pale blue dot -carl sagan-第11部分

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ecraft　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　direction　opposite　yours　while　taking　a　photograph　of　a　street　scene　from　a　moving　car。　This　may　sound　easy；　but　it's　not：　You　have　to　neutralize　the　most　innocent　of　motions。　At　zero　gravity；　the　mere　start　and　stop　of　the　on…board　tape　recorder　can　jiggle　the　spacecraft　enough　to　smear　the　picture。

This　problem　was　solved　by　sending　up　mands　to　the　spacecraft's　little　rocket　engines　（called　thrusters）；　machines　of　exquisite　sensitivity。　With　a　little　puff　of　gas　at　the　start　and　stop　of　each　data…taking　sequence；　the　thrusters　pensated　for　the　tape…recorder　jiggle　by　turning　the　entire　spacecraft　just　a　little。　To　deal　with　the　low　radio　power　received　at　Earth；　the　engineers　devised　a　new　and　more　efficient　way　to　record　and　transmit　the　data；　and　the　radio　telescopes　on　Earth　were　electronically　linked　together　with　others　to　increase　their　sensitivity。　Overall；　the　imaging　system　worked；　by　many　criteria；　better　at　Uranus　and　Neptune　than　it　did　at　Saturn　or　even　at　Jupiter。

Voyager　may　not　yet　be　done　exploring。　There　is；　of　course；　a　chance　that　some　vital　subsystem　will　fail　tomorrow；　but　as　far　as　the　radioactive　decay　of　the　plutonium　power　source　is　concerned；　the　two　Voyager　spacecraft　should　be　able　to　return　data　to　Earth　roughly　through　the　year　2015。

Voyager　is　an　intelligent　being—part　robot；　part　human。　It　extends　the　human　senses　to　far…off　worlds。　For　simple　tasks　and　short…term　problems；　it　relies　on　its　own　intelligence；　but　for　more　plex　tasks　and　longer…term　problems；　it　turns　to　the　collective　intelligence　and　experience　of　the　JPL　engineers。　This　trend　is　sure　to　grow。　The　Voyagers　embody　the　technology　of　the　early　1970s；　if　spacecraft　were　designed　for　such　a　mission　today；　they　would　incorporate　stunning　advances　in　artificial　intelligence；　in　miniaturization；　in　data…processing　speed；　in　the　ability　to　self…diagnose　and　repair；　and　in　the　propensity　to　learn　from　experience　They　would　also　be　much　cheaper。

In　the　many　environments　too　dangerous　for　people；　on　Earth　as　well　as　in　space；　the　future　belongs　to　robot…human　partnerships　that　will　recognize　the　two　Voyagers　as　antecedents　and　pioneers。　For　nuclear　accidents；　mine　disasters；　undersea　exploration　and　archaeology；　manufacturing；　prowling　the　interiors　of　volcanos；　and　household　help；　to　name　only　a　few　potential　applications；　it　could　make　an　enormous　difference　to　have　a　ready　corps　of　smart；　mobile；　pact；　mandable　robots　that　can　diagnose　and　repair　their　own　malfunctions。　There　are　likely　to　be　many　more　of　this　tribe　in　the　near　future。

It　is　conventional　wisdom　now　that　anything　built　by　the　government　will　be　a　disaster。　But　the　two　Voyager　spacecraft　were　built　by　the　government　（in　partnership　with　that　other　bugaboo；　academia）。　They　came　in　at　cost；　on　time；　and　vastly　exceeded　their　design　specifications—as　well　as　the　fondest　dreams　of　their　makers。　Seeking　not　to　control；　threaten；　wound；　or　destroy；　these　elegant　machines　represent　the　exploratory　part　of　our　nature　set　free　to　roam　the　Solar　System　and　beyond。　This　kind　of　technology；　the　treasures　it　uncovers　freely　available　to　all　humans　everywhere；　has　been；　over　the　last　few　decades；　one　of　the　few　activities　of　the　United　States　admired　as　much　by　those　who　abhor　many　of　its　policies　as　by　those　who　agree　with　it　on　every　issue。　Voyager　cost　each　American　less　than　a　penny　a　year　from　launch　to　Neptune　encounter。　Missions　to　the　planets　are　one　of　those　things—and　I　mean　this　not　just　for　the　United　States；　but　for　the　human　species—that　we　do　best。




