nasa facts our planets at a glance

8/7/2019 NASA Facts Our Planets at a Glance

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Our PlanetsAt a Glance



Our PlanetsAt a GlanceFrom our watery world we have gazed upon thecosmic ocean for un told thousands of years. Theancient astronomers observed points of light

which appeared to wander among the stars. Theycalled these objects planets, w hich m eans wan-derers, and named them afte r great mythologicaldeities— Jupiter, king of the Roman gods; Mars,the Roman god of war; Mercury, messenger of thegods; Venus, the Roman god of love and beauty;and Sa turn, father of Jupiter and god ofagriculture.

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 newdevice called the telescope a t the heavens and

made startling new discoveries.But the past 20 years have been the golden age

of 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.

Am erican expeditions have explored the Moon,our robot craft have landed on and reported fromthe surfaces of Venus and Mars, ou r spacecraft

have orbited and provided much information aboutMars and Venus, and have made close rangeobservations while flying past Mercury, Jupiter,and Saturn.

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

Future 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

Voyager 2

Pioneer Venus 1

Pioneer Ven us 2

Launch Date Arrival Date Status

Mariner 2

Mariner 4

Mariner 5

Mariner 6

Mariner 7

Mariner 9

Pioneer 10

Pioneer 11

*•

Mariner 10

Viking 1

Viking 2

Voyager 1

Venus flyby

Mars flyby

Venus flyby

Mars flyby

Mars flyby

Study Mars

from orbitJupiter flyby

Jupiter/Saturnflybys

Venus/Mercuryflybys

Unmannedlanding on Mars

Unmannedlanding on Mars

Tour of Jupiter

8/14/62

11/28/64

6/14/67

2/24/69

3/27/69

5/30/71

3/2/72

4/5/73

11/3/73

8/20/75

9/9/75

9/5/77

12/14/62

7/14/65

10/19/67

7/31/69

8/5/69

11/18/71

12/3/73

12/2/74 (Jupiter)9/1/79 (Saturn)

2/5/74 (Venus)3/29/74 (Mercury)9/21/74 (Mercury)3/1 6/75 (Mercury)

7/19/76 (inorbit)7/20/76 (landertouchdown)

8/7/76 (in orbit)

9/3/76 (landertouchdown)

3/5/79 (Jupiter)11/12/80 (Saturn)

Mission complete, craft in solar orbit.


Mission complete, craft in solar orbit



Mission complete, inoperable remains in

Martian orbit.Primary mission complete, craftcontinues to return heliosphericinformation enroute tow ard interstellarspace.

Primary mission complete, continues toreturn information enroute towardinterstellar space.


Mission complete,craft remains onsurface and in orbit. Lander continues tosend monthly weather reports, photos.

Mission complete, craft remains on

surface and inorbit. Both lander andorbiter have ceased operation.

Primary mission complete. Craftcontinues to return heliospheric

Tour of theouter planets

Orbital studiesof Venus

8/20/77

5/20/78

8/8/78

7/9/79 (Jupiter)8/25/81 (Saturn)1986 (Uranus)1989 (Neptune)

12/4/78

12/9/78

information enroute toward interstellarspace.

Mission continues. Craft has surveyedtwo of four planetary targets. On way to

Uranus

Orbiter continues to return imagesand data.

Mission complete. Probes impactedon surface.



Interplanetary Spacecraft

NASA's space probes to the planets have come inmany shapes and sizes. Wh ile they are designedto fulfil l 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 electrical

power, trajectory and orientation control, and forprocessing data and communicating with Earth.

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

Imagine that a craft ha s succes sful ly 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, spa cecraftuse stars to keep their bearings in space. With thesubsystem locked onto "fixed" points of reference,flight controllers 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 course

corrections after the spacecraft is enroute.During the f irst decade of planetary flights,NASA spacecraft were dispatched to scan theother inner plan ets—M ercury, Venus, and Mars.These worlds, and our own, are known as theterrestrial planets because of their sim ilarity toEarth's rocky composition.

For these early reconnaissance missions, NASAemployed a highly successful series of spacecraftcalled M ariners. Their flights helped shape theplanning of later missions. Between 1962 and1973, seven Mariner missions were successful.Three Mariner attempts—the first, third andeighth—failed.

