1987AJ 93. . 738T b) a) THE ASTRONOMICALJOURNAL et al1979;WetherillandWilliams1968;1976). ly collidewiththeEarth(Öpik1951,1963,1976;Shoemaker ejected fromthesolarsystem;asignificantfractioneventual- objects insuchorbitsistocollidewithamajorplanetorbe million toafewhundredyears.Theultimatefateof short comparedtotheageofsolarsystem,i.e.,afew an occasionalcomet.MostAten,Apollo,andAmoraster- to 1.3AUarecalledAmorasteroids(cf.Shoemakeretal helion distancegreaterthan1.017AUbutlessorequal cross theEarth’sorbitneartheirapheliaarereferredtoas ger responsibleformassbiologicalextinctions(Alvarezet the 10kmsizerangehavebeenpostulatedasbeingtrig- oids areinorbitsthatstableforperiodsoftime closely thananyothercelestialobject,barringtheMoonor to 1.017AUarecalledApolloasteroids,andthosewithperi- equal to1AUandwithperiheliondistancelessthanor Aten asteroids,thosewithsemimajoraxesgreaterthanor ments. Thosewithsemimajoraxeslessthan1AUandwhich groups onthebasisoftheirpresentosculatingorbitalele- proach theEarth’sorbit(Shoemakeretal.\919).Theaster- larger thanafewhundredmeterswhichcrossorcloselyap- Astronomy Observatories,operatedby AURA,Inc.,undercontractwith by theUniversityofHawaiiundercontract totheNationalAeronauticsand Collisions betweentheEarthandobjectshavingdiametersin oids comprisingthispopulationaredividedintothree the NationalScienceFoundation. VisitingAstronomer,KittPeakNational Observatory,NationalOptical Space Administration. VisitingAstronomerattheInfraredTelescope Facility,whichisoperated 738 Astron.J.93 (3),March1987 0004-6256/87/030738-09$00.90 © 1987Am. Astron. Soc.738 1979 ).AsteroidsinsuchorbitscanapproachtheEarthmore There areanestimated1500asteroidswithdiameters © American Astronomical Society • Provided by the NASA Astrophysics Data System DISCOVERY OFMCLASSOBJECTSAMONGTHENEAR-EARTHASTEROIDPOPULATION meteorites derivedfromthesenear-Earthasteroidsshouldhavelowcosmic-rayexposureages. approaching asteroidsforwhichsuchclassesexistissimilartothoseregionsofthemainbeltbetween this population.Thesimilarityinthecorrecteddistributionoftaxonomicclassesamong38Earth- objects, establishesthatallthreeofthemostcommonmain-beltasteroidclassesarerepresentedamong near-Earth asteroidpopulation.Thisdiscovery,togetherwithpreviousdetectionsofCandSclass that bothobjectsbelongtotheMtaxonomicclass,firstofthiskindberecognizedamong fluxes. Thesealbedos,togetherwiththecolorimetricand(for1986DA)near-infrareddata,establish mal modelsystemdescribedbyLebofskyetal.1986)werederivedfromthethermalinfraredandvisual model radiometricdiametersof2.3+0.1and2.0km,respectively,(ontheIRASasteroidther- March andApril1986.Modelradiometricvisualgeometricalbedosof0.14+0.020.19 Broadband colorimetry(0.36to0.85¡um),visualphotometry,near-infrared(JHK)and objects aloneshouldproduceabout1%ofthemeteoritefalls,i.e.,halfironmeteorites.Iron production basedonstrength-relatedcollisionalprocessesasteroidalsurfacespredictthatthesetwo origins amongasteroidsinthevicinityoftheseresonances.Theimpliedminerology1986DAand the 3:1(2.50AU)and5:2(2.82orbitalresonanceswithJupiter,suggestingthattheyhavetheir 10 and20/urnradiometryofthenear-Earthasteroids1986DAEBwereobtainedduring 1986 EBismostlynickel-ironmetal.Ifthisindeedthecase,thencurrentmodelsformeteorite