CHAPTER　7　AMONG　THE　MOONS　OF　SATURN


　

　

Seat　thyself　sultanically　among　the　moons　of　Saturn。

—HERMAN　MELVILLE；　MOBY　DICK；　CHAPTER　107　（1851）

There　is　a　world；　midway　in　size　between　the　Moon　and　Mars；　where　the　upper　air　is　rippling　with　electricity—streaming　in　from　the　archetypical　ringed　planet　next　door；　where　the　perpetual　brown　overcast　is　tinged　with　an　odd　burnt　orange；　and　where　the　very　stuff　of　life　falls　out　of　the　skies　onto　the　unknown　surface　below。　It　is　so　far　away　that　light　takes　more　than　an　hour　to　get　there　from　the　Sun。　Spacecraft　take　years。　Much　about　it　is　still　a　mystery—including　whether　it　holds　great　oceans。　We　know　just　enough；　though；　to　recognize　that　within　reach　may　be　a　place　where　certain　processes　ate　today　working　themselves　out　that　aeons　ago　on　Earth　led　to　the　origin　of　life。

On　our　own　world　a　long…standing—and　in　some　respects　quite　promising—experiment　has　been　under　way　on　the　evolution　of　matter。　The　oldest　known　fossils　are　about　3。6　billion　years　old。　Of　course；　the　origin　of　life　had　to　have　happened　well　before　that。　But　4。2　or　4。3　billion　years　ago　the　Earth　was　being　so　ravaged　by　the　final　stages　of　its　formation　that　life　could　not　yet　have　e　into　being：　Massive　collisions　were　melting　the　surface；　turning　the　oceans　into　steam　and　driving　any　atmosphere　that　had　accumulated　since　the　last　impact　off　into　space。　So　around　4　billion　years　ago；　there　was　a　fairly　narrow　window—perhaps　only　a　hundred　million　years　wide　in　which　our　most　distant　ancestors　came　to　be。　Once　conditions　permitted；　life　arose　fast。　Somehow。　The　first　living　things　very　likely　were　inept；　far　less　capable　than　the　most　humble　microbe　alive　today—perhaps　just　barely　able　to　make　crude　copies　of　themselves。　But　natural　selection；　the　key　process　first　coherently　described　by　Charles　Darwin；　is　an　instrument　of　such　enormous　power　that　from　the　most　modest　beginnings　there　can　emerge　all　the　richness　and　beauty　of　the　biological　world。

Those　first　living　things　were　made　of　pieces；　parts；　building　blocks　which　had　to　e　into　being　on　their　own—that　is；　driven　by　the　laws　of　physics　and　chemistry　on　a　lifeless　Earth。　The　building　blocks　of　all　terrestrial　life　are　called　organic　molecules；　molecules　based　on　carbon。　Of　the　stupendous　＇lumber　of　possible　organic　molecules；　very　few　are　used　at　the　heart　of　life。　The　two　most　important　classes　are　the　amino　acids；　the　building　blocks　of　proteins；　and　the　nucleotide　bases；　the　building　blocks　of　the　nucleic　acids。　Mist　before　the　origin　of　life；　where　did　these　molecules　e　from？　There　are　only　two　possibilities：　from　the　outside　or　from　the　inside。　We　know　that　vastly　more　ets　and　asteroids　were　hitting　the　Earth　than　do　so　today；　that　these　small　worlds　are　rich　storehouses　of　plex　organic　molecules；　and　that　some　of　these　molecules　escaped　being　fried　on　impact。　Here　I'm　describing　homemade；　not　imported；　goods：　the　organic　molecules　generated　in　the　air　and　waters　of　the　primitive　Earth。

