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es　of　stellar　evolution。

Eventually；　we　will　have　a　list　of　planetary　systems—each　perhaps　with　terrestrials　and　Jovians　and　maybe　new　classes　of　planets。　We　will　examine　these　worlds；　spectroscopically　and　in　other　ways。　We　will　be　searching　for　new　Earths　and　other　life。



ON　NONE　OF　THE　WORLDS　In　the　outer　Solar　System　did　Voyager　find　signs　of　life；　much　less　intelligence。　There　was　organic　Matter　galore—the　stuff　of　life；　the　premonitions　of　life；　perhaps　but　as　far　as　we　could　see；　no　life。　There　was　no　oxygen　in　their　atmospheres；　and　no　gases　profoundly　out　of　chemical　equilibrium；　as　methane　is　in　the　Earth's　oxygen。　Many　of　the　worlds　were　painted　with　subtle　colors；　but　none　with　such　distinctive；　sharp　absorption　features　as　chlorophyll　provides　over　much　of　the　Earth's　surface。　On　very　few　worlds　was　Voyager　able　to　resolve　details　as　small　as　a　kilometer　across。　By　this　standard；　it　would　not　have　detected　even　our　own　technical　civilization　had　it　been　transplanted　to　the　outer　Solar　System。　But　for　what　it's　worth；　we　found　no　regular　patterning；　no　geometrization；　no　passion　for　small　circles；　triangles；　squares；　or　rectangles。　There　were　no　constellations　of　steady　points　of　light　on　the　night　hemispheres。　There　were　no　signs　of　a　technical　civilization　reworking　the　surface　of　any　of　these　worlds。

The　Jovian　planets　are　prolific　broadcasters　of　radio　waves—generated　in　part　by　the　abundant　trapped　and　beamed　charged　particles　in　their　magnetic　fields；　in　part　by　lightning；　and　in　part　by　their　hot　interiors。　But　none　of　this　emission　has　the　character　of　intelligent　life—or　so　it　seems　to　the　experts　in　the　field。

Of　course　our　thinking　may　be　too　narrow。　We　may　be　missing　something。　For　example；　there　is　a　little　carbon　dioxide　in　the　atmosphere　of　Titan；　which　puts　its　nitrogen/methane　atmosphere　out　of　chemical　equilibrium。　I　think　the　CO2　is　provided　by　the　steady　pitter…patter　of　ets　falling　into　Titan's　atmosphere—but　maybe　not。　Maybe　there's　something　on　the　surface　unaccountably　generating　CO2　in　the　face　of　all　that　methane。

The　surfaces　of　Miranda　and　Triton　are　unlike　anything　else　we　know。　There　are　vast　chevron…shaped　landforms　and　crisscrossing　straight　lines　that　even　sober　planetary　geologists　once　mischievously　described　as　〃highways。〃　We　think　we　（barely）　understand　these　landforms　in　terms　of　faults　and　collisions；　but　of　course　we　might　be　wrong。

The　surface　stains　of　organic　matter—sometimes；　as　on　Triton；　delicately　hued—are　attributed　to　charged　particles　producing　chemical　reactions　in　simple　hydrocarbon　ices；　generating　more　plex　organic　materials；　and　all　this　having　nothing　to　do　with　the　intermediation　of　life。　But　of　course　we　might　be　wrong。

The　plex　pattern　of　radio　static；　bursts；　and　whistles　that　we　receive　from　all　four　Jovian　planets　seems；　in　a　general　way；　explicable　by　plasma　physics　and　thermal　emission。　（Much　of　the　detail　is　not　yet　well　understood。）　But　of　course　we　might　be　wrong。

We　have　found　nothing　on　dozens　of　worlds　so　clear　and　striking　as　the　signs　of　life　found　by　the　Galileo　spacecraft　in　its　passages　by　the　Earth。　Life　is　a　hypothesis　of　last　resort。　You　invoke　it　only　when　there's　no　other　way　to　explain　what　you　see。　If　I　had　to　judge；　I　would　say　that　there's　no　life　on　any　of　the　worlds　we've　studied；　except　of　course　our　own。　But　I　might　be　wrong；　and；　right　or　wrong；　my　judgment　is　necessarily　confined　to　this　Solar　System。　Perhaps　on　some　new　mission　we'll　find　something　different；　something　striking；　something　wholly　inexplicable　with　the　ordinary　tools　of　planetary　science—and　tremulously；　cautiously；　we　will　inch　toward　a　biological　explanation。　However；　for　now　nothing　requires　that　we　go　down　such　a　path。　So　far；　the　only　life　in　the　Solar　System　is　that　which　es　from　Earth。　In　the　Uranus　and　Neptune　systems；　the　only　sign　of　life　has　been　Voyager　itself。

