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Our Planets at a Glance. Information Summaries.National Aeronautics and Space Administration,Washington, DC. Scientific and Technical InformationBranch.NASA-PMS-010(KSC)Jul 8617p.; Photographs will not reproduce well.Superintendent of Documents, U.S. Government PrintingOffice, Washington, DC 20402.Reference Materials - General (130)

MF01/PC01 Plus Postage.*Earth Science; Physical Sciences; Science Education;Scientific and Technical Information; *SpaceExploration; *Space Sciences*Planets

People have gazed up at the cosmos for thousands ofyears and wondered about the wanderers of the heavens: the planets.The past 20 years have been the golden age of planetary explorationbecause of many enpeditions, most notably the Voyager and otherunmanned space craft. This document is a summary of the informationknown about the planets of the solar system as they are now known.There is discussion on "Interplanetary Spacecraft" and "Comparing thePlanets." A chart of 15 unmanned spacecraft denoting the mission,:aunch date, arrival date, and.status is included. (CU)

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Our PlanetsAt a GlanceFrom our watery world we have gazed upon thecosmic ocean for untold thousands of years. Theancient astronomers observed points of light whichappeared to wander among the stars. They calledthese objects planets, which means wanderers, andnamed them after great mythological deitiesJupiter, king of the Roman gods; Mars, the Romangod of war; Mercury, messenger of the gods' Venus,the Roman god of love and beauty; and Saturn,father of Jupiter and god of agriculture.

Science flourished during the European Renais-sance. Fundamental physical laws governingplanetary motion were discovered and the orbitsof the planets around the Sun were calculated. Inthe 17th Century, astronomers pointed a new devicecalled the telescope at the heavens and madestartling new discoveries.=11111111IIIMMI111b,

But the past 20 years have been the golden ageof planetary exploration. Advancements in rocketryduring the 1950s enabled mankind's machines tobreak the grip of Earth's gravity and travel to theMoon and to other planets.

American expeditions have explored the Moon,our robot craft have landed on and reported fromthe surfaces of Venus and Mars, our spacecrafthave orbited and provided much information aboutMars and Venus, and have made close range obser-vations while flying past Mercury, Jupiter, Saturnand Uranus.

These voyagers brought a quantum leap in ourknowledge and understanding of the solar system.Through the electronic sight and other "senses" ofour automated probes, color and complexionhave been given to worlds that existed for centuriesas fiszzy disks or indistinct points of light.

Jture historians will likely view these pioneer-ing flights to the planets as one of the mostremarkable human achievements of the 20thCentury.

NASA Planetary Exploration

Spacecraft Mission Launch Date Arrival Date

Mariner 2 Venus flyby 8/14162 12/14182

Mariner 4 Mars flyby 11128/64 7/14/65

Mariner 5 Venus flyby 6114/67 10/19167

Mariner 6 Mars flyby 2/24/69 7/31/69

Mariner 7 Mars flyby 3/27169 8/5169

Mariner 9 Study Marsfrom orbit

5/30/71 11)18/71

Pioneer 10 Jupiter flyby 3/2172 1213173

Pioneer 11 Jupiter/Saturnflybys

4/5/73 12/274 (Jupiter)911179 (Saturn)

Mariner 10 Venus/Mercuryflybys

1113/73 2/5174 (Venus)3/29/74 (Mercury)9/21/74 (Mercury)3/16/75 (Mercury)

Viking 1 Unmannedlanding on Mars

8/20/75 7/19/76 tin orbit)7/20/76 (Landertouchdown)

Viking 2 Unmannedlanding on Mars

9/9/75 8/7/76 (in orbit)9/3/76 Oandertouchdovvni

Voyager 1 Tour of Jupiter 915/77 3/5179 (Jupiter)11)12/80 (Saturn)

Voyager 2 Tour of theouter planets

82017 7 9 ?9 um-wt.,'8 ,:811'24'86 ILI,1989 (Noptont,)

Pioneer Venus 1 Orbital studiesof Venus

5/20178 12/4/78

Pioneer Venus 2 8/8/78 12/9/78

Status

Mission complete, craft in solar orbit.

Mission complete, craft in solar orbit.

Mission complete, craft in solar orbit

Mission complete, craft in solar orbit.

Mission complete, craft in solar orbit.

Mission complete. inoperable remains inMartian orbit.

Primary mission complete, craftcontinues to return heliosphericinformation enroute toward interstellarspace.

Primary mission complete. continues toreturn information enroute towardinterstellar space.

Mission complete, craft in solar orbit.

MICs,011 t()Mplete, Cr.if I ;entropy Onsin fact. anti in of bii But nIander and0111101 have ceasPci operat Ion

Mission complete, craft remains onsurface and in orbit. Both Lander andorbiter nave ceased operation.

Primary mission complete. Craftcontinues to return heliosphericinformation enroute toward interstellarspace

conteitii, C., turviivedof tow ptant,*w y tar twts On wait' tO

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Orbiter continues to return imagesand data

Mission complete. Probes impactedon surface.