All of the Mariner spacecraft used solar panelsas their primary power source. The f irst and thef inal 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

An Atlas Centaur rocket l if ts away from its Cape Canaverallaunch pa d carry ing the Mariner 6 space probe.

The Voyager 1 probe to the outer planets awaits f inal launchpreparations at Kennedy Space Center.



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

In 1972, NASA opened the second decade ofplanetary exploration with the launch of a Jupiter

probe. Interest was shif t ing to the outer planets,giant balls of dense ga s quite different from theterrestrial worlds we had surveyed.

Four spacecraft—two Pioneers and twoVoyagers—were sent to tour the outer regions ofou r solar system. They will eventually become thef irst human artifacts to travel to distant stars.

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

While probing new territory beyond the asteroidbelt, NASA developed highly specialized spacecraft to revisit ou r neighbors Mars and Venus.Twin Viking landers evolved from the lunar Sur-

veyor program. The Mars landers were equipped toserve as biology laboratories, seismic andweather stations. Two advanced orbiters—descendents of the Mariner craf t—were sent tostudy Martian features from above.

The Pioneer V enu s 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 information

about the planet's atmosphere before fallingvictim to its extremely high pressures andtemperatures.

Comparing the Planets

Despite their efforts to peer across the vast dis-tances of space through an obscuring atmos-phere, scientists of the past ha d only one planetthey could study closely, the Earth. But the past20 years of planetary spaceflight have given newdefinitions to "Earth" sciences like geology andmeteorology an d spawned an entirely 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

-

The Moon-like sur fac e of Mercu ry is revealed in this photo-graph taken by the approaching Mariner 10 spacecraf t.

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

Geology and weather are jus t two major areasof science benefitting from our planetary spaceprobes. Someday perhaps, terres trial biologistswill ge t 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 wa sthe primary objec tive of the Mariner 10 spaceprobe, launched fr om Kennedy Space Center inNovember 1973. Afte r a journey of nearly fivemonths, which included a flyby of Venus, th espacecraft passed within 805 kilometers (500miles) of the solar system's innermost planet onMarch 29, 1974.

The photographs Mariner 10 radioed back toEarth revealed an a ncient, hea vily cratered surfac e



on Mercury, closely resembling our own Moon.The pictures also showed huge cl i f fs crisscrossingth e planet. These apparently were created whenMercury's interior cooled and shrank, compressingth e planet's crust. The cl i f fs are as high as twokilometers (1.2 miles) and as long as 1500kilometers (932 miles).

Instruments onboard M ariner 10 discovered thatthe planet has a weak magnetic f ield and a trace

of atmosphere—a trillionth the density of theEarth's and composed chiefly of argon, neon andhelium. The space craft reported temperaturesranging from 510 degrees Celsius (950 degreesFahrenheit) on Mercury's sunlit side to -210degrees Celsius (-3 46 degrees Fahrenheit) on thedark side. Mercury literally bakes in daylight andfreezes at night.

The days and nights are long on Mercury. Ittakes 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 the

equator.Mercury, l ike the Earth, appears to have a crust

of l ight sil icate rock. Scientists believe it has aheavy iron-rich core that makes up about half ofits volume.

Mariner 10 made tw o additional f lybys of Mer-cury—on September 21 , 1974 and March 16, 1975—before control ga s used to orient the spacec raftwas exhausted and the mission was concluded.

Until the Ma riner 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 close

to the Sun that it is usually lost in the Sun's glare.When i t 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 condit ionsprior 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 probe,launched August 27, 1962, was the first of more

than a dozen su ccessful Am erican and S oviet mis-sions to study the mysterious planet. As

spacecraft 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 th e f irst spacecraft to scan anotherplanet. Its instruments made measurements ofVenus for 42 minutes. Mariner 5, launched in June1967, flew much closer to the planet. Passing

within 4,023 kilometers (2,500 miles) of Venus on

An enhanced photo taken by the Pioneer Venus Orbiter shows

th e circulat ion of the dense Venusian atmosphere.

the second U.S. f lyby, its instruments measuredthe planet's magnetic f ield, ionosphere, radiationbelts and temperatures. On its way to Mercury,Mariner 10 f lew by Venus an d returned ultravioletpictures showing cloud circulation patterns in theVenusian atmosphere.

