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Chapter 5 Asteroids（第1页）

Chapter5　Asteroids

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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）.