pale blue dot -carl sagan-第57章

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ere　a　large　asteroid　to　be　discovered　on　impact　trajectory　with　the　Earth；　presumably　no　one's　hand　would　be　stayed　by　the　niceties　of　international　diplomacy。　There　is　a　danger；　though；　that　relaxing　prohibitions　on　such　weapons　in　space　might　make　us　less　attentive　。bout　the　positioning　of　warheads　for　offensive　purposes　in　space。

We　don't　know　what　a　standoff　nuclear　explosion　would　do　to　an　asteroid。　The　answer　may　vary　from　asteroid　to　asteroid。　Some　small　worlds　might　be　strongly　held　together；　others　might　be　little　more　than　self…gravitating　gravel　heaps。　If　an　explosion　breaks；　let's　say；　a　10kilometer　asteroid　up　into　hundreds　of　1…kilometer　fragments；　the　likelihood　that　at　least　one　of　them　impacts　the　Earth　is　probably　increased；　and　the　apocalyptic　character　of　the　consequences　may　not　be　much　reduced。　On　the　other　hand；　if　the　explosion　disrupts　the　asteroid　into　a　swarm　of　objects　a　hundred　meters　in　diameter　or　smaller；　all　of　them　might　ablate　away　like　giant　meteors　on　entering　the　Earth's　atmosphere。　In　this　case　little　impact　damage　would　be　caused。　Even　if　the　asteroid　were　wholly　pulverized　into　fine　powder；　though；　the　resulting　high…altitude　dust　layer　might　be　so　opaque　as　to　block　the　sunlight　and　change　the　climate。　We　do　not　yet　know。

A　vision　of　dozens　or　hundreds　of　nuclear…armed　missiles　on　ready　standby　to　deal　with　threatening　asteroids　or　ets　has　been　offered。　However　premature　in　this　particular　application；　it　seems　very　familiar；　only　the　enemy　has　been　changed。　It　also　seems　very　dangerous。

The　problem；　Steven　Ostro　of　JPL　and　I　have　suggested；　is　that　if　you　can　reliably　deflect　a　threatening　worldlet　so　it　does　not　collide　with　the　Earth；　you　can　also　reliably　deflect　a　harmless　worldlet　so　it　does　collide　with　the　Earth。　Suppose　you　had　a　full　inventory；　with　orbits；　of　the　estimated　300；000　near…Earth　asteroids　larger　than　100　meters—each　of　them　large　enough；　on　impacting　the　Earth；　to　have　serious　consequences。　Then；　it　turns　out；　you　also　have　a　list　of　huge　numbers　of　inoffensive　asteroids　whose　orbits　could　be　altered　with　nuclear　warheads　so　they　quickly　collide　with　the　Earth。

Suppose　we　restrict　our　attention　to　the　2；000　or　so　near…Earth　asteroids　that　are　a　kilometer　across　or　bigger—that　is；　the　ones　most　likely　to　cause　a　global　catastrophe。　Today；　with　only　about　100　of　these　objects　catalogued；　it　would　take　about　a　century　to　catch　one　when　it's　easily　deflectable　to　Earth　and　alter　its　orbit。　We　think　we've　found　one；　an　as…yet…unnamed*　asteroid　so　far　denoted　only　as　1991OA。　In　2070；　this　world；　about　1　kilometer　in　diameter；　will　e　within　4。5　million　kilometers　of　the　Earth's　orbit—only　fifteen　times　the　distance　to　the　Moon。　To　deflect　1991OA　so　it　hits　the　Earth；　only　about　60　megatons　of　TNT　equivalent　needs　to　be　exploded　in　the　right　way—the　equivalent　of　a　small　number　of　currently　available　nuclear　warheads。

*　What　should　we　call　this　world？　Naming　it　after　the　Greek　Fates　or　Furies　or　Nemesis　seems　inappropriate；　because　whether　it　misses　or　hits　the　Earth　is　entirely　in　our　hands。　If　we　leave　it　alone；　it　misses。　If　we　push　it　cleverly　and　precisely；　it　hits。　Maybe　we　should　call　it　〃Eight　Ball。〃

Now　imagine　a　time；　a　few　decades　hence；　when　all　such　near…Earth　asteroids　are　inventoried　and　their　orbits　piled。　Then；　as　Alan　Harris　of　JPL；　Greg　Canavan　of　the　Los　Alamos　National　Laboratory；　Ostro；　and　I　have　shown；　it　might　take　only　a　year　to　select　a　suitable　object；　alter　its　orbit；　and　send　it　crashing　into　the　Earth　with　cataclysmic　effect。

The　technology　required—large　optical　telescopes；　sensitive　detectors；　rocket　propulsion　systems　able　to　lift　a　few　tons　of　payload　and　make　precise　rendezvous　in　nearby　space；　and　thermonuclear　weapons—all　exist　today。　Improvements　in　all　but　perhaps　the　last　can　be　confidently　expected。　If　we're　not　careful；　many　nations　may　have　these　capabilities　in　the　next　few　decades。　What　kind　of　world　will　we　then　have　made？

