nasa facts magellan

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Of all the planets in the solar system,Venu s is tile most like our own Earth insize, mass, and distance from the SunThe motions of our planetary "twin"were known to the ancients , and itsapparent changes in shape, similar tothe phases of the Moon, were firststudied by Galileo more than fourcenturies ago. In the modern era, it isby far the most visited world in thesolar system - more than 20spacecraft from the Soviet Union andthe United States have been sent theresince the early 1960's. The clouds ofVenus have been probed, the structureand composition of its atmospheremeasured, its landscape photographed ,and its rocks chemically analyzed byautomated landers.Yet . for all our fascination with Venus .we have only a sketchy, generalknowledge of the planet's surface.While the other three "terrestrial"worlds- Earth, Mercury, and Mars have long since been mapped , detailsof the face of Venus are still largelyunknown , due to the planet 's dense,constant cloud cover. The cloudsprevent us from ever photographingthe solid surface, even from space,with conventional cameras.Beginning in the early 1960s,scientists on Earth began to counterthis problem by using radar waves ,which, unlike visible light , are able topenetrate the Venusian clouds andreflect off the solid planet back toEarth . With the help of computerprocessing , these radar reflections canbe turned into pictures of the Venussurface. Earth-based radar imaging isthus extremely valuable, but it also is

limited - Venus always shows thesame hemisphere to us when it is nearenough in its orbit for high-resolutionstudy, so only a fraction of the planetcan be explored from Earth.In 1978, NASA launched the PioneerVenus spacecraft to conduct the mostthorough investigation of Venus everundertaken. Most of the experimentsconcerned the planet's atmosphere,but the spacecraft also carried a radarsystem that mapped 92 "10 of thesurface from Venus orbit with aresolution (a measure of the smallestobjects that can be seen in its map) ofabout 100 kilometers (km) (60 miles) .For the first time, planetary scientistshad a global map of Venus thatshowed the existence of itscontinent-like highlands-Aphroditeand Ishtar- its hilly plains, the large,volcano-like mountains of Beta Regio,and the flat lowlands.Five years after Pioneer, in 1983, theSoviet Union sent two Veneraspacecraft to map Venus at aresolution of approximately 2 km.Because of the nature of their orbitsaround the planet , they were only ableto map about 30 "10 of the surface nearthe north pole. Although the Venera 15and 16 images have answered some ofour questions about Venus, many moreremain unanswered. And the PioneerVenus radar map, as important as it is ,shows only large-scale features. Thehills and valleys , craters and lava flows- the telling details of Venus geology-a re as yet uncharted.In April 1988, NASA plans to launchthe Magellan spacecraft from thespace shuttle. It will arrive at Venus inJuly 1988 to begin an eight-monthmission that will map more than 90"10 ofthe planet with a resolution more than100 times better than that of PioneerVenus , and nearly ten times better thanany views of the surface taken sincethat time. More than any other singlemission, Magellan is expected tounveil the secrets of the Venusian past,just as Mariner 9 revealed theunsuspected richness of Martiangeology in 1972. In 1988, for the first

time , we will come to know the face oour planetary "twin. "The Spacecraft

A key feature of the Magellanmission is the economy and relativesimplicity of its design. To save costshardware was borrowed from otherplanetary programs, notably Voyagerand Galileo. The 975 kilogram (2150pound) spacecraft has only onescience instrument: a radar systemused for imaging , for altimetric profilinof the planet's topography, and formeasuring radiation from the Venussurface. Magellan's only moving partare a pair of 5.8 meter panels thatcollect solar energy for charging thespacecraft's nickel-cadmium batterieThese will turn around their axesduring the mission to follow the Sunwithout the spacecraft 's having torotate.Magellan's large, 3.7 meter (12 ft.)high-gain antenna dish (used for radaimaging and for communicating withEarth) and the spacecraft "bus"containing electronics subsystemswere both acquired from a spareVoyager spacecraft. The computersused for commanding, handling dataand attitude control will create the higresolution pictures required for theMagellan mapping mission. SynthetiCAperture Radar (SAR) , however, relieon a different principle to form itsimage. By combining successive radreturns taken while the sur face ofVenus passes through the beam of thantenna, the radar system will be ableto electronically reproduce theresolution of a larger antenna. This"aperture synthesis" is what gives SAits power as well as its name.

