nasa facts how we get pictures from space, revised edition

8/6/2019 NASA Facts How We Get Pictures From Space, Revised Edition

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( N F - 1 5 1 / 7 - 8 7 - R e v ) HOW WE GE PICTURES F R ~ MS P ~ C E ,REVISED EDITION (NlSA) 12 p Avail :NTIS HC A 3/nF A t ; a lso a v a i l a b l e SOD BC$1.0 as 033- -Ol005-2 CSCL 056 u n c l a s

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ince the first cavedweller ventured out togaze up at the night sky ,people have sought toknow more about themysterious images and

lights seen there . Being limited bywhat could be seen with the unaidedeye , that early stargazer rel ied onintellect and imagination to depict theuniverse, etching images in stone byhand , measuring and charting thepaths of the wanderers , and becomingas familiar with the sky as the limited

technology would allow .Although stargazers frequently tookthe wrong paths in attempting toexplain what they saw , many of themdeveloped new tools to overcometheir limitations. Galileo crafted a finetelescope for observing the heavens.His hand-drawn pictures of thesatellites of Jup iter , the "cu p handles "of Saturn , and the phases of Venus,when comb ined with the possiblereasons for those facts, shook the veryfoundations of the European soc iety inthe Middle Ages . Bigger and more

powerfu l telescopes , combined witheven newer too ls, such asspectroscopes and cameras , haveanswered most of the questions ofthose anc ient stargazers . But in do ingso, they have unfolded even newermysteries.

Beginning in the 1960s , our view ofthe heavens reached beyond theobscur ing atmosphere of Earth asunmanned spacecraft carriedcameras and other data sensors toprobe the satellites and planets of theSolar System . Images those spacecraft

sent back to Earth provided startlingclarity to details that are only fuzzymarkings on the planets ' surfaceswhen seen from Earth -basedtelescopes. Only two of the presentlyknown planets , Neptune and Pluto,remain unexplored by our cameras. InAugust 1989 , Voyager 2 will snapseveral thousand closeup frames ofthe planet Neptune and its largest

satellite , Triton. By the end of the 20thcentury , only P luto w ill not have beenvisited by one of our spacecraft.

The knowledge humans have todayof outer space would astound Galileo .Spacecraft have sent back pictures ofa cratered and moon-like surface onthe planet Mercury and revealedcirculation patterns in the atmosphereof Venus . From Mars , they have sentback images of craters, g iant canyons ,and volcanoes on the p lane t's surface .Jup ite r's atmospheric circulation hasbeen revealed , act ive volcanoes on

the Jov ian moon10

have been shownerupting , and previously unknownmoons and a ring circling the planetdiscovered. New moons were foundorbiting Saturn and the Saturn ian ringswere resolved in such detail that over1,000 concentric ring featuresbecame apparent. At Uranus , Voyagersent back deta ils of a planet that is

Planetary radio astronomyand plasma wave antenna (2)

Radio isoto pe thermoelectric

generator (3)

High -fieldmagnetometer


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covered by a featureless, bluish -greenfog. The planet is encircled by ringsdarker than charcoal and shaped byshepherding satellites, accompaniedby five large satellites , and immersedin a magnetic field .

Those discoveries, and thousands ofothers like them , were made possiblethrough the technology of telemetry,the technique of transmitting data bymeans of radio signals to distantlocations. Thus , the spacecraft not onlycarries data sensors but must alsocarry a telemetry system to convert the

Low-field magnetometer

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higher quality and contain more thantwice the amount of informationpresent in a television picture.

The most enduring image gathererin space has been the Voyager 2spacecraft. Voyager carries a dualtelev ision camera system , which canbe commanded to view an object witheither a wide-angle or telephoto lens .Th e system is mounted on a scienceplatform that can be tilted in anydirection for precise aiming . Reflectedlight from the object enters the lenses

and falls on the surface of a seleniumsulfur vidicon television tube , 11mi llimeters square. A shutter in thecamera controls the amount of lightreaching the tube and can varyexposure times from 0 .005 second forvery bright objects to 15 seconds orlonger when searching for fa intobjects such as unknown moons .

The vidicon tube temporarily holdsthe image on its surface until it can bescanned for brightness levels. Thesurface of the tube is divided into 800para llel lines , each containing 800

pixels , g iving a total of 640 ,000 . Aseach pixel is scanned for brightness, itis assigned a number from 0 to 255.

