pale blue dot -carl sagan-第15部分

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！



At　each　of　the　four　giant　planets—Jupiter；　Saturn；　Uranus；　and　Neptune—one　or　both　spacecraft　studied　the　planet　itself；　its　rings；　and　its　moons。　At　Jupiter；　in　1979；　they　braved　a　dose　of　trapped　charged　particles　a　thousand　times　more　intense　than　what　it　takes　to　kill　a　human；　enveloped　in　all　that　radiation；　they　discovered　the　rings　of　the　largest　planet；　the　first　active　volcanos　outside　Earth；　and　a　possible　underground　ocean　on　an　airless　world—among　a　host　of　surprising　discoveries。　At　Saturn；　in　1980　and　1981；　they　survived　a　blizzard　of　ice　and　found　not　a　few　new　rings；　but　thousands。　They　examined　frozen　moons　mysteriously　melted　in　the　paratively　recent　past；　and　a　large　world　with　a　putative　ocean　of　liquid　hydrocarbons　surmounted　by　clouds　of　organic　matter。

On　January　25；　1986；　Voyager　2　entered　the　Uranus　system　and　reported　a　procession　of　wonders。　The　encounter　lasted　only　a　few　hours；　but　the　data　faithfully　relayed　back　to　Earth　have　revolutionized　our　knowledge　of　the　aquamarine　planet；　its　15　moons。　its　pitch…black　rings；　and　its　belt　of　trapped　high…energy　charged　particles。　On　August　25；　1989；　Voyager　2　swept　through　the　Neptune　system　and　observed；　dimly　illuminated　by　the　distant　Sun；　kaleidoscopic　cloud　patterns　and　a　bizarre　moon　on　which　plumes　of　fine　organic　particles　were　being　blown　about　by　the　astonishingly　thin　air。　And　in　1992；　having　flown　beyond　the　outermost　known　planet；　both　Voyagers　picked　up　radio　emission　thought　to　emanate　from　the　still　remote　heliopause—the　place　where　the　wind　from　the　Sun　gives　way　to　the　wind　from　the　stars。

Because　we're　stuck　on　Earth；　we're　forced　to　peer　at　distant　worlds　through　an　ocean　of　distorting　air。　Much　of　the　ultraviolet；　infrared；　and　radio　waves　they　emit　do　not　penetrate　our　atmosphere。　It's　easy　to　see　why　our　spacecraft　have　revolutionized　the　study　of　the　Solar　System：　We　ascend　to　stark　clarity　in　the　vacuum　of　space；　and　there　approach　our　objectives；　flying　past　them；　as　did　Voyager；　or　orbiting　them；　or　landing　on　their　surfaces。

These　spacecraft　have　returned　four　trillion　bits　of　information　to　Earth；　the　equivalent　of　about　100；000　encyclopedia　volumes。　I　described　the　Voyagers　1　and　2　encounters　with　the　Jupiter　system　in　Cosmos。　In　the　following　pages；　I'll　say　something　about　the　Saturn；　Uranus；　and　Neptune　encounters。



JUST　BEFORE　VOYAGER　2　was　to　encounter　the　Uranus　system；　the　mission　design　had　specified　a　final　maneuver；　a　brief　firing　of　the　on…board　propulsion　system　to　position　the　spacecraft　correctly　so　it　could　thread　its　way　on　a　preset　path　among　the　hurtling　moons。　But　the　course　correction　proved　unnecessary。　The　spacecraft　was　already　within　200　kilometers　of　its　designed　trajectory…after　a　journey　along　an　arcing　path　5　billion　kilometers　long。　This　is　roughly　the　equivalent　of　throwing　a　pin　through　the　eye　of　a　needle　50　kilometers　away；　or　firing　your　rifle　in　Washington　and　hitting　the　bull's…eye　in　Dallas。

Mother　lodes　of　planetary　treasure　were　radioed　back　to　Earth。　But　Earth　is　so　far　away　that　by　the　time　the　signal　frog　Neptune　was　gathered　in　by　radio　telescopes　on　our　planet；　the　received　power　was　only　10…16　watts　（fifteen　zeros　between　the　decimal　point　and　the　one）。　This　weak　signal　bears　the　same　pro；　portion　to　the　power　emitted　by　an　ordinary　reading　lamp　as　the　diameter　of　an　atom　bears　to　the　distance　from　the　Earth　to　the　Moon。　It's　like　hearing　an　amoeba's　footstep。