As　we　identify　the　planets　of　other　stars；　as　we　find　other　worlds　of　roughly　the　size　and　mass　of　the　Earth；　we　will　scrutinize　them　for　life。　A　dense　oxygen　atmosphere　may　be　detectable　even　on　a　world　we've　never　imaged。　As　for　the　Earth；　that　may　by　itself　be　a　sign　of　life。　An　oxygen　atmosphere　with　appreciable　quantities　of　methane　would　almost　certainly　be　a　sign　of　life；　as　would　modulated　radio　emission。　Someday；　from　observations　of　our　planetary　system　or　another；　the　news　of　life　elsewhere　may　be　announced　over　the　morning　coffee。



THE　VOYAGER　SPACECRAFT　are　bound　for　the　stars。　They　are　on　escape　trajectories　from　the　Solar　System；　barreling　along　at　almost　a　million　miles　a　day。　The　gravitational　fields　of　Jupiter；　Saturn；　Uranus；　and　Neptune　have　flung　them　at　such　high　speeds　that　they　have　broken　the　bonds　that　once　tied　them　to　the　Sun。

Have　they　left　the　Solar　System　yet？　The　answer　depends　very　much　on　how　you　define　the　boundary　of　the　Sun's　realm。　If　it's　the　orbit　of　the　outermost　good…sized　planet；　then　the　Voyager　spacecraft　are　already　long　gone；　there　are　probably　no　undiscovered　Neptunes。　If　you　mean　the　outermost　planet；　it　may　be　that　there　are　other—perhaps　Triton…like—planets　far　beyond　Neptune　and　Pluto；　if　so；　Voyager　1　and　Voyager　2　are　still　within　the　Solar　System。　If　you　define　the　outer　limits　of　the　Solar　System　as　the　heliopause—where　the　interplanetary　particles　and　magnetic　fields　are　replaced　by　their　interstellar　counterparts—then　neither　Voyager　has　yet　left　the　Solar　System；　although　they　may　do　so　in　the　next　few　decades。*　But　if　your　definition　of　the　edge　of　the　Solar　System　is　the　distance　at　which　our　star　can　no　longer　hold　worlds　in　orbit　about　it；　then　the　Voyagers　will　not　leave　the　Solar　System　for　hundreds　of　centuries。

*　Radio　signals　that　both　Voyagers　detected　in　1992　are　thought　to　arise　from　the　collision　of　powerful　gusts　of　solar　wind　with　the　thin　gas　that　lies　between　the　stars。　From　the　immense　。power　of　the　signal　（over　10　trillion　watts）；　the　distance　to　the　heliopause　can　be　estimated：　about　100　times　the　Earth's　distance　from　the　Sun。　At　the　speed　it's　leaving　the　Solar　System；　Voyager　1　might　pierce　the　heliopause　and　enter　interstellar　space　around　the　year　2010。　If　its　radioactive　power　source　is　still　working；　news　of　the　crossing　will　be　radioed　back　to　the　stay…at…homes　on　Earth。　The　energy　released　by　the　collision　of　this　shock　wave　with　the　heliopause　makes　it　the　most　powerful　source　of　radio　emission　in　the　Solar　System。　It　makes　you　wonder　whether　even　stronger　shocks　in　other　planetary　systems　might　he　detectable　by　our　radio　telescope。

Weakly　grasped　by　the　Sun's　gravity；　in　every　direction　in　the　sky；　is　that　immense　horde　of　a　trillion　ets　or　more；　the　port　Cloud。　The　two　spacecraft　will　finish　their　passage　through

the　Oort　cloud　in　another　20；000　years　or　so。　Then；　at　last；　pleting　their　long　good…bye　to　the　Solar　System；　broken　free　of　the　gravitational　shackles　that　once　bound　them　to　the　Sun；　the　Voyagers　will　make　for　the　open　sea　of　interstellar　space。　only　then　will　Phase　Two　of　their　mission　begin。