The range (0 to 255) was chosenbecause it coincides with the mostcommon counting unit in computersystems , a unit called a byte. Incomputers , inform at ion is stored in bitsand bytes . The bit is the mostfundamental counting or storage unit,while a byte is the most useful one . Abit conta ins one of two possiblevalues , and can best be thought of asa tiny on-off switch on an electrical

circu it. A byte, on the other hand ,contains the total va lue represented by8 bits. The value can be interpreted inmany ways , such as a nume rica lvalue , an alphabet character orsymbol , or a pixel shaded betweenblack and white . In a byte , the positionof each b it represents a countingpower of 2. (By convention , bitpatterns are read from right to left.)

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Jupiter5 Mar 79

Pluto8 / 89~

The Voyager missions began from Cape Canaveral, Florida. Two probes launched 16days apart were to explore the outer planets, returning images and data. Both exploredJupiter and Saturn and their moons, but Voyager 1 veered up out of the ecliptic plane to

return data on fields and particles it encounters , while Voyager 2 flew on to encounterUranus. It sent back striking images of that planetary system which is barely visible fromEarth. Voyager 2 is no w on its way to Neptune and will arrive in late 1989 .

Binar y Table

Bit of Data 8 7 6 5 4 3 2Sequence Value 128 64 32 16 8 4 2 1Binary Value 0 0 1 0 1 1 0 1Byte Value 0 + 0 + 32 + 0 + 8 + 4 + 0 + 1 45

Seguence Value 128 64 32 16 8 4 2Brightness Values Binar:t Values

0 (black) 0 0 0 0 0 0 0 09 (dark gray) 0 0 0 0 1 0 0 1

62 (gray) 0 0 1 1 1 1 1 0183 (pale gray) 1 0 1 1 0 1 1 1255 (wh ite) 1 1 1 1 1 1 1 1


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. . . - Sun. . . - Earth

Umbriel

Sun and Earth Uranusoccultation zones

Closest approach

to Uranus (0 hrs)

c . - - - OberonPrevious planetary encounters for the Voyager spacecraft were almost leisurelyexcursions. At Saturn , for example, the spacecraft encounte red one planetary feature ata time , exploring it before focusing on the next , until it had photogr aphed all it could.However , at Uranus the planetary system is tilted almost 90 0 to the planetary plane. Forthis reason Voyager has been compared to a dart tossed at a bulfs-eye, passing thecenter, the outer rings , an d virtually everything at the same time. That meant camerashad to sc an many areas very quickly to catch even brief glimpses of features .

Thus , the first bit (the rightmost bit) ofthe eight-bit sequence represents 2 tothe zero power (2), the second bitrefers to 2 to the 1 power (21, and soon. For each bit in a byte that has aone in it, you add the value of thatpower of two (the sequence value)until all eight bits are counted. Forexample , if the byte has the bit value of00101101 , then it represents thenumber 45. The binary table at leftshows how translation of b its andbytes to numbers is done .

If all the bits in an eight-bitsequence are ones , then it willcorrespond to the value 255 . That isthe maximum value that a byte cancount to . Thus , if a byte is used torepresent shades of gray in an image,then by convention the lowest value,

zero , corresponds to pure black , whilethe highest value , 255 , corresponds topure white . All other values areintermed iate shades of gray.

When the values for all the pixelshave been assigned , they are eithersent directly to a receiver on Earth orstored on magnetic tape to be sentlater. Data are typically stored on tapeon board the spacecraft when thesignals are going to be temporarilyblocked , such as when Voyagerpasses behind a planet or a satellite .F o r e a c h i m a g e, a n d i ~ t o ~ l o f

640 ,000 pixels , 5 ,120 ,000 bits of datamust be transmitted (640 ,000 x 8) .When Voyager flew close to Jupiter ,data were transmitted back to Earth ata rate of more than 100 ,000 bits persecond. This meant that once data

began reaching the antennas onEarth 's surface , information forcomplete images was received inabout 1 minute for each transmission .

As the distance of the spacecraftfrom Earth increases, the quality of theradioed data stream decreases andthe rate of transmission of data has tobe slowed correspondingly . Thus, atthe distance of Uranus , the data has tobe transmitted some six to eight timesslower than could be done at Jupiter .That means that only one picture canbe transmitted in the time six pictureswere taken at Jupiter . However , for theUranus encounter , scientists andengineers devised a scheme to getaround that limitation . The schemewas called data compression.