The　mission　was　conceived　during　the　late　1960s。　It　was　first　funded　in　1972。　But　it　was　not　approved　in　its　final　form　（including　the　encounters　with　Uranus　and　Neptune）　until　after　the　ships　had　pleted　their　reconnaissance　of　Jupiter。　The　two　spacecraft　were　lifted　off　the　Earth　by　a　nonreusable　Titan/Centaur　booster　configuration。　Weighing　about　a　ton；　a　Voyager　would　fill　a　small　house。　Each　draws　about　400　watts　of　power—considerably　less　than　an　average　American　home—from　a　generator　that　converts　radioactive　plutonium　into　electricity。　（If　it　had　to　rely　on　solar　energy；　the　available　power　would　diminish　quickly　as　the　ship　ventured　farther　and　farther　from　the　Sun　Were　it　not　for　nuclear　power；　Voyager　would　have　returned　no　data　at　all　from　the　outer　Solar　System；　except　perhaps　a　little　from　Jupiter。）

The　flow　of　electricity　through　the　innards　of　the　spacecraft　would　generate　enough　magnetism　to　overwhelm　the　sensitive　instrument　that　measures　interplanetary　magnetic　fields。　So　the　magnetometer　is　placed　at　the　end　of　along　boom；　far　from　the　offending　electrical　currents。　With　other　projections；　it　gives　Voyager　alter　a　slightly　porcupine　appearance。　Cameras；　infrared　and　ultraviolet　spectrometers；　and　an　instrument　called　a　photopolarimeter　are　on　a　scan　platform　that　swivels　on　mand　so　these　device　can　be　aimed　at　a　target　world。　The　spacecraft　must　know　where　Earth　is　if　the　antenna　is　to　be　pointed　properly　and　the　data　rereceived　back　home。　It　also　needs　to　know　where　the　Sun　is　and　at　least　one　bright　star；　so　it　can　orient　itself　in　three　dimensions　and　point　properly　toward　any　passing　world。　If　you　can't　point　the　cameras；　it　does　no　good　to　be　able　to　return　pictures　over　billions　of　miles。

Each　spacecraft　cost　about　as　much　as　a　single　modern　strategic　bomber。　But　unlike　bombers；　Voyager　cannot；　once　launched；　be　returned　to　the　hangar　for　repairs。　The　ship's　puters　and　electronics　are　therefore　designed　redundantly。　Much　key　machinery；　including　the　essential　radio　receiver；　had　at　least　one　backup—waiting　to　be　called　upon　should　the　hour　of　need　ever　arrive。　When　either　Voyager　finds　itself　in　trouble；　the　puters　use　branched　contingency　tree　logic　to　work　out　the　appropriate　course　of　action。　If　that　doesn't　work；　the　ship　radios　home　for　help。

As　the　spacecraft　journeys　increasingly　far　from　Earth；　the　roundtrip　radio　travel　time　also　increases；　approaching　eleven　hours　by　the　time　Voyager　is　at　the　distance　of　Neptune。　Thus；　in　case　of　emergency；　the　spacecraft　needs　to　know　how　to　put　itself　into　a　safe　standby　mode　while　awaiting　instructions　from　Earth。　As　it　ages；　more　and　more　failures　are　expected；　both　in　its　mechanical　parts　and　in　its　puter　system；　although　there　is　no　sign；　even　now；　of　a　serious　memory　deterioration；　some　robotic　Alzheimer's　disease。

This　is　not　to　say　that　Voyager　is　perfect。　Serious　mission…threatening；　white…knuckle　mishaps　did　occur。　Each　time；　special　teams　of　engineers—some　of　whom　had　been　with　the　Voyager　program　since　its　inception—were　assigned　to　〃work〃　the　problem。　They　would　study　the　underlying　science　and　draw　upon　their　previous　experience　with　the　failed　subsystems。　They　would　experiment　with　identical　Voyager　spacecraft　equipment　that　had　never　been　launched；　or　even　manufacture　a　large　number　of　ponents　of　the　sort　that　failed　in　order　to　gain　some　statistical　understanding　of　the　failure　mode。