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interplanetary Spacecraft

NASA's space probes to the planets have come inmany shapes and sizes. While they are designedto fulfill separate and specific mission objectives,the craft share much in common.

Each space probe has consisted of variousscientific instruments selected for the mission,supported by basic subsystems for electricalpower, trajectory and orientation control, and forprocessing data and communicating with Earth.

Electrical power is required to operate thespacecraft instruments and systems. NASA hasused both solar energy from arrays of photovoltaiccent; and small nuclear werators as powerplants on its interplanetary probes. Rechargeablebatteries are employed for backup and supple-mental power.

Imagine that a craft has successfully journeyedmillions of miles through space to fly only oncenear a planet, and its cameras and other sensinginstruments are pointed the wrong way as itzooms past the target! To help prevent this fromhappening, a subsystem of small thrusters is usedto control interplanetary spacecraft. The thrustersare linked with devices which maintain a fix onselected stars. Just as the Earth's early seafarersused the stars to navigate the oceans, spacecraftuse stars to keep their bearings in space. With thesubsystem locked .onto "fixed" points of reference,flight con'rollers can keep scientific instrumentspointed at the target body and a spacecraft'scommunications antennas pointed toward Earth.

To ensure that a space probe encounters aplanet at the planned distance and on the propertrajectory. another major subsystem makes coursecorrections after the spacecraft is enroute.

During the first decade of planetary flights,NASA spacecraft were dispatched to scan theother inner planetsMercury, Venus, and Mars.These worlds, and our own, are known as theterrestrial planets because of their similarity toEarth's rocky composition.

For these early reconnaissance missions, NASAemployed a highly successful series of spacecraftcalled Mariners. Their flights helped shape theplanning of later missions. Between 962 and1973, seven Mariner missions were successful.Three Mariner attemptsthe first, third andeighthfailed

All of the Mariner spacecraft used solar panelsas their primary power source. The first and thefinal versions of the spacecraft had two wingscovered with photovoltaic cells. Other Marinerspace probes were equipped with four solarpanels extending from their octagonal bodies.Spacecraft in the series ranged from under 500pounds (Mariner 2 Venus probe) to more than2,000 pounds (Mariner 9 orbiter). Their basic

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An Atlas Centaur rocket lifts away from its Cape Canaverallaunch pad carrying the Mariner 6 space probe.

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The Voyager I probe to the outer planets awaits final launchpreparations at Kennedy Space Center.

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design, however, remained quite similar through-out the program. The Mariner 5 Venus probe, forexample, had originally been a backup spacecraftfor the Mariner 4 Mars flyby. The Mariner 10spacecraft sent to Venus and Mercury used com-ponents left over from the Mariner 9 Mare orbiterprogram

In 1972, NASA opened the second decade ofplanetary exploration with the launch of a Jupiterprobe. Interest was shifting to the outer planets,giant balls of dense gas quite different from theterrestrial worlds we had surveyed.

Four spacecrafttwo Pioneers and twoVoyagerswere sent to tour the outer regions ofour solar system. They will eventually become thefirst human artifacts to travel to distant stars.

Because they are traveling even farther from theSun, the outer p:anet probes operate on nuclear-generated electric power.

While probing new territory ty fond the asteroidbelt, NASA developed highly specialized spacecraft to revisit our neighbors Mars and Venus.Twin Viking lenders evolved from the lunar Sur-veyor program. The Mars lenders were equipped toserve as biology laboratories, seismic andweather stations. Two advanced orbitersdescendents of the Mariner craftwere sent tostudy Martian features from above.

The Pioneer Venus orbiter, a drum-shapedspacecraft, was equipped with a radar instrumentthat "sees" through the planet's dense cloudcover to study surface features. A separatespacecraft called the Pioneer Venus multiprobecarried four instrumented probes which weredropped to the planet's surface. The probes, andthe instrumented main body, radioed informationabout the planet's atmosphere before fallingvictim to its extremely high pressures andten iperatures.

Comparing the Planets

Despite their efforts to peer across the vast dis-tances of space through an obscuring atmos-phere, scientists of the past had only one planetthey could study closely, the Earth. But the past20 years of planetary spaceflight have given newdefinitions to "Earth" sciences like geology andmeteorology and spawned an entirety new dis-cipline called comparative planetology.

By studying the geology of planets and moons,and comparing the differences and similaritiesbetween them, we are learning more about theorigin and history of these worlds and the solarsystem as a whole.

Weather affects all of us on the Earth. in itsextremes, weather can threaten life, and long term

4

The Moonlike surface of Mercury is revealed in this photo.graph taken by the approaching Mariner 10 spacecraft,

climatic changes on the Earth could be cata-strophic. It is important to understand our com-plex weather machine. But other planets haveweather too. By studying the weather on otherworlds and comparing it to our own, we maybetter understand our Earth.

Geology and weather are just two major areasof science benefitting from our planetary spaceprobes. Someday perhaps, terrestrial biologistswill get their chance to compare the only lifeformswe've ever known, those on our Earth, to livingcreatures from another world.

