pale blue dot -carl sagan-第47部分

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　30；000　swords　of　Damocles　hanging　over　our　heads…ten　times　more　than　the　number　of　stars　visible　to　the　naked　eye　under　conditions　of　optimum　atmospheric　clarity。　Public　anxiety　might　be　much　greater　in　such　a　time　of　knowledge　than　in　our　current　age　of　ignorance。　There　might　be　irresistible　public　pressure　to　develop　means　to　mitigate　even　nonexistent　threats；　which　would　then　feed　the　danger　that　deflection　technology　would　be　misused。　For　this　reason；　asteroid　discovery　and　surveillance　may　not　be　a　mere　neutral　tool　of　future　policy；　but　rather　a　kind　of　booby　trap。　To　me；　the　only　foreseeable　solution　is　a　bination　of　accurate　orbit　estimation；　realistic　threat　assessment；　and　effective　public　education—so　that　in　democracies　at　least；　the　citizens　can　make　their　own；　informed　decisions。　This　is　a　job　for　NASA。

Near…Earth　asteroids；　and　means　of　altering　their　orbits；　are　being　looked　at　seriously。　There　is　some　sign　that　officials　in　the　Department　of　Defense　and　the　weapons　laboratories　are　beginning　to　understand　that　there　may　be　real　dangers　in　planning　to　push　asteroids　around。　Civilian　and　military　scientists　have　met　to　discuss　the　subject。　On　first　hearing　about　the　asteroid　hazard；　many　people　think　of　it　as　a　kind　of　Chicken　Little　fable；　Goosey…Lucy；　newly　arrived　and　in　great　excitement；　is　municating　the　urgent　news　that　the　sky　is　falling。　The　tendency　to　dismiss　the　prospect　of　any　catastrophe　that　we　have　not　personally　witnessed　is　in　the　long　run　very　foolish。　But　in　this　case　it　may　be　an　ally　of　prudence。



MEANWHILE　WE　MUST　STILL　FACE　the　deflection　dilemma。　If　we　develop　and　deploy　this　technology；　it　may　do　us　in。　If　we　don't；　some　asteroid　or　et　may　do　us　in。　The　resolution　of　the　dilemma　hinges；　I　think；　on　the　fact　that　the　likely　timescales　of　the　two　dangers　are　very　different—short　for　the　former；　long　for　the　latter。

I　like　to　think　that　our　future　involvement　with　near…Earth　asteroids　will　go　something　like　this：　From　ground…based　observatories；　we　discover　all　the　big　ones；　plot　and　monitor　their　orbits；　determine　rotation　rates　and　positions。　Scientists　are　diligent　in　explaining　the　dangers—neither　exaggerating　nor　muting　the　prospects。　We　send　robotic　spacecraft　to　fly　by　a　few　selected　bodies；　orbit　them；　land　on　them；　and　return　surface　samples　to　laboratories　on　Earth。　Eventually　we　send　humans。　（Because　of　the　low　gravities；　they　will　be　able　to　make　standing　broad　jumps　of　ten　kilometers　or　more　into　the　sky；　and　lob　a　baseball　into　orbit　around　the　asteroid。）　Fully　aware　of　the　dangers；　we　make　no　attempts　to　alter　trajectories　until　the　potential　for　misuse　of　world…altering　technologies　is　much　less。　That　might　take　a　while。

If　we're　too　quick　in　developing　the　technology　to　move　worlds　around；　we　may　destroy　ourselves；　if　we're　too　slow；　we　will　surely　destroy　ourselves。　The　reliability　of　world　political　organizations　and　the　confidence　they　inspire　will　have　to　make　significant　strides　before　they　can　be　trusted　to　deal　with　a　problem　of　this　seriousness。　At　the　same　time；　there　seems　to　be　no　acceptable　national　solution。　Who　would　feel　fortable　with　the　means　of　world　destruction　in　the　hands　of　some　dedicated　（or　even　potential）　enemy　nation；　whether　or　not　our　nation　had　parable　powers？　The　existence　of　interplanetary　collision　hazards；　when　widely　understood；　works　to　bring　our　species　together。　When　facing　a　mon　danger；　we　humans　have　sometimes　reached　heights　widely　thought　impossible；　we　have　set　aside　our　differences—at　least　until　the　danger　passed。

