pale blue dot -carl sagan-第19章

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red　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　out　to　be　a　lubrication　failure。　Good　to　know；　but　what　to　do　about　it？　Plainly；　it　would　be　impossible　to　overtake　Voyager　with　ail　oilcan。

The　engineers　wondered　whether　they　could　restart　the　tailed　actuator　by　alternate　heating　and　cooling；　maybe　the　resulting　thermal　stresses　would　induce　the　ponents　of　the　actuator　to　expand　and　contract　at　different　rates　and　unjam　the　system。　They　tested　this　notion　with　specially　manufactured　actuators　in　the　laboratory；　and　then　jubilantly　found　that　in　this　way　they　could　start　the　scan　platform　up　again　in　space。　Project　personnel　also　devised　ways　to　diagnose　any　additional　trend　toward　actuator　failure　early　enough　to　work　around　the　problem。　Thereafter；　Voyager　2's　scan　platform　worked　perfectly。　All　the　pictures　taken　in　the　Uranus　and　Neptune　systems　owe　their　existence　to　this　work。　The　engineers　had　saved　the　day　again。

Voyagers　1　and　2　were　designed　to　explore　the　Jupiter　and　Saturn　systems　only。　It　is　true　that　their　trajectories　would　carry　them　on　past　Uranus　and　Neptune；　but　officially　these　planets　were　never　contemplated　as　targets　for　Voyager　exploration：　The　spacecraft　were　not　supposed　to　last　that　long。　Because　of　our　wish　to　fly　close　to　the　mystery　world　Titan；　Voyager　9　was　flung　by　Saturn　on　a　path　that　could　never　encounter　any　other　known　world；　it　is　Voyager　2　that　flew　on　to　Uranus　and　Neptune　with　brilliant　success。　At　these　immense　distances；　sunlight　is　getting　progressively　dimmer；　and　the　radio　signals　transmitted　to　Earth　are　getting　progressively　fainter。　These　were　predictable　but　still　very　serious　problems　that　the　JPL　engineers　and　scientists　also　had　to　solve。

Because　of　the　low　light　levels　at　Uranus　and　Neptune；　the　Voyager　television　cameras　were　obliged　to　take　long　time　exposures。　But　the　spacecraft　was　hurtling　so　fast　through；　say；　the　Uranus　system　（at　about　35；000　miles　per　hour）　that　the　image　would　have　been　smeared　or　blurred。　To　pensate；　the　entire　spacecraft　had　to　be　moved　during　the　time　exposures　to　cancel　out　the　motion；　like　panning　in　the　directio