MercuryObtaining the first closeup views of Mercury wasthe primary objective of the Mariner 10 spaceprobe, launched from Kennedy Space Center inNovember 1973. After a journey of nearly fivemonths, which included a flyby of Venus, thespacecraft passed within 805 kilometers (500miles) of the solar system's innermost planet onMarch 29, 1974.

The photographs Mariner 10 radioed back toEarth revealed an ancient, heavily cratered surface

on Mercury, closely resembling our own Moon.The pictures also showed huge cliffs crisscrossingthe planet. These apparently were created whenMercury's interior cooled and shrank, compressingthe planet's crust. The cliffs are as high as twokilometers (1.2 miles) and as long as 1500kilometers (932 miles).

Instruments onboard Mariner 10 discovered thatthe planet has a weak magnetic field and a traceof atmospherea trillionth the density of theEarth's and composed chiefly of argon, neon andhelium. The spacecraft reported temperaturesranging from 510 degrees Celsius (950 degreesFahrenheit) on Mercury's sunlit side to 210degrees Celsius ( 346 degrees Fahrenheit) on thedark side. Mercury literally bakes in daylight andfreezes at night.

The days and nights are long on Mercury. Ittskes 59 Earth days for Mercury to make a singlerotation. It spins at a rate of about 10 kilometers(about 6 miles) per hour, measured at the equator,as compared to Earth's spin rate of about 1600kilometers (about 1,000 miles) per hour at theequator.

Mercury, like the Earth, appears to have a crustof light silicate rock. Scientists believe it has aheavy iron-rich core that makes up about half ofits volume.

Mariner 10 made two additional flybys of Mer-curyon September 21, 1974 and March 16, 1975

before control gas used to orient the spacecraftwas exhausted and the mission was concluded.

Until the Mariner 10 probe, little was knownabout Mercury. Even the best telescopic viewsfrom Earth showed Mercury as an indistinct objectlacking any surface detail. The planet is so closeto the Sun that it is usually lost in the Sun's glare.When it is visible on Earth's horizon just aftersunset or before dawn, it is obscured by the hazeand dust in our atmosphere. Only radar telescopesgave any hint of Mercury's surface conditionsprior to the voyage of Mariner 10.

VenusVeiled by dense cloud cover, our nearest neighbor-ing planet was the earliest subject of interplan-etary explorations. The Mariner 2 space orobe,launched August 27, 1962, was the first of morethan a dozen successful American and Soviet mis-sions to study the mysterious planet. Asspacecraft zoomed by, plunged into the at-mosphere, and gently landed on Venus, romanticmyths and speculations about our twin planetwere laid to rest.

Mariner 2 passed within 34,762 kilometers(21,600 miles) of Venus on December 14, 1962, andbecame the first spacecraft to scan anotherplanet. its instruments made measurements ofVenus for 42 minutes. Mariner 5, launched in June1967, flew much closer to the planet. Passingwithin 4,023 kilometers (2,500 miles) of Venus on

6

An enhanced photo taken by the Pioneer Venus Orbiter showsthe circulation of the dense Venusian atmosphere.

the second U.S. flyby, its instruments measuredthe planet's magnetic field, ionosphere, radiationbelts and temperatures. On its way to Mercury,Mariner 10 flew by Venus and returned ultravioletpictures showing cloud circulation patterns in theVenusian atmosphere.

In the spring and summer of 1978, two space-craft were launched to unravel the mystery ofVenus. On December 4, the Pioneer Venus Orbiterbecame the first spacecraft placed in orbit aroundthe planet.

Five days later, the five separate componentswhich had made up the second spacecraftthePioneer Venus Muitiprobeentered the Venusianatmosphere at different locations above theplanet. Four independent probes ,arid a main bodyradioed data about the planet's atmosphere backto Earth during their descent toward the surface.

Venus more nearly resembles Earth in size,physical composition, and density than any othiknown planet. However, spacecraft have discovered vast differences in how these planetshave evolved.

Approximately 97 percent of Venus' atmos-phere, about a hundred times as dense as Earth's,is carbon dioxide. The principal constituent ofEarth's atmosphere is nitrogen. Venus' at-mosphere acts like a greenhouse, permitting solarradiation to reach the surface but trappinq theheat which would ordinarily be radiated back intospace. As a result, surface temperatures are482 degrees Celsius (900 degrees Fahrenheit),hot enough to melt lead.

Radar aboard the Pioneer Venus orbiter pro-vided a means of seeing through Venus' dense

5

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cloud cover and determining surface features overmuch of the planet. Among the features deter-mined are two continent-like highland areas. One,about half the size of Africa, is located in theequatorial region. The other, about the size ofAustralia, is to the north.

There Is evidence of two major active volcanicareas, one larger than Earth's Hawaii - Midwayvolcanic chain (Earth's largest)with a mountainhigher than Everest (Earth's highest mountain).The concentration of lightning over these tworegions suggests frequent volcanic activity at bothplaces. Discovery of active volcanism on Venusmakes it the third solar system body known to bevolcanically active. The others are Earth and theJovian satellite lo.