Their　radio　transmitters　long　dead；　the　spacecraft　will　wander　for　ages　in　the　calm；　cold　interstellar　blackness—where　there　is　almost　nothing　to　erode　them。　Once　out　of　the　Solar　System；　they　will　remain　intact　for　a　billion　years　or　more；　as　they　circumnavigate　the　center　of　the　Milky　Way　galaxy。

We　do　not　know　whether　there　are　other　space…faring　civilizations　in　the　Milky　Way。　If　they　do　exist；　we　do　not　know　how　abundant　they　are；　much　less　where　they　are。　But　there　is　at　least　a　chance　that　sometime　in　the　remote　future　one　of　the　Voyagers　will　be　intercepted　and　examined　by　an　alien　craft。

Accordingly；　as　each　Voyager　left　Earth　for　the　planets　and　the　stars；　it　carried　with　it　a　golden　phonograph　record　encased　in　a　golden；　mirrored　jacket　containing；　among　other　things；　greetings　in　59　human　languages　and　one　whale　language；　a　12…minute　sound　essay　including　a　kiss；　a　baby's　cry；　and　an　EEG　record　of　the　meditations　of　a　young　woman　in　love；　116　encoded　pictures；　on　our　science；　our　civilization；　and　ourselves；　and　90　minutes　of　the　Earth's　greatest　hits—Eastern　and　Western；　classical　and　folk；　including　a　Navajo　night　chant；　a　Japanese　shakuhachi　piece；　a　Pygmy　girl's　initiation　song；　a　Peruvian　wedding　song；　a　3；000…year…old　position　for　the　ch'in　called　〃Flowing　Streams；〃　Bach；　Beethoven；　Mozart；　Stravinsky；　Louis　Armstrong；　Blind　Willie　Johnson；　and　Chuck　Berry's　〃Johnny　B。　Goode。〃

Space　is　nearly　empty。　There　is　virtually　no　chance　that　one　of　the　Voyagers　will　ever　enter　another　solar　system—and　this　is　true　even　if　every　star　in　the　sky　is　acpanied　by　planets。　The　instructions　on　the　record　jackets；　written　in　what　we　believe　to　be　readily　prehensible　scientific　hieroglyphics；　can　be　read；　and　the　contents　of　the　records　understood；　only　if　alien　beings；　somewhere　in　the　remote　future；　find　Voyager　in　the　depths　of　interstellar　space。　Since　both　Voyagers　will　circle　the　center　of　the　Milky　Way　Galaxy　essentially　forever；　there　is　plenty　of　time　for　the　records　to　be　found—if　there's　anyone　out　there　to　do　the　finding。

We　cannot　know　how　much　of　the　records　they　would　understand。　Surely　the　greetings　will　be　inprehensible；　but　their　intent　may　not　be。　（We　thought　it　would　be　impolite　not　to　say　hello。）　The　hypothetical　aliens　are　bound　to　be　very　different　from　us—independently　evolved　on　another　world。　Are　we　really　sure　they　could　understand　anything　at　all　of　our　message？　Every　time　I　feel　these　concerns　stirring；　though；　I　reassure　myself。　Whatever　the　inprehensibilities　of　the　Voyager　record；　any　alien　ship　that　finds　it　will　have　another　standard　by　which　to　judge　us。　Each　Voyager　is　itself　a　message。　In　their　exploratory　intent；　in　the　lofty　ambition　of　their　objectives；　in　their　utter　lack　of　intent　to　do　harm；　and　in　the　brilliance　of　their　design　and　performance；　these　robots　speak　eloquently　for　us。

But　being　much　more　advanced　scientists　and　engineers　than　we—otherwise　they　would　never　be　able　to　find　and　retrieve　the　small；　silent　spacecraft　in　interstellar　space—perhaps　the　aliens　would　have　no　difficulty　understanding　what　is　encoded　on　these　golden　records。　Perhaps　they　would　recognize　the　tentativeness　o