Unfortunately；　we　don't　know　very　much　about　the　position　of　the　early　air；　and　organic　molecules　are　far　easier　to　make　in　some　atmospheres　than　in　others。　There　couldn't　have　been　much　oxygen；　because　oxygen　is　generated　by　green　plants　and　there　weren't　any　green　plants　yet。　There　was　probably　more　hydrogen；　because　hydrogen　is　very　abundant　in　the　Universe　and　escapes　from　the　upper　atmosphere　of　the　Earth　into　space　better　than　any　other　atom　（because　it's　so　light）。　If　we　can　imagine　various　possible　early　atmospheres；　we　can　duplicate　them　in　the　laboratory；　supply　some　energy；　and　see　which　organic　molecules　are　made　and　in　what　amounts。　Such　experiments　have　over　the　years　proved　provocative　and　promising。　But　our　ignorance　of　initial　conditions　limits　their　relevance。

What　we　need　is　a　real　world　whose　atmosphere　still　retains　some　of　those　hydrogen…rich　gases；　a　world　in　other　respects　something　like　the　Earth；　a　world　in　which　the　organic　building　blocks　of　life　are　being　massively　generated　in　our　own　time；　a　world　we　can　go　to　to　seek　our　own　beginnings。　There　is　only　one　such　world　in　the　Solar　System。*　That　world　is　Titan；　the　big　moon　of　Saturn。　It's　about　5；150　kilometers　（3；200　miles）　in　diameter；　a　little　less　than　half　the　size　of　the　Earth。　It　takes　16　of　our　days　to　plete　one　orbit　of　Saturn。

*　There　could　have　been　none。　We're　very　lucky　that　there　is　such　a　world　study。　The　others　ill　have　too　much　hydrogen；　or　not　enough；　or　no　atmosphere　at　all。

No　world　is　a　perfect　replica　of　any　other；　and　in　at　least　one　important　respect　Titan　is　very　different　from　the　primitive　Earth：　Being　so　far　from　the　Sun；　its　surface　is　extremely　cold；　far　below　the　freezing　point　of　water；　around　180°　below　zero　Celsius。　So　while　the　Earth　at　the　time　of　the　origin　of　life　was；　as　now；　mainly　ocean…covered；　plainly　there　can　be　no　oceans　of　liquid　water　on　Titan。　（Oceans　made　of　other　stuff　are　a　different　story；　as　we　shall　see。）　The　low　temperatures　provide　an　advantage；　though；　because　once　molecules　are　synthesized　on　Titan；　they　tend　to　stick　around：　The　higher　the　temperature；　the　faster　molecules　fall　to　pieces。　On　Titan　the　molecules　that　have　been　raining　down　like　manna　from　heaven　for　the　last　4　billion　years　might　still　be　there；　largely　unaltered；　deep…frozen；　awaiting　the　chemists　from　Earth。



THE　INVENTION　OF　THE　TELESCOPE　In　the　seventeenth　century　led　to　the　discovery　of　many　new　worlds。　In　1610　Galileo　first　spied　the　four　large　satellites　of　Jupiter。　It　looked　like　a　miniature　solar　system；　the　little　moons　racing　around　Jupiter　as　the　planets　were　thought　by　Copernicus　to　orbit　the　Sun。　It　was　another　blow　to　the　geocentrists。　Forty…five　years　later；　the　celebrated　Dutch　physicist　Christianus　Huygens　discovered　a　moon　moving　about　the　planet　Saturn　and　named　it　Titan。*　It　was　a　dot　of　light　a　billion　miles　away；　gleaming　in　reflected　sunlight。　From　the　time　of　its　discovery；　when　European　men　wore　long　curly　wigs；　to　world　War　II；　when　American　men　cut　their　hair　down　to　stubble；　almost　nothing　more　was　discovered　about　Titan　except　the　fact　it　had　a　curious；　tawny　color。　Ground…based　telescopes　could；　even　in　principle；　barely　make　out　some　enigmatic　detail。　The　Spanish　astronomer　J。　as　Sola　reported　at　the　turn　of　the　twentieth　century　some　faint　and　indirect　evidence　of　an　atmosphere。

