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Chapter6 Traus
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&ivelysparsepopulationofasteroids，kaurs，existsbetweenJupiteraune.Somearedarkandred，similartothetarry（tholiypeasteroids，butothersarebluer，suggestingthatmuchoftheirsurfacesmaybefreshlyexposedice.Becausetheirorbitseclosetothegiaheyareingferthanabouttenmillionyears.Probably，taursareTNOsthathavebeenscatteredinerhapsbyaterwithheriionslasprobablyheminwardsuntiltheybeeshort-periodets，spendiheinnerSolarSystem，wheretheyareheatedbytheSuheirvolatilesiaculartails.
Sixtrojashavebeendiscoveredeptune’sleadingLagrangianpoint.Dynamicalargumevastdisbianpoints）arojaimesmorehanJupiter’s.
&uheKuiperbeltaNOs.OnefamilyofKuiperbeltobjectstravelin3:2orbitalresohuheclass，whicludesPluto，areknolutinos’，nottobefusedwithPlutoid，whichistheofficialIAUtermforanyToberankedasadlutoidsbeplutinos，classicalKuiperbeltobjects（lagorbitalresohune），orScatteredDiskobjeai.ClassicalKuiperbeltobjeowivelyas‘os’（‘QB1-os’）becausethefirstKuiperbeltobjecttobediscoveredafterPlutoboretheprovisioion1992QB1.
Plutoand
&iesofmostTNOsarepoorlykoanditssatellitearesuffitlylargeaohavebeeelescopicallyforseveraldecades.Frehane，anddioxidehavebeeroscopiPluto，atelesagesrevealdarkpatchesthatareprobablytholin-richresidues.Pluto’sdensitysuggeststhatrockmustbeabout70%ofitstotalmass，andmostlikelyitisinternallydiffereharockydfeasiblyaniron-riercore）overlainbyamalyofwater-icetoppedbyamorevolatile-richcrust.
Nearperihelion（ere1989），Plutohasamospherepossiblyderiton’s.BecausePluto’sgravityissoweak，animaginaryshelleng99%ofitsatmospherewouldextendto300kilometresabovethesurface，whereasfortheEarththeequivaleisonly40kilometres.MuchofPluto’satmosphereisexpectedtoseontothesurfacewhiledistaheSunincreasesfrom4.5billioperihelionto7.4billioaphelionin2113.Itisapitythatwemissedthecetostudy
Plutofredurihefly-pastbyNASA’sNewHorizonmissiontoPlutowillo2015，bywhichtimemuospheremayhavesedahe‘perma’surfaeathaseasonalshroudeo’s6.4-dayrotatiohesameastheorbitalperiodestsatellite，，whichalsorotatessynously.Thisrelationshipisaresulttides，aPlutoandahesamefacestowardseachother.Plutoismoreevenlymatsizeandmasstothaordlasestsatellite.assisabout12%ofPluto’s，anditorbitsatadistanlyabout17PlutoradiifromPluto’sparison，theMoon’smassisonly1.2%ofEarth’sanditsorbitalradiusis60Earthradii.’sproximitytoPlutoexplainswhyitremaiil1978.Pluto’stwosmallersatellites，NixandHydra，weredis2005.TheseorbitinPluto’sorbitalplaneio4:1and6:1orbitalresoh.
SeenfromPluto’ssurfawouldlookeighttimeswiderthantheMoondoesfromEarth.Becausetheirrelativemassesaresosimilar，theiroreofmass（their‘barytre’）isobutatapointihetwobodies.Althoughdoubleasteroidssuchas（90）AntiopeanddoubleKuiperbeltobjectssuchas2001QW332（200-kilometrediametertwins）areknown，Pluto-isthemostevenlymatchedpairamongbhtotasplasord>
’ssurfaier-icewithtraonia.ItsdensityislessthanPluto，butstillsuffitforasubstantialrockycore.ayturnouttobearelativelypassive，heavilycrateredglobe，lutomightimpressusallbybeinggeologicallyactive，asthevarietyinitssurfacematerialssuggests.
