万物简史英文版_比尔·布莱森-第36章

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d　through　the　atmosphere　and　punchedinto　the　earth　with　a　violence　and　suddenness　that　we　can　scarcely　imagine。　where　mansonnow　stands　became　in　an　instant　a　hole　three　miles　deep　and　more　than　twenty　miles　across。

the　limestone　that　elsewhere　gives　iowa　its　hard　mineralized　water　was　obliterated　andreplaced　by　the　shocked　basement　rocks　that　so　puzzled　the　water　driller　in　1912。

the　manson　impact　was　the　biggest　thing　that　has　ever　occurred　on　the　mainland　unitedstates。　of　any　type。　ever。　the　crater　it　left　behind　was　so　colossal　that　if　you　stood　on　oneedge　you　would　only　just　be　able　to　see　the　other　side　on　a　good　day。　it　would　make　the　grandcanyon　look　quaint　and　trifling。　unfortunately　for　lovers　of　spectacle；　2。5　million　years　ofpassing　ice　sheets　filled　the　manson　crater　right　to　the　top　with　rich　glacial　till；　then　graded　itsmooth；　so　that　today　the　landscape　at　manson；　and　for　miles　around；　is　as　flat　as　a　tabletop。

which　is　of　course　why　no　one　has　ever　heard　of　the　manson　crater。

at　the　library　in　manson　they　are　delighted　to　show　you　a　collection　of　newspaper　articlesand　a　box　of　core　samples　from　a　1991鈥�92　drilling　program鈥攊ndeed；　they　positively　bustle　toproduce　them鈥攂ut　you　have　to　ask　to　see　them。　nothing　permanent　is　on　display；　andnowhere　in　the　town　is　there　any　historical　marker。

to　most　people　in　manson　the　biggest　thing　ever　to　happen　was　a　tornado　that　rolled　upmain　street　in　1979；　tearing　apart　the　business　district。　one　of　the　advantages　of　all　thatsurrounding　flatness　is　that　you　can　see　danger　from　a　long　way　off。　virtually　the　whole　townturned　out　at　one　end　of　main　street　and　watched　for　half　an　hour　as　the　tornado　came　towardthem；　hoping　it　would　veer　off；　then　prudently　scampered　when　it　did　not。　four　of　them；　alas；didn鈥檛　move　quite　fast　enough　and　were　killed。　every　june　now　manson　has　a　weeklong　eventcalled　crater　days；　which　was　dreamed　up　as　a　way　of　helping　people　forget　that　unhappyanniversary。　it　doesn鈥檛　really　have　anything　to　do　with　the　crater。　nobody鈥檚　figured　out　a　wayto　capitalize　on　an　impact　site　that　isn鈥檛　visible。

鈥渧ery　occasionally　we　get　people　ing　in　and　asking　where　they　should　go　to　see　thecrater　and　we　have　to　tell　them　that　there　is　nothing　to　see；鈥潯�says　anna　schlapkohl；　the　town鈥檚friendly　librarian。　鈥渢hen　they　go　away　kind　of　disappointed。鈥潯�however；　most　people；including　most　iowans；　have　never　heard　of　the　manson　crater。　even　for　geologists　it　barelyrates　a　footnote。　but　for　one　brief　period　in　the　1980s；　manson　was　the　most　geologicallyexciting　place　on　earth。

the　story　begins　in　the　early　1950s　when　a　bright　young　geologist　named　eugeneshoemaker　paid　a　visit　to　meteor　crater　in　arizona。　today　meteor　crater　is　the　most　famousimpact　site　on　earth　and　a　popular　tourist　attraction。　in　those　days；　however；　it　didn鈥檛　receivemany　visitors　and　was　still　often　referred　to　as　barringer　crater；　after　a　wealthy　miningengineer　named　daniel　m。　barringer　who　had　staked　a　claim　on　it　in　1903。　barringer　believedthat　the　crater　had　been　formed　by　a　ten…million…ton　meteor；　heavily　freighted　with　iron　andnickel；　and　it　was　his　confident　expectation　that　he　would　make　a　fortune　digging　it　out。

unaware　that　the　meteor　and　everything　in　it　would　have　been　vaporized　on　impact；　hewasted　a　fortune；　and　the　next　twenty…six　years；　cutting　tunnels　that　yielded　nothing。