2 Fo r R31e by tbe Superint end ent of Documents, U.S . Go\'ernmenl PrInting Offi ceWa shington. D.C. 20402


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Orbi tal path ---,

Orbitalground track

Spacecraft

ow Synthetic Aperture Radar (SAR )o rks: Any point in the radar map imagean be loca ted by using two coordinates-he distance to the point (determined by theime delay of the returning signa/), and theunt of Doppler shift in the signal. TheAR antenna "looks" to one side of tspath to distinguish right-side fromsigna(s.As Magellan passes over the, its dish antenna will

and to the side of the's orbital path. The SAR15 miles) wide with rapid radar pulses ,record the returning signals. Eachpoint on the planet's surface can thenbe located by using two coordinates.First , a measurement of the time itthe radar signal to return toMagellan will give the spacecraft 'sto that point. Second , aDoppler

area

Lines of equalDoppler shiftLines of equaldistance

location of the point with reference tothe spacecraft's line of flight, sinceMagellan will either be approaching orreceding from the point at any giventime. (Doppler shift is a change inwavelength caused when an object ismoving relative to a wave source. Afamiliar example is the apparentchange in pitch of a car horn or a trainwhistle as it moves toward or awayfrom you . Waves from an approachingsource are bunched together andmade shorter. When the source isreceding , the waves are stretched outand become longer.)

Since each point in the radar imagewill have a unique Doppler shift andrange, these two coordinates and aknowledge of the angle of theantenna's line of sight with respect tothe surface are all that are needed toget a fix on the location of any returnedsignal. The brightness of the image atthat point then becomes an element ofthe map image.Using this technique, data will becollected by the radar instrument andradioed back to Earth, where imagesof the Venusian surface will beconstructed by computers . Theimages will have a radar resolution ofabout 120 meters for the equatorialregions of the planet (where Magel lanwill pass closest to the surface) andabout 190 meters near the poles . Inthese radar photographs, it will bepossible to distinguish features assmall as 250 meters on the surface. Bycomparison, the best existingground-based and spacecraft maps ofVenus show no features smaller than 1or 2 km .

April 1988: From Earth to VenusIn April , 1988, Magellan will becarried into low Earth orbit by thespace shuttle , then launched from theshuttle 's cargo bay into its own orbitwith an upper stage Centaur G rocketmotor attached to its base. Afterseveral revolutions around the Earth,the Centaur rocket will fire to boost theMagellan spacecraft toward Venus .The target date of April , 1988 is not

arbitrary, but is timed so that Magellancan travel an orbit joining Earth andVenus that requires a minimum amountof fuel. The period of launchopportunity lasts for 20 days,beginning Apri l 6. Venus during thattime will be approximately 90 millionkm (56 mi llion miles) from Ea rth , and itwill take just under four months forMagellan to reach its destination.Trajectory correction maneuvers alongthe way will keep the spacecraft ontime and on target.

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When it arrives at Venus in late July,a solid rocket motor attached to thespacecraft will fire to place Magellan inorbit around the planet. After a fewadjustment maneuvers , Magellan willbe in a highly elliptical orbit, with itslowest point 300 km (185 miles) fromthe surface and its high point at analtitude of 7,762 km (4,823 miles). Thetime required for Magellan to make onecomplete orbit around Venus- theorbit period-will be 3.1 hours. Sincethe orb it will be tilted five degrees tothe axis of Venus , the spacecraft willpass nearly, but not quite, over thenorth and south poles.Although Venus is very much like theEarth in size and mass, there are anumber of peculiarities about theplanet's rotation on its axis. One is thatit turns in the opposite direction fromEarth , spinning on its axis from east towest instead of west to east. Another isthat the Venusian "day" is very long - ittakes 243 of our Earth days for theplanet to make one rotation . (Strangely,it takes only 224 days for Venus tomake one complete orbit around theSun : the Venusian "day" is longer thanthe Venusian year.) Magellan will be ina fixed polar orbit around a very slowlyturning globe, and since it will take 243days for every point on the surface topass under its gaze once, themapping operations are planned totake exactly 243 days.The target date for arrival at Venus isJuly 26, 1988. This will place Magellanin Venus orbit in time to finish itsmapping mission before superiorconjunction (the lining up of the planetbehind the Sun as seen from Earth) ,during which time there would be toomuch disturbance to radiocommunications to conduct spacecraftoperations.Mapping the Veiled Planet