In the spring and summer of 1978, tw o space-craf t were launched to unravel the mystery ofVenus. On December 4, the Pioneer Ven us Orbiterbecame th e first spacecraft placed in orbit around

the planet.Five days later, the five separate componentswhich had made up the second spacecraft—thePioneer Venus Mul t iprobe—entered th e Venusianatmosphere at different locations above theplanet. Four independent probes and 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 otherknown planet. However, spacecraft have dis-covered 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 trapping theheat which would ordinarily be radiated back into

space. As a result, surface temperatures are48 2 degrees Celsius (900 degrees Fahrenheit),hot enough to melt lead.

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



The solar system's only known oasisof life, ou r planet Earth, is photo-graphed by the Apol lo 17 astronauts.

Apo l lo 15 Astrona ut David Scott ex-plores the lunar surface about 16kilometers (ten miles) east from th ebase of the Apennine Mountainsvisible in the background.



cloud cover and determining surface features overmuch of the planet. Amon g the features deter-mined are two continent-like highland areas. One,about half the size of Afr ica, 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 mountain

higher 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 362 kilometers (225 miles) per hour. Thecirculation is in the same direction— eas t to west—as Venus' slow retrograde rotation. Earth'swinds blow from west to east, the same direction

as 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 planet—E arth. The firstAmerican satellite, Explorer 1, was launched fromCape Canaveral on January 31, 1958. It discoveredan intense radiation zone, now called the Van

Allen Radiation Region, surrounding Earth. Sincethen, other research satellites have revealed thatou r planet's magnetic field is distorted into a tear-drop shape by the solar wind—the stream ofcharged particles continuously ejected from theSun. W e'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 a n extensive magnetic field, which, coupledwith the atmosphere, shields us from nearly all of

the harmful radiation coming from the Sun andother stars. Most meteors burn up in Earth's at-mosphere before they ca n 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 satellite—theMoon.

MoonThe first human footsteps upon an alien worldwere made by American astronauts on the dusty

surface of our airless, lifeless companion. Beforethe manned Apollo expeditions, the Moon wasstudied by the unmanned Ranger, Surveyor, an dLunar 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 38 2 kilograms(842 pounds) and consisting of more than 2,000separate samples.

From this material and other studies, scientistshave constructed a history of the Moon dating

back 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, like RhodeIsland or Delaware, and their collisions with theMoon created huge basins hundreds of kilometersacross.

The catastrophic bombardment died away about

4 billion years ago, leaving the lunar highlandscovered with huge overlapping cra ters 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



Hurricanes and typhoons on Earth are powered by differences in atmospheric temperatures and density. We know that similarweather condit ions occur on Mars and Jupiter. Compare the sprawling Pacif ic storm (top) photographed by the Apol lo 9 as tronautswith a Martian cyclone (lower left) and Jupiter's Great Red Spot (lower right). The Martian cyclone is about 250 kilometers (155 miles)in diameter. The Great Red Spot on Jupiter is a hurricane-like feature which has raged for centuries. This single Jovian storm system

is several t imes larger than th e Earth.

Jack Frost lives on Mars too. Light patches of f rost on the Plains of Utopia (above) were observed during the Martian winter. The Vik-ing landers became ou r first weathe r stations on another planet and scientis ts on Earth continue to get weekly updates from the Vik-ing 1 site. Had Vik ing 1's first w eath er report been aired on the 6 p.m. news, it wou ld have gone something like this: "Light winds fromthe east in the late afternoon, changing to light winds from the southeast after midnight. Maximum winds w ere 15 miles per hour.Temperatures ranged from minus 122 degrees Fahrenheit just af ter dawn to minus 22 degrees in midafternoon. Atm ospheric pressure7.70 milibars." (On Earth that same day, the lowest recorded temperature was minus 100 degrees Fahrenheit at the Soviet VostockResearch Station in the Antarctic.)