But　this　danger　never　passes。　The　asteroids；　gravitationally　churning；　are　slowly　altering　their　orbits；　without　warning；　new　ets　e　careening　toward　us　from　the　transplutonian　darkness。　There　will　always　be　a　need　to　deal　with　them　in　a　way　that　does　not　endanger　us。　By　posing　two　different　classes　of　peril—one　natural；　the　other　human…made—the　small　near…Earth　worlds　provide　a　new　and　potent　motivation　to　create　effective　transnational　institutions　and　to　unify　the　human　species。　It's　hard　to　see　any　satisfactory　alternative。

In　our　usual　jittery；　two…steps…forward…one…step…back　mode；　We　are　moving　toward　unification　anyway。　There　are　powerful　influences　deriving　from　transportation　and　munications　technologies；　the　interdependent　world　economy；　and　the　global　environmental　crisis。　The　impact　hazard　merely　hastens　the　pace。

Eventually；　cautiously；　scrupulously　careful　to　attempt　nothing　with　asteroids　that　could　inadvertently　cause　a　catastrophe　on　Earth；　I　imagine　we　will　begin　to　learn　how　to　change　the　orbits　of　little　nonmetallic　worlds；　smaller　than　100　meters　across。　We　begin　with　smaller　explosions　and　slowly　work　our　way　up。　We　gain　experience　in　changing　the　orbits　of　various　asteroids　and　ets　of　different　positions　and　strengths。　We　try　to　determine　which　ones　can　be　pushed　around　and　which　cannot。　By　the　twenty…second　century；　perhaps；　we　move　small　worlds　around　the　Solar　System；　using　（see　next　chapter）　not　nuclear　explosions　but　nuclear　fusion　engines　or　their　equivalents。　We　insert　small　asteroids　made　of　precious　and　industrial　metals　into　Earth　orbit。　Gradually　develop　a　defensive　technology　to　deflect　a　large　asteroid　or　et　that　might　in　the　foreseeable　future　hit　the　Earth；　while；　with　meticulous　care；　we　build　layers　of　safeguards　against　misuse。

Since　the　danger　of　misusing　deflection　technology　seems　so　much　greater　than　the　danger　of　an　imminent　impact；　we　can　afford　to　wait；　take　precautions；　rebuild　political　institutions—for　decades　certainly；　probably　centuries。　If　we　play　our　cards　right　and　are　not　unlucky；　we　can　pace　what　we　do　up　there　by　what　progress　we're　making　down　here。　The　two　are　in　any　case　deeply　connected。

The　asteroid　hazard　forces　our　hand。　Eventually；　we　must　establish　a　formidable　human　presence　throughout　the　inner　Solar　System。　On　an　issue　of　this　importance　I　do　not　think　we　will　be　content　with　purely　robotic　means　of　mitigation。　To　do　so　safely　we　must　make　changes　in　our　political　and　international　systems。　While　much　about　our　future　is　cloudy；　this　conclusion　seems　a　little　more　robust；　and　independent　of　the　vagaries　of　human　institutions。

In　the　long　term；　even　if　we　were　not　the　descendants　of　professional　wanderers；　even　if　we　were　not　inspired　by　exploratory　passions；　some　of　us　would　still　have　to　leave　the　Earth—simply　to　ensure　the　survival　of　all　of　us。　And　once　we're　out　there；　we'll　need　bases；　infrastructures。　It　would　not　be　very　long　before　some　of　us　were　living　in　artificial　habitats　and　on　other　worlds。　This　is　the　first　of　two　mussing　arguments；　omitted　in　our　discussion　of　missions　to　Mars；　for　a　permanent　human　presence　in　space。