In　April　1978；　almost　eight　months　after　launch；　and　while　the　ship　was　approaching　the　asteroid　belt；　an　omitted　ground　mand—a　human　error—caused　Voyager　2's　on…board　puter　to　switch　from　the　prime　radio　receiver　to　its　backup；　During　the　next　ground　transmission　to　the　spacecraft；　the　backup　receiver　refused　to　lock　onto　the　signal　from　Earth。　A　ponent　called　a　tracking　loop　capacitor　had　failed。　After　seven　days　in　which　Voyager　2　was　entirely　out　of　contact；　its　fault　protection　software　suddenly　manded　the　backup　receiver　to　be　switched　off　and　the　prime　receiver　to　be　switched　back　on。　Mysteriously—to　this　day；　no　one　knows　why—the　prime　receiver　failed　moments　later。　It　was　never　heard　from　again。　To　top　it　off；　the　on…board　puter　now　foolishly　insisted　on　using　the　failed　primary　receiver。　Through　an　unlucky　concatenation　of　human　and　robotic　error；　the　spacecraft　was　now　in　real　jeopardy。　No　one　could　think　of　a　way　to　get　Voyager　2　to　revert　to　the　backup　receiver。　Even　if　it　did；　the　backup　receiver　couldn't　receive　the　mands　from　Earth；　because　of　that　failed　capacitor。　There　were　many　project　personnel　who　feared　that　all　was　lost。

But　after　a　week　of　obdurate　unresponsiveness　to　all　mands；　instructions　to　switch　automatically　between　receivers　were　accepted　and　programmed　into　the　skittish　onboard　puter。　During　that　same　week　the　JPL　engineers　designed　an　innovative　mand　frequency　control　procedure　to　make　sure　that　essential　orders　would　be　understood　by　the　damaged　backup　receiver。

The　engineers　were　now　able　to　remunicate；　at　least　in　a　rudimentary　way；　with　the　spacecraft。　Unfortunately　the　backup　receiver　now　turned　giddy；　being　extremely　sensitive　to　stray　heat　dumped　when　various　ponents　of　the　spacecraft　powered　up　or　down。　In　the　following　months　the　JPL　engineers　devised　and　conducted　tests　that　let　them　thoroughly　understand　the　thermal　implications　of　most　spacecraft　operational　modes：　What　would　prevent　and　what　would　permit　receipt　of　mands　from　Earth？

With　this　information；　the　backup　receiver　problem　was　entirely　circumvented。　It　subsequently　acquired　all　the　mands　sent　from　Earth　on　how　to　gather　data　in　the　Jupiter；　Saturn；　Uranus；　and　Neptune　systems。　The　engineers　had　saved　the　mission。　（To　be　on　the　safe　side；　during　most　of　Voyager　2's　subsequent　flight　a　nominal　data…taking　sequence　for　the　next　planet　to　be　encountered　was　always　sitting　in　the　on…board　puters—should　the　spacecraft　again　bee　deaf　to　entreaties　from　home。）

Another　heart…wrenching　failure　occurred　just　after　Voyager　2　emerged　from　behind　Saturn　（as　seen　from　the　Earth）　in　August　1981。　The　scan　platform　had　been　moving　feverishly—pointing　here　and　there　among　the　rings；　moons；　and　the　planet　itself　during　the　all…too…brief　moments　of　close　approach。　Suddenly；　the　platform　jammed。　A　stuck　scan　platform　is　a　maddening　predicament：　knowing　that　the　spacecraft　is　flying　past　wonders　that　have　never　been　witnessed；　that　we　will　not　see　again　for　years　or　decades；　and　the　incurious　spacecraft　staring　fixedly　off　into　space；　ignoring　everything。

The　scan　platform　is　driven　by　actuators　containing　gear　trains。　So　first　the　JCL　engineers　ran　an　identical　copy　of　a　flight　actuator　in　a　simulated　mission。　This　actuator　failed　after　348　turns；　the　actuator　on　the　spacecraft　had　failed　after　352　turns。　The　problem　turned