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Chapter 5 Asteroids（第1页）

Chapter5　Asteroids

Nobookaboutplaswouldbepletewithoutadisofasteroids，becausethesearethemostoohitplaheinnerSolarSystem（whereasteroidimpactsareabouttentimesmoreonthas）。Inadditioasteroid，Ceres，isofficiallyclassifiedasad>

Shapes，sizes，andpositions

&hegoalforNASA's　Daw，whichwillspehsitin2015，havitheyearbeginninginJuly2011atVesta，theseassiveasteroid。

Afewsmallerasteroidshavealreadybeenvisitedbyspaceages（Figure　25）thattheirirregularshapes。Visualizeapock-markedpotatoscaleduptoaeresandafewhures，andyoushouldhaveaservitalimageofatypicalasteroid。Telescopicallyobservedperiodicvariationsinasteroids&#hthatmostlytheytakeoenerally，rotatilestth，sotheyrotatelikesausagestwirledonacocktailstick。

About1asteroidin50probablyhasitsowe，anditwasluckythatIda，thesedasteroidtobevisitedbyaspaceGalileoflewpastin1993，turobeowasthefirsteddiscoveryofaellite，butsubsequentlymanymorehavebeenfoundusielescopiiques，suchasadaptiveoptipeheshimmerih’satmosphere。Asteroidsatellitesraheparativelytinyuptosizessimilartothemainasteroid。Infact，theasteroidiopeappearstoutuallybodiesofindistinguishable110-kilometresize，whosetresareonlyabout170kilometresapart。Sofar，therearetwoasteroidsknowwosmallsatelliteseaeasteroidsatellitesmaybefragmentsfroma，andothersmaybecapturedobjeeithercaseisreadilyexplicable，becauseitishardtoendupwithratherthanflyingapart。

25。Imagesofasteroidsatdiffereop:Ida，a54-kilmaieroid，withitstieDactyltht。Lowerleft:Eros，a33-kilerht:Itokawa，a0。5-kilEarth-gasteroid。TherearemanyimpactcratersvisibleonIdaathemuchsmallerItokawaisboulder-strewn

Asteroiddensitieshavebeeween1。2and3。0gcm3。However，stoes，whichareclearlybitsofasteroid，havedensitiesofabout3。5gdstoeshavedeo5。0gcm3，sohemeasuredasteroidsintactsolidbody。Rather，theymustbeporousrubblepiles。Some，suchasItokawa，visitedbytheJapaneseprobeHayabusain2005（Figure25），andothersehasbeeerminedbyradar，appeartobe‘taaries’goftwomainmassesjoianarroever，thenumerousbouldersonmuchofItokawa’ssurfaceshowthatthetwomaihemselvesanypieces。

Asteroidsarenlycoloured，butbegroupedintoseveralclassesagtotheirreflecespectrum。Therearethreemaiypeshavethecharacteristicsofsilicaterodareevideerialasstoheymakeupthemajorityofasteroidswithorbitsbetweenabout2。0and2。6AUfromtheSun，whereasfrom2。6to3。4AU，C-types，havieristicsofaeteorites，arethemoston。Beyond3。4AU，asteroidstendtobedarkaredincolour。ThesearedubbedD-type，andmaybecolouredbyatarrysurfaceresidueformedfromaterialduringproloosolarradiatioheriarrysubstancesareusuallyreferredtoas‘tholins’，atermtheaGreekwordfor‘mud’bytheAmerier（1934–96）。

Scatteredhereaeroidsthatappearlargelymetallic（M-type），clearlyrelatedtoiroes，aappeartohavebasaltontheirsurfaotablyVesta，fromwhichtheytaketheirdesignatiohese，mebody，mayhaveoenoughforiingandvolis。

Asteroidorbits

Mostknownasteroids（equivalenttoabout4%oftheMoon’smass）haveorbitslyiheorbitsofMarsaheso-called‘asteroidbelt’。Over3，000maieroidshavebeeed。Morethaalmassoftheseresidesinthefestexamples，Ceres，Vesta，Pallas，ahdiametersof950，530，540，and430kilometresrespectively（VestaisdehanPallas，soismoremassivedespitebeingslightlysmaller）。Usrangedowhroughindividuallumpsofrocktodustpartievertheless，theasteroidbeltisvirtuallyemptyspadyoushouldnotthinkofitasrepletewithjostlingrocks。Allspaceprobesthathavebeehroughtheasteroidbelthavesurvivedwithoutmishap，aobesteeredcarefullytoeoughtoaostudyitinpassing。