Circl ing the planet every 3.1 hours,Magellan wil l pass closest to thesurface just north of the equator at 10degrees Venu s latitude, then will moveup over the north pole and around the4

(A) Mapp ingSpacecraft closestto Venus (10)

SouthSwath

North 80 0swath "-59 \'"

planet in a wide loop. As a result of thiselliptical orbit , Magellan will only beclose enough to the surface to conductmapping operations for about 35minutes out of each three-hour orbitalperiod . The rest of the time will bespent transmitting the raw data fromthe just-completed mapping pass andreceiving telemetry instructions fromEarth , or calibrating the spacecraft'snavigation and guidance system withreference stars.During mapping operations , thehigh-gain antenna dish will point downat the surface of Venus. In addition totaking radar imaging data, the radarsys tem will use a separate hornantenna aimed at the surface directlybeneath the spacecraft for Magellan 's

Not mapped

(C) Playbackof datato Earth

The 3.1 hour orbit of Magel/an is dividedinto distinct phases. When the spacecraftclosest to Venus , the radar antenna mapsthe surface (A) , alternating between northand south swaths on successive passes.After radar operations are comple ted, theantenna is turned to point toward Earth (B)so that data can be transmitted (C). After ape riod of calibrating the spacecraft'snavigational system (0) and another dataplayback, Magel/an turns its attention oncagain to the surface.When Magellan's elliptical orbit brings itclose to the surface of Venus , the radarinstrument will look through the clouds tomap the solid planet. Most of Jhe rest of thtime will be spent transmitting data back tEarth.

altimetry experiment. Radar pulsesfrom this fan-beam horn antenna willbounce off the surface and return tothe radar receiver. By measuring the


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time it takes for the signal to r&turn , thealtimeter wil l determine the distance tothe point directly below the spacecraft,and so wi ll construct a topographicprofile of the planet in much the samewa y that sonar is used onboard shipsto profile the ocean floo r. By mission'send , the Magellan altimeter experimentwill have produced a topographic mapshowing height variations as small as30 meters for the entire mapped part ofthe planet.Several additional types ofinformation can be gleaned fromMagellan . When the dish antenna ispo inting down at the planet, it will beused to measure the amount ofradiated thermal microwave energy,which will show temperature variationson the surface. Analysis of the way inwhich the radar signals are reflectedfrom the surface will yield informationon the electrical(conductivity andcomposition of the Venusian rocks.Also, when the spacecraft is in radiocommunication with Earth, groundtrackers will be able to analyze slightchanges in its orbital path to detectvariations in Venus' gravitational field ,which provide important clues to themakeup of the planet's interior.Because the same antenna used formapping is also used for radiocommunications, the spacecraft mustre-orient itself to transmit data back toEarth . After each mapping sequence,the spacecraft will look away fromVenus so that data from thejust-finished mapping sequence canbe beamed Earthward . Thetransmissions will be received by thelarge antennas of the Deep SpaceNetwork located at various sitesaround the world , then relayed to theJet Propulsion Laboratory inPasadena, California. Just about thetime this "call home" is completed , thespacecraft will come into position tobegin another mapping pass, and willagain point down toward the surface.

