A Short History of Nearly Everything- 第6部分

as　for　pluto　itself；　nobody　is　quite　sure　how　big　it　is；　or　what　it　is　made　of；　what　kind　ofatmosphere　it　has；　or　even　what　it　really　is。　a　lot　of　astronomers　believe　it　isn’t　a　planet　at　all；but　merely　the　largest　object　so　far　found　in　a　zone　of　galactic　debris　known　as　the　kuiperbelt。　the　kuiper　belt　was　actually　theorized　by　an　astronomer　named　f。　c。　leonard　in　1930；but　the　name　honors　gerard　kuiper；　a　dutch　native　working　in　america；　who　expanded　theidea。　the　kuiper　belt　is　the　source　of　what　are　known　as　short…period　ets—those　thate　past　pretty　regularly—of　which　the　most　famous　is　halley’s　et。　the　more　reclusivelong…period　ets　（among　them　the　recent　visitors　hale…bopp　and　hyakutake）　e　fromthe　much　more　distant　oort　cloud；　about　which　more　presently。

it　is　certainly　true　that　pluto　doesn’t　act　much　like　the　other　planets。　not　only　is　it　runty　andobscure；　but　it　is　so　variable　in　its　motions　that　no　one　can　tell　you　exactly　where　pluto　will　bea　century　hence。　whereas　the　other　planets　orbit　on　more　or　less　the　same　plane；　pluto’sorbital　path　is　tipped　（as　it　were）　out　of　alignment　at　an　angle　of　seventeen　degrees；　like　thebrim　of　a　hat　tilted　rakishly　on　someone’s　head。　its　orbit　is　so　irregular　that　for　substantialperiods　on　each　of　its　lonely　circuits　around　the　sun　it　is　closer　to　us　than　neptune　is。　formost　of　the　1980s　and　1990s；　neptune　was　in　fact　the　solar　system’s　most　far…flung　planet。

only　on　february　11；　1999；　did　pluto　return　to　the　outside　lane；　there　to　remain　for　the　next228　years。

so　if　pluto　really　is　a　planet；　it　is　certainly　an　odd　one。　it　is　very　tiny：　just　one…quarter　of　1percent　as　massive　as　earth。　if　you　set　it　down　on　top　of　the　united　states；　it　would　cover　notquite　half　the　lower　forty…eight　states。　this　alone　makes　it　extremely　anomalous；　it　means　thatour　planetary　system　consists　of　four　rocky　inner　planets；　four　gassy　outer　giants；　and　a　tiny；solitary　iceball。　moreover；　there　is　every　reason　to　suppose　that　we　may　soon　begin　to　findother　even　larger　icy　spheres　in　the　same　portion　of　space。　then　we　will　have　problems。　afterchristy　spotted　pluto’s　moon；　astronomers　began　to　regard　that　section　of　the　cosmos　moreattentively　and　as　of　early　december　2002　had　found　over　six　hundred　additional　trans…neptunian　objects；　or　plutinos　as　they　are　alternatively　called。　one；　dubbed　varuna；　is　nearlyas　big　as　pluto’s　moon。　astronomers　now　think　there　may　be　billions　of　these　objects。　thedifficulty　is　that　many　of　them　are　awfully　dark。　typically　they　have　an　albedo；　orreflectiveness；　of　just　4　percent；　about　the　same　as　a　lump　of　charcoal—and　of　course　theselumps　of　charcoal　are　about　four　billion　miles　away。

and　how　far　is　that　exactly？　it’s　almost　beyond　imagining。　space；　you　see；　is　justenormous—just　enormous。　let’s　imagine；　for　purposes　of　edification　and　entertainment；　thatwe　are　about　to　go　on　a　journey　by　rocketship。　we　won’t　go　terribly　far—just　to　the　edge　ofour　own　solar　system—but　we　need　to　get　a　fix　on　how　big　a　place　space　is　and　what　a　smallpart　of　it　we　occupy。