Venus' predominant weather pattern is a highspeed circulation of Venus' clouds which aremade up of sulphuric acid. These speeds reach ashigh as 932 kilometers (225 miles) per hour. Thecirculation is in the same directioneast to westas Venus' slow retrograde rotation. Earth'swinds blow from west to east, the same directionas its rotation.

NASA's Pioneer-Venus orbiter continues to cir-cle the planet. It is expected to send data aboutVenus to Earth for years to come.

EarthFrom our journeys into space, we have learnedmuch about our home planetEarth. The firstAmerican satellite, Explorer 1, was launched fromCape Canaveral on January 31, 1958. It discoveredan intense radiation zone, now called the VanAllen Radiation Region, surrounding Earth. Sincethen, other research satellites have revealed thatour planet's magnetic field is distorted into a tear-drop shape by the solar windthe stream ofcharged particles continuously ejected from theSun. We've learned that Earth's magnetic fielddoes not fade off into space but has definiteboundaries. And we now know that our wispyupper atmosphere, once believed calm and quies-cent, seethes with activity, swelling by day andcontracting by night. It is affected by the changesin solar activity and contributes to weather andclimate on Earth.

Satellites positioned about 35,000 kilometers(22,000 miles) out in space play a major role everyday in local weather forecasting. Their watchfulelectronic eyes warn us of dangerous storms.Continuous global monitoring provides a vastamount of useful data, as well as contributing to abetter understanding of Earth's complex weathermachine.

From their unique vantage point in space,spacecraft can survey the Earth's resources andmonitor the planet's health.

As viewed from space, Earth's distinguishingcharacteristics are its blue waters and whiteclouds. Enveloped by an ocean of air consisting of

78 percent nitrogen and 21 percent oxygen, theplanet is the only one in our solar system knownto harbor life. Circling the Sun at an averagedistance of 149 million kilometers (93 millionmiles), Earth is the third planet from the Sun andthe fifth largest in the solar system.

Its rapid spin and molten nickel-iron core giverise to an extensive magnetic field, which, coupledwith the atmosphere, shields us from nearly all ofthe harmful radiation coming from the Sun andother stars. Most meteors bum up in Earth's at-mosphere before they can strike the surface. Theplanet's active geological processes have left noevidence of the ancient pelting it almost certainlyreceived soon after it formed.

The Earth has a single natural satellitetheMoon.

MoonThe first human footsteps upon an alien worldwere made by American astronauts on the dustysurface of our airless, lifeless companion. Beforethe manned Apollo expeditions, the Moon wasstudied by the unmanned Ranger, Surveyor, andLunar Orbiter spacecraft.

The Apollo program left us a large legacy oflunar materials and data. Six two-man crewslanded on and explored the lunar surfacebetween 1969 and 1972. They returned a collectionof rocks and soil weighing 382 kilograms(842 pounds) and consisting of more than 2,000separate samples.

From this material and other studies, scientistshave constructed a history of the Moon datingback to its infancy. Rocks collected from the lunarhighlands date about 4.0 to 4.3 billion years old.It's believed that the solar system formed about4.6 billion years ago. The first few million years ofthe Moon's existence were so violent that fewtraces of this period remain. As a molten outerlayer gradually cooled and a solidified into differ-ent kinds of rock, the Moon was bombarded byhuge asteroids and smaller objects. Some of theasteroids were the size of small states, likes RhodeIsland or Delaware, and their collisions with theMoon created huge basins hundreds of kilometersacross.

The catastrophic bombardment d;ed away about4 billion years ago, leaving the lunar highlandscovered with huge overlapping craters and a deeplayer of shattered and broken rock. Heat producedby the decay of radioactive elements began tomelt the inside of the Moon at depths of about200 kilometers (124 miles) below its surface. Then,from about 3.8 to 3.1 billion years ago, greatfloods of lava rose from inside the Moon and

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Hurricanes and typhoons on Earth are powered by differences in atmospheric temperatures and density. We know that similarweather conditions nccur on Mars and Jupiter. Compare the sprawling Pacific storm (top) photographed by the Apollo 9 astronautswith a Martian cyclone (lower left) and Jupiter's Great Red Spot (lower right). The Martian cyclone Is about 250 kilometers (155miles)In diameter. The Great Red Spot on Jupiter is a hurricanelike feature which has raged for centuries. This single Jovian storm systemis several times than the Earth.

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Jack Frost lives on Mars too. Light patches of frost on the Plains of Utopia (above) were observed during the Martian winter. The Vik-ing ',tinders became our first weather stations on another planet and scientists on Earth continue to get weekly updates from the VikIry 1 sits. Had Viking l's first weather report been aired on the 8 p.m. news, it would have gone something like this: "Light winds fromthe east in the late afternoon, changing to light winds from the southeast after midnight. Maximum winds were 15 miles per hour.Temperatures ranged from minus 122 degrees Fahrenheit just after dawn to minus 22 degrees in midafternoon. Atmospheric pressure7.70 millibars." (On Earth that same day, the lowest recorded temperature was minus 100 degrees Fahrenheit at the Soviet VostokResearch Station in the Antarctic.)