Oheremaybeareasonfororeaitslarger.ThisisbecausePluto’saxialinis119.6°（beihan90°，thismeansitsrotatirade）.’sorbitisexaPluto’sequatorialplane，sosharesthehighinrelativetotheirjointorbitabouttheSuidalpullsontheSunandPlutomightfeasiblybestroocausemeltihinahisisthewearefacedwiththeintriguiofaEuropa-likesurfacefordeveiallylife-bearihit.Thebesthintwehavesofariraofobtainedin2007whidwater-ithatisstilliallineform，intrasttotheamorphoussubmicroscopicstateoficethathasbeeosolarultravioletradiationandientformorethahousahesimplestexplanationforthisisgeysers，sprayingoutfreshitheinterior，liketheplumesonEnceladus.
&
&sPlutoaotherTNOsasrahetime.Ofthese，Eris，Makemake，andHaumeaareofficiallyreizedasdlaerisflatteherbecauseofitsrapidrotatiohan4hfrom.TheseareclassicalKuiperbeltobjects，exceptErisand2007OR10（ScatteredDiskobjects），2002TC302（5:2orbitalresohune），Ixion（plutino），andSedna，whiodditywaybeyoeredDisk，inahighlyellipticalorbithelionat975AU.
&helargesttraus
ApartfromPluto，thesizesoftheseobjectsarepoorlykhoseforwhigleroundnumberisgiveheirdimeesbasedonassumptionsabouttheiralbedeoftheisunlighttheyreflect）.Iftheyarelessrefleassumed，theymustbelarger，butiftheyaremorereflective，thebesmaller.Sizeestimatesbeimprovedbymeasuringthermalradiationfromtheirsurfaces，buttheyaresocold（–230°Corless）thatthisbealybytelesspace，abovetheEarth’satmiveies，itisualloftheseobjectswillsurviveien’lists.
TNeinred（probablywidespreadtholinsacrosstheirsurfaces）toblue-grey（exposediorphouseaisoheblue-greyosmass（derivedfromtheorbitsofitssatellites）showsthatitsdensityisgreaterthanPluto’s，soitmusthavearelativelyhigh.OnQuaoar，eidammoniahydratehavebeeroscopically，suggestiresurfagargumentssimilartothoseadvancedfor）.Thiswouldrequireeithergeologicalaajorimpattogeasuffitlywidespreadtodomiru
&ween2%and3%ofTohavesatellites，whichissimilartotheabueroidswithsatellites.TheproportiohelargerTNOsandposesproblemsintryingtoatfin.
IfNASA’sNewHorizonmissionremaierits2015fly-byofPluto-，itwillbedirewardstoamoredistantThasyettobedetermiideallywillbeablue-greyobjetrastwithPluto’sreddishnature.
Atrau?
MostastrothatwehavediscoveredallthelargeobjegingtoourSolarSystely，thereothingofplaarysizehidingintheKuiperbelt.IfsuobjectwerepreseheKuiperbeltwouldbeunstable.However，thereremaintwopossibilitiesforanoutlyi（popularlydubbed‘Plahathaveegoneaway.Othereisah-massobjeindetricorbitbetween80and170AUfromtheSun.Thepresenceofsuchalargebiwardsbyaterwithune）couldexplainanobservedsuddendrop-offiionoftheKuiperbeltbeyond48AU，knownasthe‘Kuipercliff’.Itmightalsoatfortheextremesgevidencedbyobjectssua.
Thesedpossibilityarisesbeg-periodetsiiallyfromaparticularregioherthaninginfromraions.IthasbeeheseweredislodgedfromtheOortCloudbyaJupiter-massbodyabout32，000AUfromtheSun.Thiswouldbehard，butnotimpossible，todetectbytelescope.A‘pla’sofaroutbegravitationallyboucouldbejustadererthroughiellarspace，possiblyestheplaemofaar.
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