Planetary GeosciencesDivision,HawaiiInstituteofGeophysics,UniversityHawaii,Honolulu,96822 I. INTRODUCTION a) AnOverview Jet PropulsionLaboratory,CaliforniaInstituteofTechnology,Pasadena,91109 Received 10September1986;revised17November1986 a),b) Edward F.TEDESco 20 Jonathan Gradie VOLUME 93,NUMBER3 ABSTRACT ber ofdiversereasons:theyrepresentagroupobjectsfrom al. 1979). al. 1980).Forthesereasonswewillrefertoasteroidsin the nextcenturyasmankindestablishesapermanentpres- tem. with theEarthinpastandwilldosoagainfuture, which atleastsomeofthemeteoritesinourmuseumsare clude regionsfromwithintheasteroidbeltaswell,particu- replenishes thispopulationonatimescalecommensurate lived, near-Earthorbitsrequiresthattherebeasourcewhich Aten, Apollo,orAmororbitsas“near-Earth”“Earth- and areamongthemostaccessibleobjectsinoursolarsys- derived, mayharborextinctcometarynuclei,havecollided larly regionsnearthevandsecularresonances tile) cometarynuclei.Later,thissourcewasrevisedtoin- the evolutionofshort-periodcometsintodefunct(nonvola- with thoselifetimes.Öpik(1951)believedthatsourcetobe approaching” asteroids. metals, andperhapsevenhydrocarbons.Thefactthatthe oids areknowntocontainsignificantquantitiesofwater, ence inspace.Thereasonforthisliesthefactthataster- number ofcases,inorbitswhichrequirelessenergytoreach ters betweenafewhundredmetersand10km,and,in Earth-approaching asteroidsaresmall,mosthavingdiame- 3:1 and5:2commensurabilitieswithJupiter(Shoemakeret than theMoon,makesthem mosteconomicalsourceof humously, published(Herrick 1979).Sincethattime,the resources forbuildinglarge-scale structuresinspace. matter hasbeengivenconsiderable study(cf.Morrisonand Samuel Herrickin1971.Although theideawasconsidered “premature” atthetime,hiswork waseventually,andpost- 56 This lastfactwillbecomeincreasinglymoreimportantin The factthatthereareoverathousandasteroidsinshort- Near-Earth asteroidsarethereforeinterestingforanum- The firstserioussuggestionalong theselineswasmadeby MARCH 1987 1987AJ 93. . 738T a lar SystemExplorationCommitteeoftheNASAAdvisory Through Year2000:AnAugmentedProgram”inwhich, NiehofF 1979,andreferencestherein)culminatingintheSo- Council’s 1986reportentitled:“PlanetaryExploration '’IRTF—NASA’s InfraredTelescopeFacilityattheMaunaKeaObservatory;KPNO—KittPeakNational1.3mtelescope. /zm) ontheJohnsonUBVRsystem;UBVR—visualphotometry(at0.36,0.44,0.55,0.70/zmsystem. 05 April1986DAandEBEGAS reliable topermittheirroutinerecovery(Hahn1986).This the firsthalfofthiscenturyareeffectivelylostsincetheir days toseveralmonthsbuttypicallylastingafewweeks. is aconsequenceofthefactthatobservingwindowsfor received permanentnumbers,i.e.,haveelementssufficiently asteroids hadbeendiscoveredthrough1985,only47 proaching orbits,eachapproximately2kmindiameter.In ECAS—visual photometry(at0.36,0.44,0.55,0.70,and0.85(xm)ontheEight-ColorAsteroidSystem;JHK—near-infrared1.25,1.65, 24 April1986DAJHK 22 April1986DARadiometry 20 March1986DAandEBUBVR orbital elements(a=2.811 AU,#=1.166AU; member oftheAmorgroupnear-Earth asteroidssinceits tory, respectively(IAUCire.4181 and4191).1986DAisa E. ShoemakerandC. atthePalomarObserva- Thus itwasinthecaseoftwoasteroidsforwhichnew omers studynonperiodiccomets—astargetsofopportunity. reason