To do that, they reprogrammed thespacecraft en route . Instead of havingVoyager transmit the full 8 bits for eachpixel , its computers were instructed tosend back only the differencesbetween brightness levels ofsuccessive pixels . That reduced thedata bits needed for an image byabout 60 percent. Slowing thetransmission rate meant that noise didnot interfere with the image reception,and by compressing the data, a fullarray of striking images was received .The computers at NASA's JetPropulsion Laboratory restored thecorrect brightness to each pixel ,producing both black-and-white andfull-color images.

T he radio signals that aspacecraft such as Voyagersends to Earth are received

by a system of large dish antennascalled the Deep Space Network(DSN). The DSN is designed toprovide command, control, tracking ,and data acquisition for deep spacemissions . Configured around theglobe at locations approximately 120 0

apart , DSN provides 24-hour line-ofsight coverage .

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ORIG lN AL PAG ECO t . R P HO T O C :~ '\ .. ~


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ORIGINAL PAGECOLOR PHOTOG RAPH ORIGINAL PAGE

COt

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NASA's Deep Space Network consists of huge dish antennas like these position ed at three receiving stations around the globe . Thestations in Goldstone, CA; Madrid , Spain ; an d Canberra , Australia , track the spacecraft as it speeds through deep space . The fartherthe spacecraft travels away from Earth , the weaker its signal becomes . To compensate for this weaker signal, the antennas areelectronically "arrayed " so that two or more antennas focus on receiving the same signa/. Arraying not only increases the apparents trength of the signal , but also gives valuable information about the spacecraft's speed an d distance .

Stations are located at Goldstone,California , and near Madrid , Spain ,and Canberra , Australia. The DSN,managed by NASA's Jet PropulsionLaboratory in Pasadena , California ,consists of three 64-meter (210-ft)diameter dish -shaped antennas , six34-meter (111-ft) diameter antennas,and three 26-meter (8S-ft) antennas.As antennas at one station losecontact, due to Earth 's rotation,antennas at the next station rotate into

view and take over the job of receivingspacecraft data . While one station istracking a deep space mission , suchas Voyager , the other two are busytracking spacecraft elsewhere intl::le sky.

~ u r i n gVoyager 's contact withSaturn , the DSN recovered more than99 percent of the 17 ,000 imagestransmitted. That accomplishmentrequired the use of a technique knownas "antenna arraying ." Arraying for the

Saturn encounter was accomplishedby electronically adding signalsreceived by two antennas at each site.Because of the great distance Uranusis from the Earth , the signal receivedfrom Voyager 2 was only one-fourth asstrong as the s ignal received fromSaturn . A new arraying technique ,which combined signals from fourantennas , was used during the Uranusencounter to allow up to 21 ,600 bits ofdata to be rece ived each second .

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Arraying 's biggest payoff came inAustralia, whose government providedits Parkes Radio AstronomyObservatory 64-meter antenna to belinked with the DSN 's three-antennacomplex near Canberra , The mostcritical events of the encounter ,including Voyage r's closestapproaches to Uranus and itssatell ites , were designed to occurwhen the spacecraft would betransmitting to the complex inAustralia, The data were successfully

relayed to J PL through that array,

he DSN was able to trackVoyager 's position at Saturnwith an accuracy of nearly 150

kilometers (about 90 miles) during itsclosest approach , This accuracy wasachieved by using the network'srad iom et ric system, the spacecraft 'scameras , and a technique called VeryLong Baseline Interferometry , or VLBI.VLBIdetermines the direction of thespacecraft by precisely measuring theslight difference between the time of

arrival of the signal at two or moreground antennas, The same techniquewas used at U ranus to aim thespacecraft so accu rately that thedeflection of its trajectory caused bythe planet's gravity would send it onto Neptune ,

When the D SN antennas receive theinformation from the spacecraft,computers at the Jet PropulsionLaboratory store it for future use andreassemble it into images, To recreatea pictu re from data that has been sentacross the vacuum o f space,computers read the da ta bit by bit,calculating the values for each pixeland converting the value into a smallsquare of light. The squares aredisplayed on a television screen thatduplicates the vidicon screen on thespacecraft. The resulting image is ablack-and-white facsimile of the objectbeing measured ,

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OR lmNAL P AGEC O L O R ~ HO TO G RA P H