now　the　bad　news；　i’m　afraid；　is　that　we　won’t　be　home　for　supper。　even　at　the　speed　oflight；　it　would　take　seven　hours　to　get　to　pluto。　but　of　course　we　can’t　travel　at　anything　likethat　speed。　we’ll　have　to　go　at　the　speed　of　a　spaceship；　and　these　are　rather　more　lumbering。

the　best　speeds　yet　achieved　by　any　human　object　are　those　of　the　voyager　1　and2　spacecraft；which　are　now　flying　away　from　us　at　about　thirty…five　thousand　miles　an　hour。

the　reason　the　voyager　craft　were　launched　when　they　were　（in　august　and　september1977）　was　that　jupiter；　saturn；　uranus；　and　neptune　were　aligned　in　a　way　that　happens　onlyonce　every　175　years。　this　enabled　the　two　voyagers　to　use　a　“gravity　assist”　technique　inwhich　the　craft　were　successively　flung　from　one　gassy　giant　to　the　next　in　a　kind　of　cosmicversion　of　“crack　the　whip。”　even　so；　it　took　them　nine　years　to　reach　uranus　and　a　dozen　tocross　the　orbit　of　pluto。　the　good　news　is　that　if　we　wait　until　january　2006　（which　is　whennasa’s　new　horizons　spacecraft　is　tentatively　scheduled　to　depart　for　pluto）　we　can　takeadvantage　of　favorable　jovian　positioning；　plus　some　advances　in　technology；　and　get　there　inonly　a　decade　or　so—though　getting　home　again　will　take　rather　longer；　i’m　afraid。　at　allevents；　it’s　going　to　be　a　long　trip。

now　the　first　thing　you　are　likely　to　realize　is　that　space　is　extremely　well　named　and　ratherdismayingly　uneventful。　our　solar　system　may　be　the　liveliest　thing　for　trillions　of　miles；　butall　the　visible　stuff　in　it—the　sun；　the　planets　and　their　moons；　the　billion　or　so　tumblingrocks　of　the　asteroid　belt；　ets；　and　other　miscellaneous　drifting　detritus—fills　less　than　atrillionth　of　the　available　space。　you　also　quickly　realize　that　none　of　the　maps　you　have　everseen　of　the　solar　system　were　remotely　drawn　to　scale。　most　schoolroom　charts　show　theplanets　ing　one　after　the　other　at　neighborly　intervals—the　outer　giants　actually　castshadows　over　each　other　in　many　illustrations—but　this　is　a　necessary　deceit　to　get　them　allon　the　same　piece　of　paper。　neptune　in　reality　isn’t　just　a　little　bit　beyond　jupiter；　it’s　waybeyond　jupiter—five　times　farther　from　jupiter　than　jupiter　is　from　us；　so　far　out　that　itreceives　only　3　percent　as　much　sunlight　as　jupiter。

such　are　the　distances；　in　fact；　that　it　isn’t　possible；　in　any　practical　terms；　to　draw　the　solarsystem　to　scale。　even　if　you　added　lots　of　fold…out　pages　to　your　textbooks　or　used　a　reallylong　sheet　of　poster　paper；　you　wouldn’t　e　close。　on　a　diagram　of　the　solar　system　toscale；　with　earth　reduced　to　about　the　diameter　of　a　pea；　jupiter　would　be　over　a　thousand　feetaway　and　pluto　would　be　a　mile　and　a　half　distant　（and　about　the　size　of　a　bacterium；　so　youwouldn’t　be　able　to　see　it　anyway）。　on　the　same　scale；　proxima　centauri；　our　nearest　star；would　be　almost　ten　thousand　miles　away。　even　if　you　shrank　down　everything　so　that　jupiterwas　as　small　as　the　period　at　the　end　of　this　sentence；　and　pluto　was　no　bigger　than　amolecule；　pluto　would　still　be　over　thirty…five　feet　away。