theyarestudiedinmuchthesamewayasastron- recovered, evenbywide-fieldSchmidttelescopes.Forthis orbital elementsaretoopoorlyknowntoallowthembe Hence, manyofthenear-Earthasteroidsdiscoveredduring asteroids inorbitsofthistypeareshort,rangingfromafew and poorlyknownorbits.Forexample,although88such strained bythegenerallylongtimebetweencloseapproaches and 5:2commensurabilitieswithJupiter. of taxonomicclassesamongthenear-Earthasteroidsiscon- and 2.2/zm)ontheJHKsystem;Radiometry—radiometry(at1020ytzm)AandQsystems;UBVR—visualphotometry(0.36,0.45,0.55,0.70 data arepresentedhere. sistent withtheirorigininthemainasteroidbeltnear3:1 addition, wewilldemonstratethattheinferreddistribution phasis ours).Inthispaperwewillpresentevidenceforthe worth morethanSItrillionatcurrentmarketprices”(em- existence ofnotone,buttwo,suchasteroidsinEarth-ap- oid onekilometerindiameter{ifsuchathingexists)couldbe potential sourcesoflargeamountsmetals.Ametalaster- num groupmetals(0.001to0.01percent).Suchobjectsare trace amountsofotherelements,includinggoldandplati- nickel (fivetotenpercent)andcobalt(0.6percent),with They arepredominantlyiron(about90percentbyweight), using ironmeteoritesasanexample,itstates(p.159)that, a discussionontheeconomicvalueofasteroidalmaterials “Iron meteoritesareactuallychunksofiron-nickel-alloy. 13 March1986DAandEBRadiometry ( 1986)Objects(s) 17 March1986EBRadiometry 12 March1986EBRadiometry 1986 byM.Kizawa,Shizuoka, Japan, and4March1986by 739 E.F.TEDESCOANDJ.GRADIE:MCLASSNEAR-EARTHASTEROIDS UT Date The physicalstudyofEarth-approachingasteroidsiscon- © American Astronomical Society • Provided by the NASA Astrophysics Data System 1986 DAandEBwerediscovered on16February b) ThePresentStudy 3 Technique Table I.Logoftheobservations. 3 Telescope/ Observatory KPNO KPNO IRTF IRTF IRTF IRTF IRTF not theEarth,while1986EBisamemberofAtengroup MPC* 10628,1986)causeittocrosstheorbitofMarsbut Earth (and,incidently,Venus)eventhoughitssemimajor since itsorbitalelements{a=0.974AU,Q1.247AU; be transformedintozv—wcolorindexprovided,aswasthe color indicesontheECASandUBVsystemsinTableIII.A nights onwhichthesedatawereacquiredassignedto lowed byadiscussionoftheseresultsinSec.IVandsum- axis islessthanunity. MPC 10625,1986)aresuchthatitcrossestheorbitof Tedesco etal.(1982).TheJohnsonRband(0.70//m; tems isgiveninTableIV. summary ofthemeancolorsforbothasteroidsinsys- magnitude observationsarepresentedinTableIIandthe Pluto washighenoughintheskytoobserve.Thevisual- events. Theobservationsreportedhereweremadebefore obtain visuallightcurvesofPluto-Charonmutualeclipse dier/a/. 1978;Tedescoeitf/.1982)on5April1986UT.The of theEight-ColorAsteroidSurvey(ECAS)system(Gra- photometric systemon20March1986UTusingtheKitt mary inSec.V. two asteroids.WegivethederivedresultsinSec.Ill,fol- Peak NationalObservatory1.3mreflectorandinfivebands ECAS ubvcolorindices. 