As Magellan orbits the slowly turningplanet, the surface will be mapped insuccessive, overlapping strips. Eachstrip, or swath as it is called , will beabout 25 km (15 miles) wide and about16 ,000 km (9 ,942 miles) long. Closer tothe north pole, where successiveswaths will naturally converge, there isno need for all of them to overlap, so an

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alternating pattern of northern andsouthern mapping passes will be use(Figure X) . On one pass, the radar wilbegin mapping in southern latitudesand continue all the way to the northpole. Then, on the next pass, themapping swath will begin farthersouth, and will end before reaching thpole. In this way, all of the mappingstrips will overlap in middle latitudes tensure complete coverage near theequator, and every other strip willoverlap near the poles to give fullcoverage at higher latitudes. Theseimage strips will then be combined bcomputers on Earth into photomosaicimages covering large parts of theVenus surface.

Sweltering un der a perpetual cloud cover,Venus reveals no surface deta ils even inultraviolet light (A), but a radar instrumenton the Pioneer Venus orbiter was able todisclose fo r the first time the large-scalegeography of the planet (B). Blue areasrepresent the planet's lowlands, whilehighlands are green and yel/ow in this falscolor map .


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ORIGINAL PAGECOLOR PHOTOGRAPH

Venus ' la rgest elevated landmass ,Aphrodite, has two major mountain regionson opposite sides of the "continent." Inaddition to the rough mountainous regions,Aphrodite also has the lowest elevations onVenus - in the trenches of Diana Chasma ,which may be a rift valley caused by crustalextension.

Magellan's ell iptical orbit makes itimpossible to obtain full coverage ofboth poles during the course of a 243day mission, so mission designers hadto choose whether the northern orsouthern hemisphere would be fully

mapped. Because the large"continent" of Ishtar, which seems tohave a number of significant geologicprovinces, appears on Pioneer Venusmaps to extend into high northernlatitudes, it was decided to provide fullcoverage of the northern hemisphere;mapping coverage of the southernhemisphere will extend to about -67degrees latitude.Thus , about eight times each day, for243 days, Magellan will take radarimages of the surface of Venus , withthe result that, by project's end ,approximately 90 %of the planet willhave been mapped .

The Planet VenusAlthough the Earth and Venus havevery similar bulk properties, there areimportant differences between the twoplanets. They are most likely made ofthe same type of silicate rock, and theirinteriors are probably similar, butVenus does not appear to have amagnetic field as Earth does. Venus iscloser to the Sun than we are, andreceives almost twice as much solarradiation . Both planets haveatmospheres . The Venusianatmosphere, however, is much denserthan our own , and is composed almost

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ORIGINA _ PAGER PHOTOGRAPH


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a .IGII AL PAGECOLOR PHOTOGRAP h

A portion of the "continent" of Ishtar isshown in this com puter-processed PioneerVenus image. At the center is Mt. Maxwell ,taller than Mt . Everest, and believed to bean active volcano. The Lakshmi plateau,rising 4-5 km above the mean level ofVenus , is bordered by mountain ranges tothe north and northwes t. The plateau isbelieved to consist of thin lavas over/yingan uplifted section of older crust. SovietVenera radar data suggest that thedepression called Colette is a collapsedvolcanic crater. On Ishtar's southern flankare the Ut and Vesta cliffs, which descendto vast lowlands.

entirely of carbon d ioxide , with ahigh-altitude covering of cloudscomposed of sulfuric acid . It is thisthick blanket of carbon dioxide thattraps incoming thermal radiationbetween the solid planet and theatmosphere and makes Venus aperpetual furnace , where surfacetemperatures reach 480C (gOOF) andthe atmospheric pressure is 90 timesthat of Earth . Any water that might haveonce existed has long sincedisappeared: Venus today is bone-dry.We know some things aboutVenusian geology from past spaceprobes and from Earth-based radarstudies. Soviet Venera lander photosand chemical analysis experimentsperformed on the surface have shownthat the rocks of the highland areasnear the landers are heavy andbasaltic, like the rocks of Earth 's oceanfloor or the rocks that form from oozingvolcanic lava flows .The large-scale geography of theplanet has been disclosed by radarstudies from Earth, and more fully, bythe Pioneer Venus Orbiter in 1978.Most of the planet consists of eitherrolling upland plains (these areapparently composed of older crustalrock) or smooth lowland areas. Thereare two major continents , or elevatedplateaus - Aphrodite, named for theRoman equivalent of the goddessVenus, and Ishtar, named for theBabylonian equivalent-that appear tobe younger geologically. Ishtar is about