8



Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

Mean Distance From Sun

(Millions of Kilometers)

Period of Revolution

Rotation Period

Inclination of Axis

Inclination of Orbit To

Ecliptic

Eccentricity of Orbit

Equatorial Diameter

(Kilometers)

Atmosphere

(Main Components)

Known Satellites

57.9

88 Days

59 Days

2°

7°

.206

4,880

VirtuallyNone

0

108.2

224.7 Days

- 243 DaysRetrograde

3°

3.4°

.007

12,104

CarbonDioxide

0

149.6

365.26 Days

23 Hours56 Minutes4 Seconds

23°27'

0°

.017

12,756

NitrogenOxygen

1

227.9

687 Days


25°12'

1.9°

.093

6,787

CarbonDioxide

2

778.3

11. 86 Years


3°5'

1.3°

.048

142,800

Hydrogen.Helium

16—1 Ring

1,427

29.46 Years


26°44'

2.5°

.056

120,400

Hydrogen.Helium

21—23 Rings

2,869

84.01 Years

- 23.9 HoursRetrograde

97°55'

.8°

.047

51,800

Helium.Hydrogen,Methane

5 Rings

4.496

164.1

22 Hoursor Less

28°48'

1.8°

.009

49,500

Hydrogen.Helium,Methane

2

5.900

247.7 Years

- 6 Days, 9 Hours18 MinutesRetrograde

60°?

17.2°

.25

3.500

NoneDetected

1

Large Martian chan nels (left) start near the volcano Elysium Mons and wind their way to the northwest fo r several hundred ki lometers.Their origin is controversial : Did they form from lava f lows or wate r released f rom th e melting of ground ic e during volcanic eruptions?Compare the Martian channels with the Skylab photograph of the Rio de la Plata river in Uruguay (right).



Impact craters are formed when a planetary surface is st ruckby 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, al though somemore recent craters remain discernable. Compare the lunarcraters (top) with craters on Mars (bottom). Volcanoes are vents in a planet's crust that permit th e escape

of 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 surpr ise to scient ists that Jupiter 's moon lo is volcanical lyactive (bottom). Voyager 1 photographed a volcanic plume(visible above the limb of lo) about 1 1 hours before its clos estapproach. Researchers believe tidal forces result ing fromJupiter 's massive size are responsible for the internal heatingof lo. The active volcanoes on lo 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.

10



A rock-littered, rolling landscape is seen by the Viking 1 lander after its touchdown on Mars. Parts of the spacecraf t are visible in theforeground. This is a portion of the f irst panoramic view returned to Earth by the robot spacecraft .

poured ou t over its surfac e, filling in the large im -pact basins to form the dark parts of the Moon-called m aria 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 wil l 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 about

evenly matched in strengths an d 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 su rface. Theseobservations—later determined to be opticalillusions—led to the popular notion that intelligentbeings had constructed a system of irrigation

canals on the planet. In 1938, when Orson Welles

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

Another reason fo r 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 Vikingspacecraf t—each consisting of an orbiter and alander—were 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 landers 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 stil l don't knowwhether life exists on M ars. Small sa mples of thered Martian soil were specially treated in three

different experiments designed to detect biolog-ical processes. While some of the tests indicatedbiological activit ies 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 Marinerspacecraft—three which f lew by the planet and

1 1



one which was placed into Martian orbit-

surveyed the planet extens ively before the Vikingmissions.

Mariner 4, launched in late 1964, flew past theplanet on Ju ly 14 , 1965. It approac hed to w ithin9,656 kilometers (6,000 miles) of the surface.Returning 22 close-up pictures of Mars, it foundno evidence of artificial canals or f lowing wa ter.Mariners 6 and 7 fol lowed during th e summer of1969, returning about 20 0 pictures showing adiversi ty of surface condit ions during their f lybys.Earl ier atmospheric data were confirmed andrefined. On May 30, 1971, M ariner 9 was launchedon a mission to study th e Martian surface fromorbit for nearly a year. It arrived five and a halfmonths after l if toff, only to find Mars in the midstof a planet-wide dust storm which made surfacephotography impossible for several weeks. Butafter the storm cleared, Mariner 9 began return-ing the first of 7,000 pictures which revealedpreviously unknown Martian features, includingevidence that rivers, and possibly seas, couldhave once existed on the planet. The Mariner mis-

sions to Mars were followed up with the VikingProject—the first American soft landing on thesurface of another planet, excluding our ownMoon.