20 March.Weusedtheformulas giveninTedescoetal. III wereportthev—wcolorindexforobservationsfrom served throughtheV=v,w,andRfilters.Hence,inTable case, thatasufficientnumberofECASstandardswereob- FWHM 0.22¿¿m)issimilarenoughtotheESASwband infrared, andthermalinfrared.InTableIwepresentalogof the observationsfollowedbyresultstherefrom. Planet Center,Commission20ofthe International AstronomicalUnion, Smithsonian AstrophysicalObservatory, Cambridge,MA. *MPC—Minor PlanetCirculars.Amonthly circularissuedbytheMinor 1986 EBwereobtainedinthreespectralregions:visible,near (1982) fortransformingbetween theJohnsonUBVand (0.70yum; FWHM0.06/¿m)toallow2lV—Rcolorindex In Sec.IIwepresenttheobservationsobtainedonthese The standardstarsusedforthesenightsweretakenfrom Photometric observationswereobtainedontheUBVR Ground-based physicalobservationsof1986DAand a) VisibleWavelengthObservations II. OBSERVATIONS J. GradieandE.Tedesco J. GradieandE.Tedesco J. GradieandE.Tedesco R. NelsonandE.Tedesco E. Tedesco J. GradieandE.Tedesco M. BuieandE.Tedesco Observers 739 1987AJ 93. . 738T uncertainties determinedorestimatedforeachnight. uncertainties includebothphotometricandtransformation have notbeencorrectedtomonochromaticmagnitudes.The listed inTableVIaregivenmagnitudesonthatsystemand tometric systemdefinedbyRiekeetal(1985).Thevalues following, theavailabilityofcometanditsstandardstars. and W.Golish.Ourobservationsweremadepriorto, dard-star observationsmadeduringthatnightbyD.Griep International HalleyWatchprogramandutilizedthestan- vations weremadeontelescopetime“borrowed”fromthe These resultsaresummarizedinTableVI.TheAprilobser- of faintandunusualasteroids(GradieTedesco1987). three nightsinMarchaspartofanextensiveongoingsurvey DA andintheirtransformationontothestandardsystem. certainties reflectthescatterinmeasurementsof1986 ondary standardontheEliasetalsystem.Thequotedun- numerous occasionsinthepastandwhichweuseasasec- observed thesolar-likestarSAO120107,ameasuredon for extinctioncoefficients.Asaconsistencycheckwealso ble tomeasureanetworkofstandardsoveranappreciable Elias etal(1982)JHKstandards(HD77281andHD vations weremadedifferentiallywithrespecttotwonearby range ofairmasses,asisourcustomarypractice,theseobser- were madebeforePlutohadrisen.Becauseitwasnotpossi- measurement ofaPluto-Charonmutualeclipseeventand 1986 DAandEBwereobtainedattheNASAIRTFon one hourofanightonwhichtheprimaryprogramwas marized inTableV.Theseobservationsweremadeduring April 1986UT.Theresultsoftheseobservationsaresum- at theNASAInfraredTelescopeFacility(IRTF)on24 106965), andnominalvalues(0.1mag/airmass)wereused 740 E.F.TEDESCOANDJ.GRADIE:MCLASSNEAR-EARTHASTEROIDS The 10and20//mobservationswerereducedonthepho- Ten- andtwenty-micrometerradiometricobservationsof JHKphotometric observationsof1986DAwereobtained © American Astronomical Society • Provided by the NASA Astrophysics Data System 1986 DA 1986 EB Object 1986 DA 1986 EB Object c) 10and20¡imThermalRadiometry b) Near-Infrared(JHK)Photometry 860405.188 860320.189 860405.181 860320.219 UT Date 860405.218 860405.158 860320.189 860320.219 860405.181 UT date 0.04 ±0.02 0.09 ±0.04 0.03 ±0.02 0.07 ±0.04 Table II.Visualphotometryof1986DAandEB. Table III.Colorimetryof1986DAandEB. (mag) ±0.02 15.57 15.50 14.48 14.17 14.29 V 0.02 ± 0.05 ±0.03 0.03 ±0.02 0.04 ±0.03 from Sun Distance (AU) 1.245 1.246 1.245 1.166 1.187 jects. Thealbedosanddiametersreportedherewerederived Infrared AstronomicalSatellite{IRAS).Thismodelissimi- body? lar tothestandard(TRIADsystem)modeldescribedby Morrison andLebofsky(1979),exceptthattheabsolutevi- for