*　Not　because　he　thought　it　remarkably　large。　but　because　in　Greek　mythology　members　of　the　generation　preceding　the　Olympian　gods—Saturn；　his　siblings；　and　his　cousins—were　called　Titans。

In　a　way；　I　grew　up　with　Titan。　I　did　my　doctoral　dissertation　at　the　University　of　Chicago　under　the　guidance　of　Gerard　P。　Kuiper；　the　astronomer　who　made　the　definitive　discovery　that　Titan　has　an　atmosphere。　Kuiper　was　Dutch　and　in　a　direct　line　of　intellectual　descent　from　Christianus　Huygens。　In　1914；　while　making　a　spectroscopic　examination　of　Titan；　Kuiper　was　astonished　to　find　the　characteristic　spectral　features　of　the　gas　methane。　When　he　pointed　the　telescope　at　Titan；　there　was　the　signature　of　methane。　When　he　pointed　it　away；　not　a　hint　of　methane。*　But　moons　were　not　supposed　to　hold　onto　sizable　atmospheres；　and　the　Earth's　Moon　certainly　doesn't。　Titan　could　retain　an　atmosphere；　Kuiper　realized；　even　though　its　gravity　was　less　than　Earth's；　because　its　upper　atmosphere　is　very　cold。　The　molecules　simply　aren't　moving　fast　enough　for　significant　numbers　to　achieve　escape　velocity　and　trickle　away　to　space。

*　Titan's　atmosphere　has　no　detectable　oxygen；　so　methane　is　not　wildly　out　of　chemical　equilibrium—as　it　is　on　Earth—and　its　presence　is　in　no　way　a　sign　of　life。

Daniel　Harris；　a　student　of　Kuiper's；　showed　definitively　that　Titan　is　red。　Maybe　we　were　looking　at　a　rusty　surface；　like　that　of　Mars。　If　you　wanted　to　learn　more　about　Titan；　you　could　also　measure　the　polarization　of　sunlight　reflected　off　it。　Ordinary　sunlight　is　unpolarized。　Joseph　Veverka；　now　a　fellow　faculty　member　at　Cornell　University；　was　my　graduate　student　at　Harvard　University；　and　therefore；　so　to　speak；　a　grandstudent　of　Kuiper's。　In　his　doctoral　work；　around　1970；　he　measured　the　polarization　of　Titan　and　found　that　it　changed　as　the　relative　positions　of　Titan；　the　Sun；　and　the　Earth　changed。　But　the　change　was　very　different　from　that　exhibited　by；　say；　the　Moon。　Veverka　concluded　that　the　character　of　this　variation　was　consistent　with　extensive　clouds　or　haze　on　Titan。　When　we　looked　at　it　through　the　telescope；　we　weren't　seeing　its　surface。　We　knew　nothing　about　what　the　surface　was　like。　We　had　no　idea　how　fat　below　the　clouds　the　surface　was。

So；　by　the　early　1970s；　as　a　kind　of　legacy　from　Huygens　and　his　line　of　intellectual　descent；　we　knew　at　least　that　Titan　has　a　dense　methane…rich　atmosphere；　and　that　it's　probable　enveloped　by　a　reddish　cloud　veil　or　aerosol　haze。　But　what　kind　of　cloud　is　red？　By　the　early　1970s　my　colleague　Bishun　Khare　and　I　had　been　doing　experiments　at　Cornell　in　which　we　irradiated　various　methane…rich　atmospheres　with　Ultraviolet　light　or　electrons　and　were　generating　reddish　or　brownish　solids；　the　stuff　would　coat　the　interiors　of　our　reaction　vessels。　It　seemed　to　me　that；　if　methane…rich　Titan　had　red…brown　clouds；　those　clouds　might　very　well　be　similar　to　what　we　were　making　in　the　laboratory。　We　called　this　material　tholin；　after　a　Greek　word　for　〃muddy。〃　At　the　beginning　we　had　yen　little　idea　what　it　was　made　of。　It　was　some　organic　stew　made　by　breaking　apart　our　starting　molecules；　and　allowing　the　atoms—carbon；　hydrogen；　nitrogen—and　molecular　fragments　to　rebi