want to measu

particle sizes and densities, and

Size, density, and compositionof the core. The core is thought

to be about the size of Earth,but 15 or 20 times as dense.

to determine if material other

Rings

The ring designations were assigned chronologically by their

date of discovery, and have nothing

to do with relative positions. This

summary will list them beginningwith the outermost ring.

C- or crepe ring; barely visiblein small telescopes, it lies inside

the B-ring.

than ice is present. Ho w and why

the rings formed is also a major

question. The manner in which

sunlight is scattered depends on

the area and size of particles and

their numbers. Jupiter's ring forward scatters because it contains

particles about 0.0005 centimete

(0.002 inches) in diameter. Saturn

E-ring; extending to about eight

Saturn radii (480,000 kilometers)

A D-ring may exist between

the Coring and the cloud tops,

Experiment

COsmic Ray

Imaging SCience

Infrared

In terierometerSpectrometer

LOW-EnergyCha rgedParticles

Magnetometer

Photopolarimeter

Planetary RadioAstronomy

Plasma

Plasma Wave

Radio SCience

UltraViOletSpectrometer

VOYAGER SCIENTIFIC

Principal Investigator INVESTIGATIONS

R.E . VOgt Primary MC a l i f o r n i ~ I . eaSUlementsTechnology nstltUte of ;ndergy spectra and isot .1i n energeti °PIC compo . .

earn Leader. c particles in OUter I SIUon of COsmic ra .Bradford Smi th I . P anetary magnetos hYparticlesUmversit f ' . maglng of planet Peres.TUcson y 0 ATlzona, not Possible f r o ~ a;d satellites at resoluf

Rudolf Hanel structures. arth. Atmospheric d ~ ~ ~ ~ i ~ ~ phase anglesGOddard S ' and surfaceCenter pace Flight Energy balance of

pera ture fields C planets. Atmosph .f'hnM.Krimigis lite Surfaces a ~ d ~ m p O S i t i o n and p h y s ~ ~ I ~ c::mpOSition and tetJ . Hopkin; Energy sp aturn's rings. a c aracteristics of s a t ~ ; -

mversity, A I' ectra and is -PhYSics L a b o ~ ~ o l r y e d particles In planetary ~ ~ P g I C composition of low

N netosphe -energy chorman N res and Inter I arged

GOddard S p : ~ s e Fl ' p anetary Space.Center Ight Planetary d

an InterplanetarArthur L. Lane Y magnetIc fields.

Jet Propulsion'Laboratory

James Warwick,RadIOphysics In, c.

Herbert· BridgeMassachu 'of Techn Isetts Institute

o ogy

Frederick L S fTRW . car

Systems G r ~ u p Team Leader.G. Len Tyler,'

Stanford University

UA

. ~ Y l e BroadfootOIversitYof '

SOuthern C 1'£ .a hOrnla

Methan e, ammo'spheres. Com osnla, molecular hYdro

faces and S a t ~ ' s l ~ ~ ~ and phYSical ch!an't and .aerOsols in atmPI --"65. Censtlcs f 0-

anetary radio ' . 0 satellite sur-

magnetospheres emISSIons and plasmaE' resonances .nergy spectra f In planetary

charged particle 0 . solar-Wind electrterstellar hydr s In planetary env ' ons and ions lowD ogen. Ironments a ci . . energyynamics of I ' n lomzed in-

tion p h e n o m e ~ anctary magnetosph

Interactions. D e ~ e ~ S m S o C i a t e d With ch::ge:dand satellite perturbaPh . lnatlOn of I partlcle_ I -

YSlcal properti i c ectron density p asma wavesatellite masses es 0 atmospheres and ' .Saturn's rin ' denSities and . IOnospheres PI

gs. 'gravity field . anet andAtmospheric com ' . s. Structure of

Thermal s t r u c t u r / ~ S l t J o n including hydrIn Interplanetary and upper atmospheres ~ g ~ n to helium ratio

Interstellar space. ' y rogen and helium'



backscatter strongly, bu t

had no way to measure ifis also forward scattering.

Radio signals sent by the spaceto Earth through the rings

help determine the size ofalthough photos

will not show individual

The radio signals can

particles 30 to 100 cen-

Saturn's atmosphere appears to ha ve beltszones similar to Jupiter's, but they are

bscured by a thick , high-altitude haze. A thing and a wide but diffuse E-ring lie

utside the A-ring. Both are only visible edge on .