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Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

Mean Distance From Sun(Million of Kilometers)

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88 days

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149 6 277.9 778 3 1,477 2,870 4,497 5,900..,....--........---,..Period of Revolution 365 days 687 days 11,86 years 29 46 veers 84 years 165 years 248 years

Rotation Period 59 days 243 daysRetrograde

23 hours56 minutes

24 hours37 minutes

9 hours55 minutes

10 hours40 minutes

17.2 hoursRetrograde

18 hours30 min 17)

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6 days 9 hours18 minutesRetrograde

Inclination of Axis Near 0° 3° 23°27' 75°12, 305' 26044' 97055' 28 °48. 7

Inclination of Orbit toEcliptic

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Eccentricity of Orbit 206 007 017 093 048 056 047 009 254

Equatorial Mame let(Kilometers)

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Atmosphere(Main Components)

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Large Martian channels (left) start near the volcano Elysium Mons and wind their way to the northweet for several hundred kilometers.Their origin is controversial: Did they form from lava flows or wale- released from the melting of ground ice during volcanic eruptions?Compare the Martian channels with the Skylab photograph of the Rio de la Rata river in Uruguay (right).

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Impact craters are formed when a planetary surface is struckby a meteorite. Mercury, our Moon, and many of the icy, rockysatellites of the outer solar system are characterized by heavi-ly cratered surfaces. On Earth, geological processes tend todestroy evidence of ancient crater impacts, although somemore recent craters remain discernable. Compare the lunarcraters (top) with craters on Mars (bottom).

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Volcanoes are vents In a planet's crust that permit the escapeof internal heat. The geologically active Earth has hundreds ofvolcanoes, like this one In New Zealand (top). Compare it toone of the large shield-type volcanoes on Mars (center). it wasa surprise to scientists that Jupiter's moon to Is volcanicallyactive (bottom). Voyager 1 photographed a volcanic plume(visible above the limb of to) about 11 hours before its closestapproach. Researchers believe tidal forces resulting fromJupiter's massive size are responsible for the Internal heatingof lo. The active volcanoes on to are the only ones knownin the solar system other than Earth's. in addition to Mars,evidence of volcanic activity in the past has been found onthe Moon, Mercury and Venus.

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A rock-littered, rolling landscape Is seen by the Viking 1 lender after its touchdowa on Mars. Parts of the spacecraft are visible in theforeground. This is a portion of the first panoramic view returned to Earth by the robot spacecraft.

poured out over its surface, filling in the large im-pact basins to form the dark parts of the Mooncalled maria or seas. Explorations show that therehas been no significant volcanic activity on theMoon for more than 3 billion years and, sincethen, the lunar surface has been altered only bythe rare impacts of large meteorites and by theatomic particles from the Sun and stars.

If our astronauts had landed on the Moon abillion years ago, it would have looked very muchas it does today, and thousands of years fromnow, the footsteps left by the Apollo crews will re-main sharp and clear.

One question about the Moon that remains un-solved is where did it come from? Three theoriesattempt to explain its existence: that it formednear the Earth as a separate body, that it sepa-rated from the Earth, or that it formed somewhereelse and was captured by the Earth. The notionthat the Moon may have once been part of theEarth now appears less likely than the other sug-gestions because of the difference between thetwo bodies in chemical composition, such as theabsence of water either free or chemically com-bined in rocks. The other two theories are aboutevenly matched in strengths and weaknesses. Theorigin of the Moon remains a mystery.

MarsOf all the planets, Mars has long been consideredthe solar system's prime candidate for harboringextraterrestrial life. Astronomers observing the redplanet through telescopes saw what appeared tobe straight lines crisscrossing its surface. Theseobservationslater determined to be opticalIllusionsled to the popular notion that intelligentbeings had constructed a system of Irrigationcanals on the planet In 1938, when Orson Welles

broadcast a radio drama based on the science fic-tion classic "War of the Worlds," enough peoplebelieved in the tale of invading Martians to cause anear panic.

Another reason for scientists to expect life onMars arose from apparent seasonal color changeson the planet's surface. That led to speculationthat conditions might support a bloom of Martianvegetation during the warmer months and causeplant life to become dormant during colderperiods.

In August and September 1975, two Vikingspacecrafteach consisting of an orbiter and alanderwere launched from Kennedy SpaceCenter, Florida on a mission designed to answerseveral questions, including: is there life on Mars?Nobody expected the spacecraft to spot Martiancities but it was hoped the biology experiments onboard the lenders would at least find evidence ofprimitive life, past or present.