Only very general conclusions about thegeology of a planet - Venus or Earth -canbe drawn from radar images with theresolution obtained by Pioneer Venus . The

more sensitive Magellan spacecraft willgive scientists a better understanding ofprocesses that have shaped the surfaceand the interior of Venus .

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the size of Australia; Aphrodite is abouttwice as large, or approximately thesame size as South America. Juttingup from the Ishtar highlands is one ofthe highest mountains in the solarsystem, 10 ,800 meter-high (35,400 ft)Mount Maxwell, which is most likely anactive volcano. Two other highlandareas of apparent volcanic origin ,Alpha Regio and Beta Regio, alsostand out conspicuously on thePioneer Venus map.The Earth and Venus: Twins?

Some 4.6 billion years ago, theplanets of the solar system condensedas large, individual knots in a whirlpoolof solid material revolving around theSun . Heavier elements like iron andsilicon remained in the inner solarsystem to form the rocky planetsMercury, Venus, Earth , and Mars. Thelighter gasses-hydrogen and helium-went to form the giant planetsbeyond the asteroid belt. The largestrocky planet, Earth, was extremely hotin those millennia after it condensedinto a tight ball , and in its early historythe planet shed heat through greatviolent volcanoes that covered itssurface. The Earth still sheds heattoday, but now on a low simmer, withisolated chains of volcanoes spewinghot material from the interior.The Earth's upper crust is dividedinto pieces - tectonic plates - thatmove around the planet's curvedsurface, driven by convection cells inthe hot fluid rock underneath the solidcrust. One of the most importantquestions for the study of Venus iswhether a similar process of platetectonics has shaped the surface ofour planetary "twin. " Virtually all of thelarge-scale features of the Earth,including the mountain chains and theocean basins , are a result of themovement of these plates. Whencontinental plates collide , mountains

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such as the Himalayas and the Alpsare thrust upward. Where the platespull apart, rift valleys and ocean basinsform. Earthquakes and volcanoes, themajor geologic upheavals on ourplanet, occur primarily at plateboundaries where pieces of the crustare stretching apart or crunchingtogether.Although we might logically expectEarth 's "twin" to have similar processesshaping its surface, we have yet to seeevidence of planetwide plate tectonicson Venus. On Earth , where plates arepushing away from each other in themiddle of the Atlantic Ocean, there is avolcanic ridge thousands of kilometersin length where a great deal of theplanet 's internal heat is vented. Nosuch conspicuous plate boundariesappear in the Pioneer Venus map,however, suggesting that if a system ofplate tectonics does exist on Venus, itmust be of a different kind than Earth 's.(It should be noted , though , thatevidence of plate tectonics on ourplanet would only be marginally visibleat the resolution of Pioneer.)

Questions For Magellan'sExploration of VenusVolcanoes

One of the most important tasks forMagellan during its mapping missionwill be to take an inventory of volcaniccraters on the Venusian surface so thatscientists can reconstruct the planet'sgeologic history. Ground-based andVenera radar images have shown theexistence of volcanic craters on theIshtar plateau. Electrical dischargesdetected by the Pioneer Venusspacecraft in the clouds above Mt.Maxwell, high concentrations of sulfurdioxide in the atmosphere, and otherdata suggest that Maxwell may be anactive volcano, possibly one of many stillactive on Venus. By counting how manyvolcanoes are on Venus ' surface andshowing where and what kind they are ,Magellan will provide data on theplanet's internal processes.