All four spacecraft, tw o orbiters and two landers,exceeded by large margins their design lifetime of90 days. The four spa cecra ft were launched in 1975and began Mars operation in 1976. The f i rst to failwas O rbiter 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 attitude con-

trol gas. Lander 1 is still operating.Photos sent from the Plain of Chrys e— where

Viking 1 landed on July 20 , 1976—show a bleak,rusty red landscape. A panorama returned bythe robot explorer pictures a gently rolling plain,l ittered with rocks and graced by rippled sanddunes. Fine red dust from the Martian soil givesthe sky a pinkish hue. Viking 2 landed on the Plainof Utopia, arriving several weeks after its tw in. Thelandscape it viewed is more roll ing than that seenby Viking 1, and there are no dunes visible.

Both Viking landers became weather stations,recording wind velocity and direction, temperatures

and atmospheric pressure.

The four largest Mart ian volcanoes stand out as dist inc t ivedark circular features in this photograph taken from a distanceof 575,000 kilometers (350,000 miles) by the approachingVik ing 1 spacecraf t .

As days became weeks, the Martian weatherchanged little. The highest atmospheric tempera-ture recorded by either lander was -21 degreesCentigrade (-1 7 degrees Fahrenheit) at the Viking1 site in midsummer.

The lowest tempe rature,-124 degrees Celsius(-191 degrees Fahrenheit), was recorded at themore northerly Viking 2 site during winter. Windspeeds near hurricane force were measured at thetw o Martian weather stations during global duststorms. Viking 2 photographed light patches offrost—probably water ice—during i ts second

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

12



bu t even this small amount ca n 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 suggest

that great rivers once existed on the planet.Mars has two small, irregularly shaped moons,

Phobos and Deimos, with ancient, crateredsurfaces.

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 th e colossal worlds ofgas and their moons of rock and ice. For eachprobe, Jupiter was the first port of call.

Pioneer 10, wh ich lifted off from K ennedy SpaceCenter March 2, 1972, was the first spacecraft topenetrate th e Asteroid Belt and travel to the outerregions of the solar system. In December 1973, itreturned the f i rst closeup pictures of Ju piter as i tf lew within 130,354 kilometers (81,000 miles) of theplanet's banded cloudtops. Pioneer 11 fol lowed ayear later. Voyagers 1 and 2 were launched in thesumm er of 1977 and returned spec tacu lar photo-graphs of Jupiter and its 16 satellites during f lybysin 1979.

During their visi ts these explor ing space craftfound Jup i ter to be a whir l ing ball of l iquid hydro-

gen, topped w ith 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 c rystals. Scien-tists believe it l ikely that between th e planet'sfr igid cloud tops and the warmer hydrogen oceanthat l ies 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 l ived.

The Great Red Spot has been observed for

centuries through Earth-based telescopes. It is atremendous atmospheric storm, similar to Earth'shurricanes, which rotates counterclockwise.

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

Jupiter looms ahead of the Voyager 1 sp acecraf t . The GreatRed Spot is visible at the lower left. Slightly above the feature,and to the right, is the volcanical ly act ive moon lo .

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

Voyager 1 returned th e f i rst evidence of a ringencircling Jupiter. Photographs returned by thespacecraf t 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 pace—once every 9 hours, 50minutes and 30 seconds. I t takes the mass iveplanet almost 12 Earth years to complete a jour-ney around the Sun. The planet is something of amini solar system, with 16 known moons orbi t ingabove its clouds.

A new mission to Jupi ter— the Gal ileo Project-is being readied for the late 1980s. An atmos-pheric probe will descend into Jupiter's cloudlayers whi le another spacecraft orbi ts th e planet.

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Galilean Satellites

In 1610, Galileo Galilei aimed his telescope atJupiter and spotted four points of light orbitingthe planet. For the first t ime, humans ha d 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 look

at 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 identifiedat least eight active volcanoes on the moon.Plumes extended as far as 250 kilometers (155miles) above the moon's s urfa ce. The sa tellite'spizza-colored surface, rich in hues of oranges an dyellow, is probably the result of sulphur-rich

materials which have been brought to the surfaceby volcanic activity.Europa, approximately the same size as our

Moon, is the brightest Galilean satellite. Itssurface displays a complex array of streaks thatindicate the crust has been fractured.