thereductionofasteroidobservationsobtainedby they haveformedcoevallyordoshareacommonparent properties suchastheir0.36to0.85fimcolorindices(Table using thethermalmodel(Lebofskyetal1986)developed absolute visualmagnitudesaswellothersimilarphysical albedos (p)andmodelradiometricdiametersforbothob- photometry tocomputemodelradiometricgeometricvisual IV) andtheiralbedosdiameters(TableVII).Could EB. Itiscuriousthattheseobjectshaveformallyidentical sults invaluesof#of15.94for1986DAand such valuesdeterminedbyTedesco(1986). for Gofbothobjectsinkeepingwiththeschemeadopting value ofGisadopted.Inthiscaseweadopteda0.25 observations spanningawiderangeinphaseangle),then and EB,thereisinsufficientdatatodetermineG(i.e.,several parameter.” If,asistruewithourobservationsof1986DA tion: H,theabsolutevisualmagnitude,andG,“slope magnitudes usedinreducingtheIRASasteroiddata(cf. MPC 10193).Thissystemusesatwo-parameterphasefunc- employed byTedesco(1986)incomputingtheabsolute less bodiesderivedbyBowelletal(1987)andwasthat tem isbasedontheempiricalphaserelationforatmosphere- Astronomical Union(IAU)inNovember1985.Thissys- tude systemadoptedbyCommission20oftheInternational 0.07 ±0.02 v 0.06 ±0.02 0.07 ±0.02 0.09 ±0.01 Applying thisprocedureusingthedatafromTableIIre- The thermalobservationswerecombinedwiththevisual The Vobservationswerereducedtotheabsolute-magni- from Earth Distance 0.344 (AU) 0.344 0.273 0.208 0.230 0.15 ±0.02 0.14 ±0.02 b) AlbedosandDiameters a) AbsoluteMagnitudes III. DERIVEDRESULTS Phase (deg) 21.01 angle 38.79 38.73 33.98 30.65 0.20 ±0.02 0.22- 0.25- 0.24 ±0.04 U-B ; 0.02 0.03 0.69 ±0.02 0.72 ±0.02 0.70 ±0.02 0.71 ±0.03 F(l,«) (mag) ±0.02 B- V 17.41 16.82 17.11 17.34 17.25 740 1987AJ 93. . 738T p arethreeobservablepropertiesthatcanbeusedtocon- 741 E.F.TEDESCOANDJ.GRADIE:MCLASSNEAR-EARTHASTEROIDS al.). Also,thecorrectiontomonochromaticmagnitudeis formulation ofthestandardmodeltoaccountfornu- diameters observedforCeresandPallaswereusedtorevise sual magnitudeHandslopeparameterGhavereplacedthe reason theIRASsystemalbedosreportedinTableVIIcan- merical changeintheabsolutemagnitude.Oneeffectofthis now anintegralpartofthemodel. the adoptedvalueforbeamingfactor(cf.Lebofskyet reformulation hasbeenachangeinthealbedoscale.Forthis tem. not bedirectlycomparedwithalbedosfromtheTRIADsys- Bowell etal.(1978),Zellner(1979),andGradie v nomic systemsfortheasteroids.Thetaxonomicsystemof Tedesco (1982)haveusedthesepropertiestodefinetaxo- strain thecompositionalcharacteristicsofasteroids. the TRIADtaxonomicsystem(asextendedbyGradieand TRIAD) aregiveninTableVII.Becausewewillbeusing TRIAD system.Thealbedosonbothsystems{IRASand mean valueforMclassasteroidsfoundbyBowelland fore derivedF(1,0)absolutemagnitudesfor1986DAand the 33near-Earthasteroidsforwhichthesedataareavail- herein willbeontheTRIADsystemunlessnotedotherwise. Tedesco 1982)throughoutthispaper,albedosreferredto Lumme 1979)andusedthemtocomputealbedosonthe since thatwasthescaleemployedinthesesystems.Wethere- tems notedabove,theTRIADalbedoscalemustbeused gradations throughtheasteroidbelt.Inthissystem,nearly Gradie andTedescohasbeenusedtostudycompositional major classes,C,S,orM,showninFigs.1and2. 