OTHER　PLANETARY　SYSTEMS　must　face　their　own　impact　hazards—because　small　primordial　worlds；　of　which　asteroids　and　ets　are　remnants；　are　the　stuff　out　of　which　planets　form　there　as　well。　After　the　planets　are　made；　many　of　these　planetesimals　are　left　over。　The　average　time　between　civilization…threatening　impacts　on　Earth　is　perhaps　200；000　years；　twenty　times　the　age　of　our　civilization。　Very　different　waiting　times　may　pertain　to　extraterrestrial　civilizations；　if　they　exist；　depending　on　such　factors　as　the　physical　and　chemical　characteristics　of　the　planet　and　its　biosphere；　the　biological　and　social　nature　of　the　civilization；　and　of　course　the　collision　rate　itself。　Planets　with　higher　atmospheric　pressures　will　be　protected　against　somewhat　larger　1mpactors；　although　the　pressure　cannot　be　much　greater　before　greenhouse　warming　and　other　consequences　make　life　improbable。　If　the　gravity　is　much　less　than　on　Earth；　impactors　will　make　less　energetic　collisions　and　the　hazard　will　be　reduced—although　it　cannot　be　reduced　very　much　before　the　atmosphere　escapes　to　space。

The　impact　rate　in　other　planetary　systems　is　uncertain。　Our　system　contains　two　major　populations　of　small　bodies　that　feed　potential　impactors　into　Earth…crossing　orbits。　Both　the　existence　of　the　source　populations　and　the　mechanisms　that　maintain　the　collision　rate　depend　on　how　worlds　are　distributed。　For　example；　our　Oort　Cloud　seems　to　have　been　populated　by　gravitational　ejections　of　icy　worldlets　from　the　vicinity　of　Uranus　and　Neptune。　If　there　are　no　planets　that　play　the　role　of　Uranus　and　Neptune　in　systems　otherwise　like　our　own；　their　Oort　Clouds　may　be　much　more　thinly　populated。　Stars　in　open　and　globular　stellar　clusters；　stars　in　double　or　multiple　systems；　stars　closer　to　the　center　of　the　Galaxy；　stars　experiencing　more　frequent　encounters　with　Giant　Molecular　Clouds　in　interstellar　space；　may　all　experience　higher　impact　fluxes　at　their　terrestrial　planets。　The　etary　flux　might　be　hundreds　or　thousands　of　times　more　at　the　Earth　had　the　planet　Jupiter　never　formed—according　to　a　calculation　by　George　Wetherill　of　the　Carnegie　Institution　of　Washington。　In　systems　without　Jupiter…like　planets；　the　gravitational　shield　against　ets　is　down；　and　civilization…threatening　impacts　much　more　frequent。

To　a　certain　extent；　increased　fluxes　of　interplanetary　objects　might　increase　the　rate　of　evolution；　as　the　mammals　that　flourished　and　diversified　after　the　Cretaceous…Tertiary　collision　wiped　out　the　dinosaurs。　But　there　must　be　a　point　of　diminishing　returns：　Clearly；　some　flux　is　too　high　for　the　continuance　of　any　civilization。

One　consequence　of　this　train　of　argument　is　that；　even　if　civilizations　monly　arise　on　planets　throughout　the　Galaxy；　few　of　them　will　be　both　long…lived　and　non…technological。　Since　hazards　from　asteroids　and　ets　must　apply　to　inhabited　planets　all　over　the　Galaxy；　if　there　are　such；　intelligent　beings　everywhere　will　have　to　unify　their　home　worlds　politically；　leave　their　planets；　and　move　small　nearby　worlds　around。　Their　eventual　choice；　as　ours；　is　spaceflight　or　extinction。




CHAPTER　19　　　REMAKING　THE　PLANETS

　

　

Who　could　deny　that　man　could　somehow　also　make　the　heavens；

could　he　only　obtain　the　instruments　and　the　heavenly　material？

—MARSILIO　FICINO；　〃THE　SOUL　OF　MAN〃　（CA。　1474）

In　the　midst　of　the　Second　World　War；　a　young　American　writer　named　Jack　Williamson　envisioned　a　populated　Solar　System。　In　the　twenty…second　centur