Like Europa, the other two Galilean moons-Ganymede an d Callisto— are frozen worlds of iceand rock. Ganymede is the largest satellite in thesolar system—larger than the planet Mercury. It iscomposed of about 50 percent water or ice andthe rest rock. Callisto, only slightly smaller thanGanymede, has the lowest density of any Galileansatellite, implying that it has large amounts of

water 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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Saturn

No planet in the solar system is adorned likeSaturn. 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 markings

are often muted by high altitude haze.Three American spa cecra ft have visited Saturn.Pioneer 11 zipped by the planet and its moonTitan in 1979, returning the first closeup pictures.Voyager 1 followed in November 1980,sendingback breathtaking photographs that revealed forthe first t ime the complexities of Saturn's ringsystem and moons. Voyager 2 f lew by the planetand its moons in August 1981.

The spac ecraft discovered that there areactually thousands of ringlets encircling Saturn.

Saturn's rings are composed of countless ow-density particles orbiting individually around theequator at progressive distances from the planet's

cloud tops. Analysis of radio waves passingthrough the rings sh owed 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.

Unable either to form into a moon or to driftaway from each other, individual ring particlesappear to be held in place by the gravitational

pulls 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 conning from the planet's ringsystem rather than its atmosphere. No lightningwa s observed in Saturn's a tmosphere. 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 Titanma y be a place where rain or snow falls from

Voyager 2 photographs of the Saturn ring system during itsAugust 1981approach. The shadow of the planet's exquisi tering 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. How ever, thiscon-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 NeptuneThe first spacecraft to reach these bluish greengiants will be Voyager 2, scheduled for a 1986encounter with Uranus and a 1989 survey ofNeptune. The two planets are roughly the same inmass and diameter. They are colder and signif-icantly denser than Saturn. The atmospheres ofboth Uranus and Neptune are known to contain

methane, which gives the planets their unusualgreenish color. Both planets are believed topossess deep atmospheres which are principallycomposed of hydrogen and helium. Scientiststheorize the planets have cores of rock and metalsurrounded',t>7 layers of ice, liquid hydrogen andgaseous hydrogen. Uranus has a system of faintrings discovered in 1977. Uranus has five knownmoons, Neptune, two. .';•' ^\,

Pluto

Pluto is art oddity of the solar system. It hasnothing in commori with the gas giants. It travelsfarther from the Sun than any of the rest, yet theeccentr ic ity of its orbit periodically carries it insideof Neptune's orbit. Pluto's orbit is also highly in-clined; that is, it is well above an d below the planeof the other planets' orbits.

Discovered in 1930 after years of searching byastronomers looking for a predicted ninth planet,Pluto appears to be little more than a celestialsnowball. Its diameter was once thought to begreater than Mercury's bu t later observationscalculate it between 3,000 and 3,500 kilometers(1,864 and 2,175 miles), which is about the same

size or somewhat smaller than our Moon. Earth-based observations indicate Pluto's surface iscovered with methane ice.

Small as it is, Pluto has a satellite. The moonwas discovered in 1978and named Charon.

There are no plans for sending a probe to Pluto.Of the nine planets, it will be the only one notvisited this century by automated spacecraft,providing the Voyager 2 flies near Uranus andNeptune as planned.

Beyond

There may still be undiscovered planets that orbitthe Sun.Some scientists believe there is evidencefor a tenth planet beyond the orbit of Pluto. Thesearch for new worlds in our solar system con-tinues with Earth-based telescopes.

Soon the search for unknown planets will beextended to our neighboring stars with a neworbiting observatory— the Space Telescope. Thisunmanned telescope will be carried into orbit bythe Space Shuttle in the mid 1980s.

This fuzzy image is the planet Uranus, photographed througha 36-inch telescope carr ied aloft in a high alt i tude bal loon.

U.S. Naval Observatory photo of the spiral galaxy NGC 6946.

Unimpaired by Earth's image-distorting atmos-phere, the Space Telescope will enable astron-omers to peer much farther into space an d viewobjects much dimmer than they can with Earth-based telescopes.

Scientists may be able to spot planets orbitingnearby stars, proving what most researchersalready believe—that the formation of star-orbitingplanets occurs commonly in the universe.

As we study new worlds and expand the infantscience known as comparative planetology, weare gradually piecing together age-old puzzles.

Automated probes to the planets are helpingunravel the mystery of how the solar systemformed and how it evolved into its present state.The missions are also telling us about our homeplanet, by studying the weather, geology and otherconditions of our neighbors in space.

nasa facts our planets at a glance

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