. Until now, Saturn's satellites have remain edto us because of their vas t distances

Ea rth . Voyager 1will photograph eight ofmoons, shown here in a size compari

with our Earth and Moon , and may revea l

imilarly striking features .

NORTH EQUATORIAL BELT

C-RING __ ---,..

B-RING

A

CASSINI DIVISION

' - - - - ENCKE DIVISION

SOUTH EQUATORIAL BELT

timeters (1 to 3 feet) in diameter.Measurements of the signals' attentuation by the rings will giveinformation on particles that arelarger than a few centimeters.

Scientists want to determine

why density apparently differsfrom ring to ring, and if there arewaves or clumps of particles

within the rings. Waves may formin the ring material by gravitational forces from satellites out

side the rings. Large objects in the

rings themselves may also cause

local variations in density.

Satellites

The satellites of Saturn, other than

Titan, which is a class by itself,are different from any circling the

other planets. They are smaller

than Jupiter'S Galilean satellites,

but larger than Amalthea and the

tiny Martian satellites Phobos an

Deimos. Titan's size is thought to

be anywhere from 5 percent to

20 percent larger than the planet

Mercury, which makes Titan the

largest satellite discovered in our

solar system.Scientists are uncertain about

the densities of the Saturnian

satellites; they could range fromice bodies to a mixture of ice and

rock. For example, Titan could

either be all ice, or ice mixed wit

up to 15 percent rock.There are also uncertain ies

about atmospheric pressure on th

satellites. The atmospheric pressure is estimated to be between 2

millibars and 2,000 millibars at

the surface of Titan .In comparison

the surface atmospheric pressure

is about 1,000 millibars on Earth

and about 10 millibars on Mars .

____ NORTH

._-----

SOUTH TEMPERATE B

SOUTH SOUTH TEMPERATE BE

SOUTH POLAR REGION



These questions about size,composition, and atmospheric

pressure will be partially answered

by the Voyager spacecraft, which

is also expected to discover new,

smaller satellites outside the

A-ring.

Magnetosphere

Scientists want to know whySaturn's magnetosphere behavesdifferently from the magnetospheresof Earth and Jupiter. Pioneer datashow that Saturn's magnetic poleis offset by less than one degreefrom the rotational pole, whereas

the magnetic poles of Earth,Jupiter, and the Sun are offsetsubstantially.

Voyager has determined with

high precision the length of a dayon Saturn: 10 hours, 39.4 minutes.

That rotation period refers to the

B

interior of Saturn and was measured

with Voyager radio-astronomy

data obtained since January 1980.Earth observations had shown

similar periods from temperate

and polar regions of Saturn, but amuch shorter (10 hours, 14 minutes)

period near the equator; indicating

the presence of a high-veloci ty

equatorial jet stream.

o APETUS

o HYPERION

TITAN

0 RHEA

0 DIONE

0 TETHYS

0 ENCELADUS

MIMAS

EART

MOON



VOYAGER AT JUPITER

e two Voyager spacecraft pro-more knowledge (and more

about Jupiter, its fiveajor satellites, and the magnetic

d radiation environment ofhe Jovian system during the first

months of 1979 than hadaccumulated in the 369 years

e Galileo originated the scien-ific method and began telescopic

A few of the major scientificincluded at least eight

volcanoes on the sulfur-s satellite 10 ; a doughnut-shaped

ionized material orbiting

that is fed by lo's volcanoesd, in turn, pumps material across

he length and breadth of the entire

system; a ring of particl es

nding Jupiter; and auroras and

superbolts of lightning crackling

through the atmosphere.

A. Jupiter and its four planet-sized moons,called th e Galilean satellites, were photographedin March 1979 by Voyager 1 and assembled intothis collage. They are not to scale but are in

th eir relative position s.

B

B. This historic, ultraviolet photog raph of a vocanic explosion on 10 was taken by Voyager 1 in

March 1979, from a distance of490,000 kilomet

C. Voyager 1 photographed Jupiter from th edark side of the planet as the spacecraft proceedeon to Saturn.



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voyager at saturn, 1980

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