by　the　standards　of　today；　crater　research　in　the　early　1900s　was　a　trifle　unsophisticated；　tosay　the　least。　the　leading　early　investigator；　g。　k。　gilbert　of　columbia　university；　modeledthe　effects　of　impacts　by　flinging　marbles　into　pans　of　oatmeal。　（for　reasons　i　cannot　supply；gilbert　conducted　these　experiments　not　in　a　laboratory　at　columbia　but　in　a　hotel　room。）somehow　from　this　gilbert　concluded　that　the　moon鈥檚　craters　were　indeed　formed　byimpacts鈥攊n　itself　quite　a　radical　notion　for　the　time鈥攂ut　that　the　earth鈥檚　were　not。　mostscientists　refused　to　go　even　that　far。　to　them；　the　moon鈥檚　craters　were　evidence　of　ancientvolcanoes　and　nothing　more。　the　few　craters　that　remained　evident　on　earth　（most　had　beeneroded　away）　were　generally　attributed　to　other　causes　or　treated　as　fluky　rarities。

by　the　time　shoemaker　came　along；　a　mon　view　was　that　meteor　crater　had　beenformed　by　an　underground　steam　explosion。　shoemaker　knew　nothing　about　undergroundsteam　explosions鈥攈e　couldn鈥檛：　they　don鈥檛　exist鈥攂ut　he　did　know　all　about　blast　zones。　oneof　his　first　jobs　out　of　college　was　to　study　explosion　rings　at　the　yucca　flats　nuclear　test　sitein　nevada。　he　concluded；　as　barringer　had　before　him；　that　there　was　nothing　at　meteorcrater　to　suggest　volcanic　activity；　but　that　there　were　huge　distributions　of　other　stuff鈥攁nomalous　fine　silicas　and　magnetites　principally鈥攖hat　suggested　an　impact　from　space。

intrigued；　he　began　to　study　the　subject　in　his　spare　time。

working　first　with　his　colleague　eleanor　helin　and　later　with　his　wife；　carolyn；　andassociate　david　levy；　shoemaker　began　a　systematic　survey　of　the　inner　solar　system。　theyspent　one　week　each　month　at　the　palomar　observatory　in　california　looking　for　objects；asteroids　primarily；　whose　trajectories　carried　them　across　earth鈥檚　orbit。

鈥渁t　the　time　we　started；　only　slightly　more　than　a　dozen　of　these　things　had　ever　beendiscovered　in　the　entire　course　of　astronomical　observation；鈥潯�shoemaker　recalled　some　yearslater　in　a　television　interview。　鈥渁stronomers　in　the　twentieth　century　essentially　abandonedthe　solar　system；鈥潯�he　added。　鈥渢heir　attention　was　turned　to　the　stars；　the　galaxies。鈥�

what　shoemaker　and　his　colleagues　found　was　that　there　was　more　risk　out　there鈥攁　greatdeal　more鈥攖han　anyone　had　ever　imagined。

asteroids；　as　most　people　know；　are　rocky　objects　orbiting　in　loose　formation　in　a　beltbetween　mars　and　jupiter。　in　illustrations　they　are　always　shown　as　existing　in　a　jumble；　butin　fact　the　solar　system　is　quite　a　roomy　place　and　the　average　asteroid　actually　will　be　abouta　million　miles　from　its　nearest　neighbor。　nobody　knows　even　approximately　how　manyasteroids　there　are　tumbling　through　space；　but　the　number　is　thought　to　be　probably　not　lessthan　a　billion。　they　are　presumed　to　be　planets　that　never　quite　made　it；　owing　to　theunsettling　gravitational　pull　of　jupiter；　which　kept鈥攁nd　keeps鈥攖hem　from　coalescing。

when　asteroids　were　first　detected　in　the　1800s鈥攖he　very　first　was　discovered　on　the　firstday　of　the　century　by　a　sicilian　named　giuseppi　piazzi鈥攖hey　were　thought　to　be　planets；　andthe　first　two　were　named　ceres　and　pallas。　it　took　some　inspired　deductions　by　theastronomer　william　herschel　to　work　out　that　they　were　nowhere　near　planet　sized　but　muchsmaller。　he　called　them　asteroids鈥攍atin　for　鈥渟tarlike鈥濃�攚hich　was　slightly　unfortunate　asthey　are　not　like　stars　at　all。　sometimes　now　they　are　more　accurately　called　planetoids。

finding　asteroids　became　a　popular　activity　in　the　1800s；　and　by　the　end　of　the　centuryabout　a　thousand　were　known。　the　problem　was　that　no　one　was　systematically　recordingthem。　by　the　early　1900s；　it　had　often　bee　impossible　to　know　whether　an　asteroid　thatpopped　into　view　was　new　or　simply　one　that　had　been　noted　earlier　and　then　lost　track　of。　bythis　time；　too；　astrophysics　had　moved　on　so　much　that　few　astronomers　wanted　to　devotetheir　lives　to　anything　as　mundane　as　rocky　planetoids。　only　a　few　astronomers；　notablygerard　kuiper；　the　dutch…born　astronomer　for　whom　the　kuiper　belt　of　ets　is　named；took　any　interest　in　the　solar　system　at　all。　thanks　to　his　work　at　the　mcdonald　observatoryin　texas；　followed　later　by　work　done　by　others　at　the　minor　planet　center　in　cincinnati　andthe　spacewatch　project　in　arizona；　a　long　list　of　lost　asteroids　was　gradually　whittled　downuntil　by　the　close　of　the　twentieth　century　only　one　known　asteroid　was　unaccounted　for鈥攁nobject　called　719　albert。　last　seen　in　october　1911；　it　was　finally　tracked　down　in　2000　afterbeing　missing　for　eighty…nine　years。