The radar images should be able todiscriminate between successive,overlapping lava flows so as todetermine the sequence of volcanicevents that shaped the surface. Byexamining the slopes and shapes ofthese volcanic flows , it also will bepossible for scientists to makejudgments about the composition ofthe lava, which again gives cluesabout the makeup of the planet'sinterior and the thickness of the crust.Earlier spacecraft data have shownthat the gravitational field of Venus isstrongest over the planet's elevatedplateaus - further evidence thatyoung , volcanic rocks are being piledup in these highland areas. Magellan 'sgravity survey will collect moreinformation on this correlation betweengravity and topography.

Impact CratersMeteorite impact craters alsoappear in radar images of Venus, andanother major task for Magellan will beto distinguish these impact scars fromvolcanic craters and to take count ofhow many are still preserved on thesurface and where they exist. This isan important point to establish , sincethe more cratered a surface is , and thelarger the craters it contains , the olderit is likely to be. Earth 's surface, for themost part, is young and uncratered.Although meteorites have struck ourplanet in the past , the evidence hasbeen erased by wind and watererosion and by the constant motion of

tectonic plates through time - thesurface of Earth is a slate that hasbeen drawn on, wiped clean , andreworked again over millions ofcenturies. Venus, on the other hand ,appears to retain evidence of acomparatively distant past. Magellan'sglobal inventory of craters should havemuch to tell scientists about the


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This processed Synthetic Aperture Radar(SAR) image of the Mt. St . Helens region ofWashington was taken by the SEASAToceanographic satellite. Magellan's SARinstrument will return radar images ofVenus with comparable resolution.

ORIGINAL P GEBLACK AND WH ITE PHOTOGRAPH

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planet's history and the age of differentgeologic provinces. The rate of surfacecratering will also provide informationon how dense the planet 's atmospherehas been through time.At the best resolutions obtained todate, it is unclear whether certaincircular features seen on Venus are thescars of old impacts, or whether theyare collapsed remnants of volcaniccraters or domes of rock somehowwarped upward by tectonic forces.Magellan's resolution should help clearup the mystery. If the radar imagesshow large stretches of old , crateredterrain , it would argue against tectonicmotion in those regions, sincecompression or extension of the crustwould destroy old craters. It would alsoindicate that the processes of erosionproceed much more slowly on Venusthan on Earth.Faults , Ridges, and Mountains

Th ere evidently is some kind ofcrustal movement at work on Venus,because mountain-like folded ridges

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and rift-like valleys appear in radarimages of Ishtar Terra. These featuresare most probably caused by thecompression or extension of the crust.Magellan will reveal the details of thesefeatures , allowing scientists tocharacterize how Venusian tectonicswork. It has been proposed that thehigh surface temperatures on Venusplaya part in the distortion of the crust ,and Magellan will provide new data totest that theory Large rift valleys suchas Devana Chasma in Beta Regio willbe scrutinized to see whether theywere formed by volcanic processes orby tectonic motion.Water and Wind

Another critical question aboutVenus is whether it once had water onits surface. Modern-day ratios ofdeuterium to hydrogen in Venus'atmosphere suggest that at some pointin the past there was more water in theplanet 's atmosphere. Magellan will belooking for evidence of ancient marineterraces , rive r channels and deltas, orother geologic features that might point

to the existence of ancient oceans . Thiswould have profound implications for thevolution of the planet's atmosphere aswell as its surface. Although the mostrecent Venera radar pictures ha ve showno signs of large-scale erosion by windon Venus , and surface winds there arebelieved to be slower than on Earth,there may be large windblown dunes onthe surface that would show up in highresolution Magellan images.

Th e Venera spacecraf t haveinspected less than one third of thesurface of Venus at high resolution .Over the course of one Venusian year,Magellan will map nearly the entireglobe with ten times the detail of thesebest previous spacecraft images, andwill reveal , if they exist, signs ofvolcanic, wind , water, and meteoriteimpact forces : in short, all theprocesses that dictate a planet'shistory and shape its face. By doingthat, the Magel lan mission should helpto answer the question of why Venus,our planetary "twin ," is at the sametime so much of a stranger.

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