so　the　solar　system　is　really　quite　enormous。　by　the　time　we　reach　pluto；　we　have　e　sofar　that　the　sun—our　dear；　warm；　skin…tanning；　life…giving　sun—has　shrunk　to　the　size　of　apinhead。　it　is　little　more　than　a　bright　star。　in　such　a　lonely　void　you　can　begin　to　understandhow　even　the　most　significant　objects—pluto’s　moon；　for　example—have　escaped　attention。

in　this　respect；　pluto　has　hardly　been　alone。　until　the　voyager　expeditions；　neptune　wasthought　to　have　two　moons；　voyager　found　six　more。　when　i　was　a　boy；　the　solar　system　wasthought　to　contain　thirty　moons。　the　total　now　is　“at　least　ninety；”　about　a　third　of　which　havebeen　found　in　just　the　last　ten　years。

the　point　to　remember；　of　course；　is　that　when　considering　the　universe　at　large　we　don’tactually　know　what　is　in　our　own　solar　system。

now　the　other　thing　you　will　notice　as　we　speed　past　pluto　is　that　we　are　speeding　pastpluto。　if　you　check　your　itinerary；　you　will　see　that　this　is　a　trip　to　the　edge　of　our　solarsystem；　and　i’m　afraid　we’re　not　there　yet。　pluto　may　be　the　last　object　marked　onschoolroom　charts；　but　the　system　doesn’t　end　there。　in　fact；　it　isn’t　even　close　to　endingthere。　we　won’t　get　to　the　solar　system’s　edge　until　we　have　passed　through　the　oort　cloud；　avast　celestial　realm　of　drifting　ets；　and　we　won’t　reach　the　oort　cloud　for　another—i’m　sosorry　about　this—ten　thousand　years。　far　from　marking　the　outer　edge　of　the　solar　system；　asthose　schoolroom　maps　so　cavalierly　imply；　pluto　is　barely　one…fifty…thousandth　of　the　way。

of　course　we　have　no　prospect　of　such　a　journey。　a　trip　of　240；000　miles　to　the　moon　stillrepresents　a　very　big　undertaking　for　us。　a　manned　mission　to　mars；　called　for　by　the　firstpresident　bush　in　a　moment　of　passing　giddiness；　was　quietly　dropped　when　someone　workedout　that　it　would　cost　450　billion　and　probably　result　in　the　deaths　of　all　the　crew　（their　dnatorn　to　tatters　by　high…energy　solar　particles　from　which　they　could　not　be　shielded）。

based　on　what　we　know　now　and　can　reasonably　imagine；　there　is　absolutely　no　prospectthat　any　human　being　will　ever　visit　the　edge　of　our　own　solar　system—ever。　it　is　just　too　far。

as　it　is；　even　with　the　hubble　telescope；　we　can’t　see　even　into　the　oort　cloud；　so　we　don’tactually　know　that　it　is　there。　its　existence　is　probable　but　entirely　hypothetical。

about　all　that　can　be　said　with　confidence　about　the　oort　cloud　is　that　it　starts　somewherebeyond　pluto　and　stretches　some　two　light…years　out　into　the　cosmos。　the　basic　unit　ofmeasure　in　the　solar　system　is　the　astronomical　unit；　or　au；　representing　the　distance　fromproperly　called　the　opik…oort　cloud；　it　is　named　for　the　estonian　astronomer　ernst　opik；　who　hypothesized　itsexistence　in　1932；　and　for　the　dutch　astronomer　jan　oort；　who　refined　the　calculations　eighteen　years　later。

the　sun　to　the　earth。　pluto　is　about　forty　aus　from　us；　the　heart　of　the　oort　cloud　about　fiftythousand。　in　a　word；　it　is　remote。

but　let’s　pretend　again　that　we　have　made　it　to　the　oort　cloud。　the　first　thing　you　mightnotice　is　how　very　peaceful　it　is　out　here