90% ofthe500classifiableasteroidsfellintoonethree F(1,0) usedinthe“standardmodel”andoccultation 1986 EB(usingphasecoefficientsof0.031mag/deg,the 1986 DAÖÖ4ÖÖ3008014021070860424.27312.86+0.030.460.020.60 1986 EB0.060.040.070.150.240.71 Object u—vbwxUBVUTdateJHK The redefinitionoftheabsolutemagnituderequiredare- Broadband UBVcolorsandthevisualgeometricalbedo Figure 1isaplotoftheU—B\sBVcolorindicesfor To classify1986DAandEBinthetaxonomicsys- © American Astronomical Society • Provided by the NASA Astrophysics Data System 1986 DA 1986 EB Object Table IV.Meancolorsof1986DAandEB. + 0.02 c) TaxonomicClassifications 860313.46 860317.48 860313.52 860312.47 860422.48 860422.26 UT date Table VI.10and20¡im(AQ)observationsof1986DAEB. from Sun Distance (AU) 1.240 1.238 1.237 1.180 1.179 1.205 from Earth Distance (AU) 0.257 0.246 0.262 0.257 0.211 0.211 an uncertainties inthecolorindicesisgivenlowerright- able (seeTableVIII).Thepositionsof1986DAand recognized amongtheEarth-approachingasteroidpopula- for thesecolorsnearzero.Thereisthereforenodoubtthat albedos (0.07albedo of1986DAaredistinctlythoseanMclassobject class. Also,thecombinationof/—ATcolorandgeometric demonstrate, confirmstheMclassification. Both ofthesetypesdataareavailableandeach,aswewill reflectivity atwavelengthsgreaterthanthatoftheVband. do informationandthesecondemploysdataonrelative this classificationisnotunambiguous.Thereareatleasttwo M regionsandtheuncertaintyincolorindices,however, of anMclassification.DuetotheoverlapbetweenCand ites (Zellner1979).RadarobservationsoftheMclassaster- statite-metal assemblageoftheenstatite-chondritemeteor- analogous tothemetallic(iron-nickel)meteoritesoren- of theMclassasteroidsareconsistentwithcompositions oid 16PsychebyOstroetal.(1985){U—V=0.96,p tions byOstroetal.(1985)ofoneotherMclassasteroid,97 containing amixtureofenstatiteandmetal.Radarobserva- nearly entirelymetallicincomposition,similargrosscom- Psyche, coupledwithsimilarcolorsandalbedosfor1986 trations, includingallmetal.Theradarobservationsof16 DA and1986EB,suggestthattheselatterobjectsarealso Klotho, areconsistentwithobjectshavingarangeofconcen- position totheironmeteorites.Wecanthereforestatewith 1986 DAandEBareMclassobjects,thefirstsuchtobe (0.84 <£/—F1.05)thatplacethemsecurelyintheM (cf. Veederetal.1982,1983).Finally,thev—wandx ( 1979),respectively,showaradarreflectanceindicativeof v = 0.094;Bowelletal.(1979)andMorrisonZellner The spectralreflectancepropertiesandgeometricalbedos As showninFig.2,both1986DAandEBhave Table V.JHKphotometryof1986DA. Phase (deg) angle 27.6 31.4 31.4 18.5 16.4 16.4 IV. DISCUSSION 4.23 ±0.04 4.26 +0.04 4.62 +0.04 3.18 +0.04 3.79 +0.04 3.55 +0.04 (mag) N 2.39 +0.05 2.71+0.09 2.41+0.06 2.03 +0.10 1.57 +0.12 1.79 +0.15 741 1987AJ 93. . 738T approaching asteroids.Thet/2?Fcolorsandalbedosusedto Earth asteroidpopulationusingclassificationsof38Earth- 742 E.F.TEDESCOANDJ.GRADIE:MCLASSNEAR-EARTHASTEROIDS cal calculations,Wisdom(1983,1985)suggestedthatthe3:1 Kirkwood gapwasapossiblesourceregionaswell. source bodiesinthemainbelt.Baseduponcelestialmechani- larities, notallEarth-approachingasteroidshadidentifiable also concludedthat,duetothelackofdetailedspectralsimi- from themainbelt,inparticular5:2Kirkwoodgap.They main-belt asteroidsarguedthatasizablefractionmustcome cluded thattheclosesimilaritybetweenspectral(0.35to Wetherill 1979),arerequired.McFaddenetal.