This image of the coma of Comet Halley was sent to Earth from the Pioneer Venusspacecraft and was compiled from more than 20,000 separate vertical ultraviolet scans,The coma, or cloud of gases surrounding the nucleus, is 20 million kilometers indiameter. Concentric areas show decreasing brightness from the comet 's centeroutward, Data were collected an d beamed to Earth on February 2-5, 1986,


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olor images can be made bytaking three black-and-white

L ! ! ! ~! ! ! I Iframes in succession andnding ("registering ") them on one

another in the three color-planes of atelevision screen. In order for that towork , however , each of the threeframes has to be taken by the cameraon board the spacecraft throughdifferent filters . On Voyager , one frameis taken through a blue filter, onethrough a green , and one through anorange .

Filters have varying effects on theamount of light being measured . Forexample , light that passes through ablue filter will favor the blue values inthe image making them appearbrighter or transparent, whereas red ororange values will appear muchdarker than normal. On Earth the threeimages are given the appropriatecolors of the f ilters through which theywere measured and then blendedtogether to give a color image .

An important feat the interplanetaryspacecraft must accompl ish isfocusing on its target while traveling atextremely high speeds . Voyager spedpast Uranus at more than 40 ,000miles an hour. To get an unblurredimage , the cameras on board had tosteadily track their target wh ile thecamera shutters were open. Thetechnique to do this , called imagemot ion compensation , involves rotatingthe entire spacecraft under the control

RIGINAt.: PAGECO LOR P H OTOG RAP H

In January 1986 , Voyager 2 made it sclosest encounter with the planetUranus . It returned never-before -seenclose-up views of a planet barelyvisible from the Earth 's surface .

of the stabilizing gyroscopes . Thestrategy was used successfully both atSaturn 's satellite Rhea and at Uranus .Both times , cameras tracked theirtargets without interruption.

Once the image is reconstructed bycomputers on Earth , it sometimeshappens that objects appearnondescript or that subtle shades inplanetary details such as cloudtopscannot be discerned by visualexamination alone. This can beovercome , however , by adding a final"contrast enhancement " to theproduction . The process of contrastenhancement is like adjusting thecontrast and brightness controls on atelevision set. Because the shades ofthe image are broken down intopicture elements , the computer canincrease or decrease brightnessvalues of individual pixels , therebyexaggerating their differences andsharpening even the tiniest details.

For example , suppose a portion ofan image returned from space revealsan area of subtle gray tones . Data fromthe computer ind icates the range inbrightness values is between 98 and120 , and all are fairly evenlydistributed . To the unaided eye , theport ion appears as a blurred graypatch because the shades are toonearly similar to be discerned. Toeliminate this visual handicap , thebrightness values can be assignednew numbers . The shades can bespread farther apart , say five shadesapart rather than the one currently

being looked at. Because the data arealready stored on computers , it is afairly easy task to isolate the twentythree values and assign them newones: 98 could be assigned 20 , 99assigned 25 , and so on . The resultingimage is "enhanced " to the unaidedeye, while the information is the sameaccurate data transmitted from thevicinity of the object in space .

he past 25 years of spacetravel and exploration havegenerated an unprecedented

q of data from planetarysystems . Images taken in space andtelemetered back to Earth have greatlyaided scientists in formulating betterand more accurate theories about thenature and origin of our Solar System .Data gathered at close range, andfrom above the distorting effects ofEarth's atmosphere , produce imagesfar more detailed than pictures takenby even the largest Earth -boundtelescopes .

In our search to understand theworld as well as the universe in whichwe live , we have in one generationreached farther than in any othergeneration before us . We haveovercome the limitations of lookingfrom the surface of our planet andhave traveled to others . Whateveryearning drew those first stargazersfrom the security of their caves to lookup at the night sky and wonder stilldraws men and women to the stars .

