pale blue dot -carl sagan-第59章

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　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　century；　he　imagined；　Venus　would　be　settled　by　China；*　Japan；　and　Indonesia；　Mars　by　Germany；　and　the　moons　of　Jupiter　by　Russia。　Those　who　spoke　English；　the　language　in　which　Williamson　was　writing；　were　confined　to　the　asteroids…and　of　course　the　Earth。

*　In　the　real　world；　Chinese　space　officials　are　proposing　to　send　a　two…person　astronaut　capsule　into　orbit　by　the　turn　of　the　century。　It　would　be　propelled　by　a　modified　Long　March　2E　rocket　and　be　launched　from　the　Gobi　Desert。　If　the　Chinese　economy　exhibits　even　moderate　continuing　growth—much　less　the　exponential　growth　that　marked　it　in　the　early　to　mid…1990s—China　may　be　one　of　the　world's　leading　space　powers　by　the　middle　of　the　twenty…first　century。　Or　earlier。

The　story；　published　in　Astounding　Science　Fiction　in　July　1942；　was　called　〃Collision　Orbit〃　and　written　under　the　pseudonym　Will　Stewart。　Its　plot　hinged　on　the　imminent　collision　of　an　uninhabited　asteroid　with　a　colonized　one；　and　the　search　for　a　means　of　altering　the　trajectories　of　small　worlds。　Although　no　one　on　Earth　was　endangered；　this　may　have　been　the　first　appearance；　apart　from　newspaper　ic　strips；　of　asteroid　collisions　as　a　threat　to　humans。　（ets　impacting　the　Earth　had　been　a　staple　peril。）

The　environments　of　Mars　and　Venus　were　poorly　understood　in　the　early　1940s；　it　was　conceivable　that　humans　could　live　there　without　elaborate　life…support　systems。　But　the　asteroids　were　another　matter。　It　was　well　known；　even　then；　that　asteroids　were　small；　dry；　airless　worlds。　If　they　were　to　be　inhabited；　especially　by　large　numbers　of　people；　these　little　worlds　would　somehow　have　to　be　fixed。

In　〃Collision　Orbit；〃　Williamson　portrays　a　group　of　〃spatial　engineers；〃　able　to　render　such　barren　outposts　clement。　Coining　a　word；　Williamson　called　the　process　of　metamorphosis　into　an　Earth…like　world　〃terraforming。〃　He　knew　that　the　low　gravity　on　an　asteroid　means　that　any　atmosphere　generated　or　transported　there　would　quickly　escape　to　space。　So　his　key　terraforming　techno