(1985)con- as asteroidsindynamicallyunstableregionsofthebelt,e.g., due toperturbationsbytheterrestrialplanets,sourcessuch Earth asteroidpopulation. reasonable confidencethatironsarefoundamongthenear- Kirkwood gaps(cf.Wetherill1985),orextinctcomets 1986 DA15.940250142T16.180.0310A2 1.1 ¡am)propertiesofsevennear-Earthasteroidsandsome 1986 EB15.940.250.192.016.170.0310.17 Object HGp(km)F(1,0)ß v dices areindicatedbytheerrorbarin lowerright-handcorner. are fromBowelletal.(1978).Typical uncertaintiesinthecolorin- delineating theouterlimitsofC,S, andMtaxonomicclassfields Fig. 1.U—B\sB—Vfor33Earth-approaching asteroids.Boxes We nextexaminetheissueofsource(s)near- Since theorbitsofmostnear-Earthasteroidsareunstable Table VII.Meanalbedosanddiametersof1986DAEB. © American Astronomical Society • Provided by the NASA Astrophysics Data System eter ±0.02 ±0.1 IRAS Diam-TRIAD B-V tion bycomparingthatdistribution withthedistribution similar tothatfoundamongthe Earth-approachingpopula- asteroid belthadarelativedistribution oftaxonomicclasses classes as“other.” tance. Theonlydifferenceisthattheyassignedspecific This classificationschemeisidenticaltothatemployedby classification couldnotbeassignedwerecalled“Others.” umn. ThoseasteroidsforwhichanunambiguousC,S,orM which thosedatawerepublishedisgivenintheremarkscol- permanent number,theprovisionaldesignationunder sample, whereasherewesimply refertoallsuchnon-C-S-M classes (e.g.,D,E,F,P,R,or U) toeachasteroidintheir of thetaxonomicclassesasafunctionheliocentricdis- Gradie andTedesco(1982)intheirstudyofthedistribution data werepublishedbeforeanasteroidhadbeenassigneda was done,itisnotedintheremarkscolumn.Incaseswhere sulted andusedasthebasisforaclassification.Wherethis or ledtoambiguousclassifications,othersourceswerecon- are presentedinTableVIII.Wherethesedatawerelacking, assign theasteroidstoaclass,andclassadoptedforeach, the Mclass. DA and1986EBinthisparameterspaceplacesthemsquarelyamong objects. Similardataformain-beltasteroids(fromFig.1inZellner adopted classwasC,thesebeingthemeanalbedosforSandC plotted at/v=0.16iftheiradoptedclasswasSand0.037 which onlytheU—Visknown(lightcircles).Thelatterhavebeen circles) andfornineadditionalEarth-approachingasteroids proaching asteroidsforwhichsuchdataarecurrentlyavailable(dark Fig. 2.Visualgeometricalbedo(p)vsU—Vfor24Earth-ap- 1979) areplottedassmalldarkcircles.Notethatthelocationof1986 v We nextsoughttodeterminewhich, ifany,regionofthe 742 1987AJ 93. . 738T Asteroid 2368 Beltrovata 2100 Ra-Shalom 2062 Aten 2608 Seneca 2340 Hathor 2212 Hephaistos 2201 Oljato 2061 Anza 743 3199 Nefertiti 3102 1981QA 1566 Icarus A—Veeder cia/.(1984). 3288 Seleucus 3200 Phaethon 1580 Betulia 1036 Ganymed 6— B ell andBrown(1987). 2— cf.McFaddenetal.( 1985). 4— Thispaper. 1915 Quetzalcoatl 1866 Sisyphus 1862 Apollo 1685 Toro 1627 Ivar 1620 Geographos 5— L . LebofskyandE.Tedesco(unpublished). 3— B ell etal.(1987). 2— BowellandHarris,IAU Circ.No.3436. 1943 Anteros 1916 Boreas 1865 Cerberus 1864 Daedalus 1863 Antinous 3— Harris,IAUCirc.No. 3450. 1977 VA 1982 XB 1980 AA 1978 CA 1980 WF 1979 VA 1986 EB 1986 DA 1983 SA 1— Othermeansnotanunequivocal C,S,orM. 1— Tedesco(1986)(Acombination ofBowelletal.