The results sent back by the two unmannedlaboratories, which soft-landed on the planet,were teasingly inconclusive. We still don't knowwhether life exists on Mars. Small samples of thered Martian soil were specially treated in threedifferent experiments designed to detect biolog-ical processes. While some of the tests indicatedbiological activities were occurring, the sameresults could be explained by the planet's soilchemistry. There was a notable absence ofevidence that organic molecules exist on Mars.

Despite the inconclusive results of the biologyexperiments, we know more about Mars than anyother planet except Earth. Six U.S. missions to thered planet have been carried out. Four Marinerspacecraftthree which flew by the planet and

12 11

one which was ;Amid into Martian orbitsurveyed the planet extensively before the Vikingmissions.

Mariner 4, launched in late 1984, flew past theplanet on July 14, 1965. It approached to within9,656 kilometers (6,000 miles) of the surface.Returning 22 close-up pictures of Mars, it folndno evidence of artificial canals or flowing water.Mariners 8 and 7 followed during the summer of1969, returning about 200 pictures showing adiversity of surface conditions during their flybys.Earlier atmospheric data were coni:rmed andrefined. On May 30, 1971, Mariner 9 was launchedon a mission to study the Martian qurface fromorbit for nearly a year. It arrived five and a halfmonths after liftoff, only to lind Mars 17 midstof a planet-wide dust storm which madephotography impossible for several weeks. bu:after the storm cleared, Mariner 9 began return-ing the first of 7,O( pictures which revealedpreviously unknow.. Martian features, includingevidence that rivers, and possitvy seas, couldhave orl'A existed on the planet. The Mariner mis-sions to Mars were followed up with the VikingProjectthe fir,. American soft landing on thesurfer of another planet, excluding our awnMoon.

All four spacecraft, two orbiters and two lenders,exceeded by large margins their de:4n lifetime of90 days. The four spacecraft were launched in 1975and began Mars operation in 1976. The first to fallwas Orbiter 2 which stopped operating in July 24,1978 when its attitude control gas was depletedbecause of a leak. Lander 2 operated until April 12,1980 when it was shut down due to batterydegeneration Orbiter 1 operated until August 7,1980, when it too used the last of its atnt,le con-trol gas. Lander 1 ceased operating . Nova. mbe:

13, 1983.Photos sent from the Plain of Cilybewhere

Viking 1 landed on July 20, 19n--show a bleak,rusty red iandsc.ape. A panorama returned bythe robot explorer pictures a gently roiling plain,littered with rocks and graced by rippled sanddunes, Fine red dust ?tom the Medlar, soil givesthe sky a pinkish hue. Viking 2 landed on the Plainof Utopia, arriving sevt.nA! weeks after Its twin. Thelandscape it viewed is more rolling than that seenby Viking 1, and there are no dunes visible.

Both Viking lenders became weather stations,recording wind velocity and direction, temperaturesand atmospheric pressure.

12

The four largest an stand out as distinctivedark circular features In this pvtograph take from a distanceof 575,000 kilometers (350,0W miles) by the approachingViking 1 spacecraft.

As days became weeks, the Martian tvcstherchanged little. The highest atmospheric tempera-ture recorded by either lander was 21 degreesCentigrade ( -17 degrees Fahrenheit) at the Viking1 site in midsummer.

The lowest temperature, 124 degrees Celsius( -191 degrees Fahrenheit), was recorded at themore northerly Viking 2 site during winter. Windspeeds near hurricane force were measured at thetwo Martian weather stations during global duststorms. Viking 2 photographed light patches offrostprobably water ice curing its secondwinter on the planet.

The Martian atmosphere, like that of Venus, isprimarily carbon dioxide. Present in small percent-ages are nitrogen, oxygan and argon, with traceamounts of krypton and xenon. Martian air con-tains only about 1/1000 as much water as Earth's

13

but even this small amount can condense out andform clouds which ride high in the atmosphere, orswirl around the slopes of towering Martian vol-canoes. Local patches of early morning fog canform in valleys.

There is evidence that in the past, a denser Mar-tian atmosphere may have allowed water to flow onthe planet. Physical features closely resemblingshorelines, gorges, riverbeds and islands suggestthat great rivers once existed on the planet.

Mars has two small, irregularly shaped moons,Phobos and Deimos, with ancient, cateredsurfaces.

JupiterOutward from Mars and beyond the Asteroid Beltlie the giants of our solar system.

In March 1972, NASA dispatched the first offour space probes to survey the colossal worlds ofgas and their moons of rock and ice. For eachprobe, Jupiter was the first port of call.

Pioneer 10, which lifted off from Kennedy Spacetenter March 2, 1972, was the first spacecraft topenetrate the Asteroid Belt and travel to the outerregions of the solar system. in December 1973, itreturned the first closeup pictures of Jupiter as itflew wilt in 132,252 kilometers (81,168 miles) of theplanet's banded cloudtops. Pioneer 11 followed ayear later. Voyagers 1 and 2 were launched in thesummer of 1977 and returned spectacular photo-graphs of Jupiter and its 16 satellites during flybysin 1979.