We　have　a　tendency　to　minimize　the　dangers　of　new　technologies。　A　year　before　the　Chernobyl　disaster；　a　Soviet　nuclear　power　industry　deputy　minister　was　asked　about　the　safety　of　Soviet　reactors；　and　chose　Chernobyl　as　a　particularly　safe　site。　The　average　waiting　time　to　disaster；　he　confidently　estimated；　was　a　hundred　thousand　years。　Less　than　a　year　later　。　。　。　devastation。　Similar　reassurances　were　provided　by　NASA　contractors　the　year　before　the　Challenger　disaster：　You　would　have　to　wait　ten　thousand　years；　they　estimated；　for　a　catastrophic　failure　of　the　shuttle。　One　year　later　。　。　。　heartbreak。

Chlorofluorocarbons　（CFCs）　were　developed　specifically　as　a　pletely　safe　refrigerant—to　replace　ammonia　and　other　refrigerants　that；　on　leaking　out；　had　caused　illness　and　some　deaths。　Chemically　inert；　nontoxic　（in　ordinary　concentrations）；　odorless；　tasteless；　non…allergenic；　nonflammable；　CFCs　represent　a　brilliant　technical　solution　to　a　well…defined　practical　problem。　They　found　uses　in　many　other　industries　besides　refrigeration　and　air　conditioning。　But；　as　I　described　above；　the　chemists　who　developed　CFCs　overlooked　one　essential　fact—that　the　molecules'　very　inertness　guarantees　that　they　are　circulated　to　stratospheric　altitudes　and　there　cracked　open　by　sunlight；　releasing　chlorine　atoms　which　then　attack　the　protective　ozone　layer。　Due　to　the　work　of　a　few　scientists；　the　dangers　may　have　been　recognized　and　averted　in　time。　We　humans　have　now　almost　stopped　producing　CFCs。　We　won't　actually　know　if　we've　avoided　real　harm　for　about　a　century；　that's　how　long　it　takes　for　all　the　CFC　damage　to　be　pleted。　Like　the　ancient　Camarinans；　we　make　mistakes。*　Not　only　do　we　often　ignore　the　warnings　of　the　Oracles；　characteristically　we　do　not　even　consult　them。

*　There　is　of　course　a　wide　range　of　other　problems　brought　on　by　the　devastatingly　powerful　technology　we've　recently　invented。　But　in　most　cases　they're　not　Camarinan　disasters…damned　if　you　do　and　damned　if　you　don't。　Instead　they're　dilemmas　of　wisdom　or　timing…for　example；　the　wrong　refrigerant　or　refrigeration　physics　out　of　many　possible　alternatives。

The　notion　of　moving　asteroids　into　Earth　orbit　has　proved　attractive　to　some　space　scientists　and　long…range　planners。　They　foresee　mining　the　minerals　and　precious　metals　of　these　worlds　or　providing　resources　for　the　construction　of　space　infrastructure　without　having　to　fight　the　Earth's　gravity　to　get　them　up　there。　Articles　have　been　published　on　how　to　acplish　this　end　and　what　the　benefits　will　be。　In　modern　discussions；　the　asteroid　is　inserted　into　orbit　around　the　Earth　by　first　making　it　pass　through　and　be　braked　by　the　Earth's　atmosphere；　a　maneuver　with　very　little　margin　for　error。　For　the　near　future　we　can；　I　think；　recognize　this　whole　endeavor　as　unusually　dangerous　and　foolhardy；　especially　for　metal　worldlets　larger　than　tens　of　meters　across。　This　is　the　one　activity　where　errors　in　navigation　or　propulsion　or　mission　design　can　have　the　most　sweeping　and　catastrophic　consequences。

The　foregoing　are　examples　of　inadvertence。　But　there's　another　kind　of　peril：　We　are　sometimes　told　that　this　or　that　invention　would　of　course　not　be　misused。　No　sane　person　would　be　so　reckless。　This　is　the　〃only　a　madman〃　argument。　Whenever　I　hear　it　（and　it's　often　trotted　out　in　such　debates）；　I　remind　myself　that　madmen　really　exist。　Sometimes　they　achieve　the　highest　levels　of　political　power　in　modern　industrial　nations。　This　is　the　century　of　Hitler　and　Stalin；　tyrants　who　posed　the　gravest　dangers　not　just　to　the　rest　of　the　human　family；　but　to　their　own　people　as　well。　In　the　winter　and　spring　of　1945；　Hitler　ordered　Germany　to　be　destroyed—even　〃what　the　people　need　for　elementary　survival〃—because　the　surviving　Germans　had　〃betrayed〃　him；　and　at　any　rate　were　〃inferior〃　to　those　who　had　already　died。　If　Hitler　had　had　nuclear　weapons；　the　threat　of　a　counterstrike　by　Allied　nuclear　weapons；　had　there　been　any；　is　unlikely　to　have　dissuaded　him。　It　might　have　encouraged　him。

Can　we　humans　be　trusted　with　civilization…threatening　technologies？　If　the　chance　is　almost　one　in　a　thousand　that　much　of　the　human　population　will　be　killed　b