Jupiter’sgravityhassiderableinfluenaieroidorbits。Notably，itprevetlingintoorbitswhoseperiodswouldbeinresosowuallynoasteroidswhoseorbitalperiodsaresimple4:1，3:1，5:2，or2:1ratiosofJupiter’s。ThesedtedistaheSun（orbitalsemi-majoraxes）of2。06，2。50，2。82，3。28AU，respectively，whiownastheKirks，afterDanielKirkwood，aronomerwhodisdexplaihemin1886。Notallorbitalresoablewithrespecttoasteroidorbits，andinfactthereisasmallfamilyofasteroidswhoseorbitalperiodsaretwo-thirdsthatofJupiter（a3:2orbitalresonance）。

ThereareagreatmaeroidswiththesameorbitalperiodasJupiter。Theremaybemorethanamillioerthareihaassaboutohatofthemaiheseolyclosetolos60°aheadof，and60°behisorbit。ThosearespecialplaceswherethebiatioheSunandJupiterallowssmallobjectstoorbitstably，andareknownastheleadingandtrailingLagrangianpoints。AsteroidsisarebygivennamesofheroesfromtheTrreeknames60°aheadofJupiterandTrojannames60°behind），butarecollectivelyreferredtoas‘Trojanasteroids’。

We'realldoomed!

&eroidsareknowninsimilar‘trojaionshiptoMars，butEarthhasnotrojanpaniohereareasteroidswhoseorbitscrossours，kh-gasteroids。Ifyouareworriedabouts，thismays，butasteroidorbitstendtobeiheecliptiostalasseither‘above’or‘below’uswhentheycrossourorbit。OnlyasubsetofEarth-crardedasPotentiallyHazardousAsteroids（PHAs），beipasswithin0。05AUoftheEarth（arangesuffitlyclosethatperturbationscausedbyvariousthirdbodiesgabouta）andthatarelargerthanabout150metresier（bigenoughtosurvivepassagethroughtheatmospherewithundimiheendof2009，about1，100PHAshadbeeed，plusfeotentiallyHazardousets。

&calculatedapproachbyaPHAisbyApophis（35）thatwilleveryFriday13April2029。Soosdis2004，itsorbitwassuffitlypoorlyknownthattherewasace（estimatedat2。7%）ofa，butsubsequentlyalongerseriesofobservatiowillpasssafelyabout30，000kilometresabovethesurface。Itwillbebaon13April2036，andbecausewedolyhowcloseitwillpassiklyhowmuchitstrajectorywillbeaffectedbytheEarth’sgravitydurier。However，thecesofin2036arevanishinglysmall。

Apemospherewithundiminishedspeedisverydangerous。Onhitticouldcauseatsunami，andifithitslaesacratermuchlargerthanitselfaesthesurroundingarea。A2。2-million-year-old，130-kilometreamedEltaninhasbeendisdertheflooroftheBellinghausehesouthernmostPacifi，apparentlycausedbyanasteroidseveralkilometresihiswouldhavebarelybeeheo，letalomosphere，beftheseabed。Agtoodels，theresultingtsunamiwouldhavedevastatedtheetresabovesea-levelinsouthernd60metresabovesea-levelihequantityofwateranddustthrowmhteveriggerforclimategeleadiionofouraors，Homeerectus，outofAfrica，ataboutthisdate。Themostretbetweehanda10-kilometre‘dinosaur-killer’asteroidhappened65millithe200-kilometre-diameterChicxulubowburiedbeianpeninsulaofMexico。Thiscausedaglobalealupheavalthatiswidelycreditedastheassexti’whenabout75%ofspeciesoflifeohwerewipedout。

Catastrophesofthatmagnitudearemercifullyrare，butstatisticsshowthatasteroidimpakalongsidevolis，earthquakes，ahereventsaspotentialcausesofdeath。A1-kilometreasteroidabletodevastateetresfromthepointofimpactstrikestheoaverageaboutevery200，000years，whereasa200-metreasteroidwithasignifitlysmallertsunamidangerradiusmightbeexpectedaboutevery10，000years。