During their visits these exploring spacecraftfound Jupiter to be a whirling ball of liquid hydro-gen, topped with a uniquely colorful atmospherewhich is mostly hydrogen and helium. It containssmall amounts of methane, ammonia, ethane,acetylene, phosphene, germanium tetrahydrideand possibly hydrogen cyanide. Jupiter's cloudsalso contain ammonia and water crystals. Scien-tists believe it likely that between the planet'sfrigid cloud tops and the warmer hydrogen oceanthat lies below, there are regions where methane,ammonia, water and other gases could react toform organic molecules. Because of Jupiter'satmospheric dynamics, however, these organiccompounds, if they exist, are probably short lived.

The Great Red Spot has been observed forcenturies through Earth-based telescopes. it is atremendous atma,pheric storm, similar to Earth'shurricanes, which rotates counterclockwise.

Our space probes detected lightning in Jupiter'supper atmosphere and observed auroral emissions

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Jupiter looms ahead of the Voyager 1 spacecraft. The GreatRed Spot is visible at the lower left. Slightly above the feature,and to the right, is the iolcanictily active moon lo.

similar to Earth's northern lights in the Jovianpolar regions.

Voyager 1 returned the first evidence of a ringencircling Jupiter. Photographs returned by thespacecraft and its companion Voyager 2 showed anarrow ring too faint to be seen by Earth's tele-scopes.

Largest of the solar system's planets, Jupiterrotates at a dizzying paceonce every 9 hours, 55minutes and 30 seconds. It takes the massiveplanet almost 12 Earth years to complete a jour-ney around the Sun. The planet is something of amini solar system, with 16 known moons orbitingabove its clouds.

A new mission to Jupiterthe Galileo Projectis being readied for the late 1980s. An atmos-pheric probe will descend into Jupiter's cloudlayers while another spacecraft orbits the planet.

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Ga Mean Satellites

In 1610, Galileo Geniei aimed his telescope atJupiter and spotted four points of light orbitingthe planet. For the first time, humans had seenthe moons of another planet. The four worldswould become known as Galilean satellites, inhonor of their discoverer. But Galileo might hap-pily have traded his moment in history for a lookat the dazzling photographs returned by the Voy-ager spacecraft as they flew past Jupiter's fourplanet-sized satellites.

One of the most remarkable findings of theVoyager mission was the discovery of active vol-canoes on the Galilean moon lo. It was the firsttime volcanic eruptions were observed on a worldother than Earth. The Voyager cameras identifibdat least eight active volcanoes on the moon.Plumes extended as far as 250 kilometers (155miles) above the moon's surface. The satellite'spizza-colored surface, rich in hues of oranges andyellow, is probably the result of sulphur-richmaterials which have been brought to the surfaceby volcanic activity.

Europa, approximately the same size as ourMoon, is the brightest Galilean satellite. Itssurface displays a complex array of streaks thatindicate the crust has been fractured.

Like Europa, the other two Galilean moonsGanymede and Callistoare frozen worlds of iceand rock. Ganymede is the largest satellite in thesolar systemlarger than the planet Mercury. It iscomposed of about 50 percent water or ice andthe rest rock. Cellist°, only slightly smaller thanGanymede, has the lowest density of any Galileansatellite, implying that it has large amounts ofwater in its composition. More detailed studies ofthe Galilean satellites will be performed by thenext orbiting spacecraft scheduled to be sent toJupiter.

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I o

Europa

SaturnNo planet in the solar system is adorned iikeSaturn. its exquisite ring system is unrivalled. LikeJupiter, Saturn is composed mostly of hydrogen.But In contrast to the vivid colors and wild tur-bulence found in Jupiter's clouds, Saturn has amore subtle, butterscotch hue and its markingsare often muted by high altitude haze.

Three American spacecraft have visited Saturn.Pioneer 11 zipped by the planet and its moonTitan in 1979, returning the firs closeup pictures.Voyager 1 followed in November 1980, sendingback breathtaking photographs that revealed forthe first time the complexities of Saturn's ringsystem and moons. Voyager 2 flew by the planetand its moons in August 1981.

The spacecraft discovered that there areactually thousands of ringlets encircling Saturn.

Saturn's rings are composed of countless low-density particles orbiting individually around theequator at progressive distances from the planet'scloud tops. Analysis of radio waves passingthrough the rings showed that the particles varywidely in size, ranging from dust to boulders. Mostof the material is ice and frosted rock.

Scientists believe the rings resulted, either froma moon or a passing body which ventured tooclose to Saturn and was torn apart by great tidalforces, or the incomplete coalescence of primor-dial planetary material, or from collisions withlarger objects orbiting the planet.

tleible either to form into a moon or to drirtaway from each other, individual ring particlesappear to be held in place by the gravitationalpulls of Saturn and Its satellites.

Radio emissions quite similar to the staticheard on an AM car radio during an electricalstorm were detected by the Voyager spacecraft.These emissions are typical of lightning but arebelieved to be coming from the planet's ringsystem rather than its atmosphere. No lightningwas observed in Saturn's atmosphere. But as theyhad at Jupiter, the Voyager spacecraft saw a ver-sion of Earth's northern and southern lights nearSaturn's poles.