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Br ief History of Pictures By Unmanned Spacecraft

Name Year

Pioneer 4 1959

Range r 7 1964

Range r 8 1965

Range r 9 1965

Surveyor 1 1966

Surveyor 3 1967

Surveyor 5 1967

Surveyor 6 1967

Surveyor 7 1968

Lunar Orb iter 1 1966

I unar Orbiter 2 1966Lunar Orbiter 3 1967

I unar Orbiter 4 1967Lunar Orbiter 5 1967

Mariner 4 1964

Mariners 6 and 7 1969

Mariner 9 1971

Mar iner 10 1973

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Mission

Moon : measured particles and fields in a flyby , entered heliocent ric orbit

Moon : 4 ,3 16 high-resolution TV pictures of Sea of Clouds ; impacted

Moon : 7 ,137 pictures of Sea of Tranquility ; impacted

Moon: 5 ,814 pictures of Crater Alphonsus ; impacted

Moon : 11 ,237 pictures, soft landing in Ocean of Storms

Moon : 6 ,315 pictures , first soil scoop ; soft landed in Sea of Clouds

Moon : more than 19,000 pictures ; first alpha scatter analyzed chemicalstructure ; soft landed in Sea of Tranquill ity

Moon : 30 ,065 pictures ; first lift off from lunar surface , moved sh ip 10 feet , softlanded in Central Bay region

Moon : returned telev ision pictures, performed alpha scatter , and took surfacesample ; first soft landing on ejecta blanket beside Crater Tycho

Moon medium and high-resolution pictures of 9 possible land ing sites ; first orb itof another planetary body ; impacted

Moon : 211 frames (422 medium and high-resolution pictures) ; impacted

Moon : 211 frames including picture of Surveyor 1 on lunar surface ; impacted

Moon : 167 frames ; impacted

Moon : 212 frames , including 5 possible land ing sites and micrometeoroid data ;impacted

Mars : 21 pictures of cratered moon-like surface , measured planet's thin , mostlycarbon dioxide atmosphere ; flyby

Mars : verified atmospheric findings : no nitrogen present , dry ice near polarcaps ; both flybys

Mars : 7,400 pictures of both satellites and planet's surface ; orbited

First multiple planet encounter

Venus : first full-disc pictures of planet ; ultraviolet images of atmosphere ,revealing circulation patterns ; atmosphere rotates more slowly than planetarybody ; flyby

Mercury : pictures of moon-like surface w ith long , narrow valleys and cliffs ; flyby;three Mercury encounters at 6-month intervals


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Name

Pioneer 10

Pioneer 11(Pioneer Saturn)

Pioneer Venus 1

P ioneer Venus 2

Viking 1

Viking 2

Voyager 1

Voyager 2

Year

1972

1973

1978

1978

1975

1975

1977

1977

Mis s ion

Jupiter : first close-up pictures of Great Red Spot and planetary atmosphere ;carries plaque with interga lactic greetings from Earth

Jupiter : pictures of planet from 42,760 km (26 ,72 5 mil above cloudtops ; onlypictures of polar regions ; used Jupiter's gravity to swing it back across the SolarSystem to Saturn

Saturn: pictures of planet as it passed through ring plane with in 21,400 km(13 ,30 0 mil of cloudtops; new discoveries were made ; spacecraft renamedPioneer Saturn after leaving Jup iter

Venus : studied cloud cover and planetary topography ; orbited

Venus: multi probe , measuring atmosphere top to bottom ; probes designed toimpact on surface but continued to return data for 67 minutes

Mars : first surface pictures of Mars as we ll as color pictures ; landed July 20 ,1976 ; rema ined operating until November 1982

Mars: showed a red surface of oxidized iron ; landed September 3 , 1976

Jupiter : launched after Voyager 2 but on a faster trajectory ; took pictures ofJupiter 's rapidly changing cloudtops; discovered ring circling planet , activevolcano on 10 , and f irst moons with color : 10 , orange ; Europa, amber ; andGanymede , brown ; flyby

Saturn : pictures showed atmosphere similar to Jupiter 's, but with many morebands and a dense haze that obscured the surface; found new rings withinrings ; increased known satellite count to 17 ; flyby

Jupiter : color and black-and-white pictures to complement Voyager 1 ; timelapse movie of volcanic action on 10 ; flyby

Saturn : cameras with more sensitivity resolved ring count to more than 1 ,00 0 ;time-lapse movies studied ring spokes ; distinctive features seen on severalmoons ; 5 new satel lites were discovered ; flyby

Uranus : first encounter with this distant planet ; photographed su rfaces ofsatellites , resolved r ings into multicolored bands showing anticipatedshepherding satellites ; discovered 10 new moons , 2 new rings , and a tiltedmagnetic field ; flyby

Neptune: encounter scheduled for 1989

For sale by the Superintendent of Do cume nts , U.S . Government Pr inting OfficeWa shing ton, D.C. 20402

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nasa facts how we get pictures from space, revised edition

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