TocategorizethehazardposedbyeachPHA，astronomersuseanumericalsystemcalledtheTorinoScale（agreedatameetinginTurihehisbihatwouldbedeliveredandtheprobabilityofintoasinglenumberfrom0to10，where0meansnegligibleceofandortoosmalltopemosphere，and10isimpactbya‘dinosaur-killer’gglobalostPHAsexg150metresierareraher0or1whentheyarediscovered，andthe1sareusuallydowo0whehasbeeelydetermined。ApophisholdstherectemporarilyheldaTashighas4（‘eritiionbyastronomers;〉1%reaterceofcapableioion’），butwasdowo0in2006。

Asemi-formallinkioriesknoaceguardhasassumedthetaskoflodcategallPHAs。Thisisimportantbelikemostsortsofnaturaldisasterwhenallweitigatetheeffects，itwouldbepossibletopreventabyaPHA。Toachievethis，itisogeeitherthePHA’sspeedoritsdireoftravel。Thelongerihisisdohesmallertherequiredge。Therearevariouswaystfromthebrute-forcemethodofequippiharocketmotor，tothemoresubtleployofgofonesideiivesubstasolarradiatiohejobforyou。UsioblastapartaniningPHAisnotasmartidea，belessyoucouldguaraallthefragmentswouldbetoosmalltopemosphere，youmightmaketheproblemworsebygmultipleimpacts。

Asteroidmining

Thereisasilverlining，inthatasteroidscouldbevaluablesouraterials。A1-kilometreM-typeasteroidorenidironthantheworld’sannualptiomassiveexample，Psytaiolastformillioeroids，especiallyM-types，alsopreetalslikeplatinum。

&menttobeginmiasteroidwouldbeverygreat，butthepoteurnsareimmeremainstobeseehemaieroidsturnsouttobesupplyofrawmaterialstoEarthortospace-basedindustries。Somehobjectsareprobablydefuhremnantwater-icesurviviheirdustysurfaces，whichcouldbevaluableaspropellantandradiationshielding，aswellasf。

Namesandprovisioions

By1891，332asteroidshadbeendiscoveredvisually，butphotographyhadboostedthetallyto464withiherearenowover100，000ksofalltypes，eachofwhieedstobeidentifiediheIAUoverseesasystemofprovisioiooeaewdiscovery。Thissistsoftheyearofdiscoveryplusatwo-lettercodecoupledwithnumericalsubscripts，dieandsequenceofdiscovery。Thefirstletter（A–Y，omittingI）specifieswhichhalf-monththediscoverywasmadein（AforJanuary1–15，BforJanuary16–31，andsoon，uptoYfor16–31December），thesedletter（A–Z，omittingI，whichgives25options）isawardedtoeachdissequendanumberedsubscriptallowsthecycleof25toberepeatedasmanytimesashus，2011BAwouldbethefirstbodydistheperiodJanuary16–312011;2011BBwouldbethe2nd;2011BA1wouldbethe26th，andsoon。Whe’sorbithasbeeermined（whichmaytakeseveralyears），itbeaermaname，whichreplacestheprovisioion。Forexample，Apinallyhadtheprovisioion2004MN4（signifyihdisc16–30June2004）。

Theprivilegeofsuggestinameisgiventothedis，thoughsomeautomatedsurveysrevealsomasthatmanagersaregladofsuggestionamesareanameprecededbyanumber，addedinsequenceaseaameisadded。Soformallywehave（1）Ceres，（4）Vesta，（99942）Apophis，andsoon。Availablemythologiamesaretoofewforalltheseobjedprettymugisallowed，exceptthatbeinoffensiveahretpolitiilitaryactivity。Ikronomerswhohavehadasteroidshem（bycolleagues;youameoeryourself），andthereisosenaat’a’i，whis‘flyingrotheNavaholaheoIhavehadahandinnamingis（57424）edin2007tooratethe50thanniversaryofBBCtelevision’slprogrammeTheSkyatNight，whiLatiu。Wepickeditfromalistbecauseitshedateofthefirstbroadcast，ril24（57424）。