The probes also studied Saturn's moons,detected undiscovered moons, found some thatshare the same orbit, and determined that Titanhas a nitrogen-based atmosphere.

A large constituent of Titan's atmosphere ismethane. The surface temperature of Titan ap-pears to be around the "triple" point of methane,meaning methane may be present on Titan in allthree states: liquid, gaseous, and solid (ice).Methane, therefore, may play the same role onTitan that water plays on Earth.

Although the spacecraft's cameras could notpeer through the dense haze that obscures thesurface of Titan, measurements indicate Titanmay be a place where rain or snow falls from

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v

Voyager 2 photographs of the Saturn ring system during itsAugust 1981 approach. The shadow of the planet's exquisitering system can be clearly seen in the equatorial region.

methane clouds and rivers of methane cutthrough methane glaciers.

Continuing photochemistry due to solar radia-tion may be converting Titan's methane to ethane,acetylene, ethylene, and, in combination withnitrogen, hydrogen cyanide. The latter is a buildingblock to amino acids. Titan's temperature isbelieved to be too low to permit progress beyondthis stage of organic chemistry. However, this con-dition may be similar to that which occurred in theatmosphere of the primeval Earth between threeand four billion years ago.

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Uranus and NeptuneFour and a half years after visitir.g Saturn, the

Voyager 2 spacecraft completed the first close-upobservation of the Uranian system. The six-hourflyby revealed more information about Uranus andits retinue of icy moons than :lad been gleanedfrom ground observations since its discovery overtwo centuries ago by the English astronomer WilliamHerschel.

Uranus, third largest of the planets, is the odd-ball of the solar system. Unlike the other planets,it lies tipped on its side with its north and southpoles alternately facing the sun during its 84-yearswing around the solar system. During Voyager'sflyby, the south pole faced the sun.

Voyager found that the planet's magnetic fielddoes not follow the usual north-south axis found onthe other planets. Instead, it is tilted 60 degrees, andoffset from the planet's center, a phenomenon thaton Earth would be like having one magnetic polein New York and the other in Jakarta.

Uranus's atmosphere consists mainly of hydro-gen, with about 12 per cent helium and smallamounts of ammonia, methane and water vapor.Wind speeds range up to 200 meters per second(447 mph), and blow from the west instead of theeast as previously expected. Temperatures near thecloudtops measure 200 degrees C. (-329 degreesF.)

The sunlit south pole is shrouded in a kind ofphoto-chemical "smog" believed to be a combi-nation of acetylene, ethane and other sunlight-generated chemicals. Surrounding the planet'satmosphere and extending thousands of kilometersinto space is a mysterious ultraviolet sheen called an"electroglow."

About 8,000 kilometers (5,000 miles) belowUranus's cloudtops there is thought to be a scaldingocean of water and dissolved ammonia some 10,000kilometers (6,000 miles) deep. Beneath this oceanis an earth-sized molten core of heavier materials.

Voyager discovered 10 new moons orbitingUranus, each about 40-170 kilometers (24-102miles) in diameter. The planet's five knownmoons Titania, Ariel, Miranda, Umbriel andOberon range in size from 480-1600 kilometers(300.1000 miles) across. The half-ice, half-rockspheres are a geological showcase, featuring twelve-mile-high mountains, jagged cliffs and canyons,crater-pocked plains and winding valleys possiblycarved out by glaciers.

The planet was thought to have nine dark rings;Voyager found eleven. In contrast to Saturn'srings, which are composed of bright grain-sizedparticles, Uranus's rings are made up of bouldersized chunks.

Voyager 2 will complete its Grand Tour of thesolar system on August 25, 1989, when it sweeps towithin about 1,280 kilometers (800 miles) ofNeptune. The planet has two known invhis, Nereidand Triton. The latter will be observed and photo-graphed during the flyby. Neptune is the fourthlargest of the planets and is believd to be a twin ofUranus.

Uranus as seen by Voyager 2's wide-angle camera.The images, taken through violet, orange andmethane filters, show the concentric patternsof the planet's cloud layers. The view is towardthe south pole.

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The scarred face of Miranda, the smallest ofUranus's five major moons, indicates a violentpast.

PlutoPluto is the most distant of the planets, yet the

ecc..ntricity of its orbit periodically carries it insidethat of Neptune's. The orbit also is highly inclined

well above and below the orbital plane of theother planets.

Discovered in 1930, Pluto appears to be littlemore than a celestial snowball. Its diameter iscalculated to be between 3,000 and 3,500 kilo-meters (1,864 and 2,175 miles), about the same asEarth's moon. Ground-based observations indicatethat its .urface is covered with methane ice.

The planet has one known satellite, Charon,discovered in 1978. There are no plans to send a

7probe to Pluto.'U.S. G.P.O. 1989-261-446100060

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