so　from　the　point　of　view　of　asteroid　research　the　twentieth　century　was　essentially　just　along　exercise　in　bookkeeping。　it　is　really　only　in　the　last　few　years　that　astronomers　havebegun　to　count　and　keep　an　eye　on　the　rest　of　the　asteroid　munity。　as　of　july　2001；twenty…six　thousand　asteroids　had　been　named　and　identified鈥攈alf　in　just　the　previous　twoyears。　with　up　to　a　billion　to　identify；　the　count　obviously　has　barely　begun。

in　a　sense　it　hardly　matters。　identifying　an　asteroid　doesn鈥檛　make　it　safe。　even　if　everyasteroid　in　the　solar　system　had　a　name　and　known　orbit；　no　one　could　say　what　perturbationsmight　send　any　of　them　hurtling　toward　us。　we　can鈥檛　forecast　rock　disturbances　on　our　ownsurface。　put　them　adrift　in　space　and　what　they　might　do　is　beyond　guessing。　any　asteroid　outthere　that　has　our　name　on　it　is　very　likely　to　have　no　other。

think　of　the　earth鈥檚　orbit　as　a　kind　of　freeway　on　which　we　are　the　only　vehicle；　but　whichis　crossed　regularly　by　pedestrians　who　don鈥檛　know　enough　to　look　before　stepping　off　thecurb。　at　least　90　percent　of　these　pedestrians　are　quite　unknown　to　us。　we　don鈥檛　know　wherethey　live；　what　sort　of　hours　they　keep；　how　often　they　e　our　way。　all　we　know　is　that　atsome　point；　at　uncertain　intervals；　they　trundle　across　the　road　down　which　we　are　cruising　atsixty…six　thousand　miles　an　hour。　as　steven　ostro　of　the　jet　propulsion　laboratory　has　put　it；鈥渟uppose　that　there　was　a　button　you　could　push　and　you　could　light　up　all　the　earth…crossingasteroids　larger　than　about　ten　meters；　there　would　be　over　100　million　of　these　objects　in　thesky。鈥潯�in　short；　you　would　see　not　a　couple　of　thousand　distant　twinkling　stars；　but　millionsupon　millions　upon　millions　of　nearer；　randomly　moving　objects鈥斺�渁ll　of　which　are　capableof　colliding　with　the　earth　and　all　of　which　are　moving　on　slightly　different　courses　throughthe　sky　at　different　rates。　it　would　be　deeply　unnerving。鈥潯�well；　be　unnerved　because　it　isthere。　we　just　can鈥檛　see　it。

altogether　it　is　thought鈥攖hough　it　is　really　only　a　guess；　based　on　extrapolating　fromcratering　rates　on　the　moon鈥攖hat　some　two　thousand　asteroids　big　enough　to　imperilcivilized　existence　regularly　cross　our　orbit。　but　even　a　small　asteroid鈥攖he　size　of　a　house；say鈥攃ould　destroy　a　city。　the　number　of　these　relative　tiddlers　in　earth…crossing　orbits　isalmost　certainly　in　the　hundreds　of　thousands　and　possibly　in　the　millions；　and　they　are　nearlyimpossible　to　track。

the　first　one　wasn鈥檛　spotted　until　1991；　and　that　was　after　it　had　already　gone　by。　named1991　ba；　it　was　noticed　as　it　sailed　past　us　at　a　distance　of　106；000　miles鈥攊n　cosmic　termsthe　equivalent　of　a　bullet　passing　through　one鈥檚　sleeve　without　touching　the　arm。　two　yearslater；　another；　somewhat　larger　asteroid　missed　us　by　just　90；000　miles鈥攖he　closest　pass　yetrecorded。　it；　too；　was　not　seen　until　it　had　passed　and　would　have　arrived　without　warning。

according　to　timothy　ferris；　writing　in　the　new　yorker；　such　near　misses　probably　happentwo　or　three　times　a　week　and　go　unnoticed。

an　object　a　hundred　yards　across　couldn鈥檛　be　picked　up　by　any　earth…based　telescope　untilit　was　within　just　a　few　days　of　us；　and　that　is　only　if　a　telescope　happened　to　be　trained　on　it；which　is　unlikely　because　e