(1979)andUBVderivedcolorindicesfromZellner1985)). 1984 KB 10—This paper. 433 Eros 887 Alinda 9— L.LebofskyandE. Tedesco(unpublished). A—Lebofsky etal.(1978). 7— L.Lebofsky(personal communicationinMcFaddenetal.1985). 6— Lebofskyetal.(1981). 8— Veedercia/.(1984). 5— G.Veeder(personal communicationinMcFaddenetal.1985). 3— MorrisonandZellner(1979)(polarimetricalbedo,oralbedolimitsetbyapolarimetricparameter, e.g.,a). 2— GradieandTedesco(1987)(convertedtoTRIADsystem). 1— MorrisonandZellner(1979)(radiometricalbedo). © American Astronomical Society E. F.TEDESCOANDJ.GRADIE:MCLASSNEAR-EARTHASTEROIDS Apollo Amor Type Amor Apollo Apollo Apollo Amor Apollo Amor Apollo Amor Amor Amor Apollo Apollo Amor Amor Amor Apollo Aten Apollo Apollo Aten Aten Amor Amor Amor Amor Apollo Aten Apollo Apollo Amor Amor Apollo Amor Apollo Amor Table VIII.AdoptedtaxonomicclassesforAten,Apollo,andAmorasteroids. Pv 0.18 0.23 0.21 0.03 0.18 0.18 0.18 0.18 0.14 0.23 0.19 0.15 0.05 0.20 0.22 0.19 0.17 0.22 0.17 0.12 0.18 Pv Ref. 10 10 Notes toTableVIII—UBVReferences Notes toTableVIII—pReferences v Notes toTableVIII—Remarks Provided bythe NASA Astrophysics Data System 0.54 0.43 0.52 0.27 0.42 0.50 0.47 0.47 0.50 0.41 0.40 0.50 0.37 0.36 0.31 0.46 0.35 0.45 0.52 0.41 0.52 0.50 0.41 0.38 0.14 0.50 U-B 0.37 0.28 0.53 0.45 0.50 0.24 0.21 B-V 0.80 0.90 0.84 0.84 0.87 0.66 0.82 0.88 0.89 0.93 0.79 0.83 0.77 0.72 0.76 0.84 0.85 0.82 0.94 0.84 0.83 0.77 0.77 0.92 0.80 0.81 0.85 0.72 0.70 0.86 0.94 0.66 0.71 UBV Ref. class S Other S S C Other S S c C S Other Other C Other S S Other S S S s s C M Other Adopted M S Remarks 2 2 3 Possibly M 1971 FA 1947 XC=1979XA 1978 RA 1976 A 1978 DA 1977 RA 1982 RA 1976 UA 1978 SB 1982 DV 1983 TB,4 743 1987AJ 93. . 738T 41406 6 9 6 ber ofasteroidsinthenear-Earthpopulationisgivenas number observedineachclass,thefractionoftotalfor found byGradieandTedescoformain-beltasteroids. mined byGradieandTedesco(1982)fortheasteroidpopu- percentages ofC,S,M,and“other”classasteroidsasdeter- estimated relativeabundance.Thesenewestimatesaregiven number ofCobjectsfromfiveto20andrecalculatingthe rate betweentheCandSMclassesbyincreasing the factorof4differencesinalbedoand,hence,discovery dard statisticalmethods.Wehaveattemptedtoaccountfor each class,andtheuncertaintyinthatnumberusingstan- AU), and2:1(3.277AU)resonances. as “corrected”values.Thesevaluesarecomparedwiththe found nearthe3:1and5:2Kirkwoodgapsissuggestiveofan ture oftheEarth-approachingpopulationandasteroids lation nearthe4:1(2.064AU),3:1(2.5005:2(2.824 proaching objects,anideaoriginallysuggestedbyWetherill asteroidal ratherthancometarysourcefortheEarth-ap- tent paucityoflow-albedoCclassobjectsinthenear-Earth group relativetotheirabundanceinthevicinityof3:1 bodies presentlylocatedneartheseunstablezonesor,more from theabundanceofK/range90X10to and 5:2resonancesmaybeduetoasmallernumberofsource clusterings forcertainirongroupsat400+100X10the 2.3 X10yr(Voshage1967;Buchwald1975)withdistinct bits. Ifthesourceofnear-Earthpopulationisprimarily major classesandisthereforetheleastlikelytosurvive others, theCclassmaterialismostfriableofthree likely, assuggestedbyGreenbergandChapman(1983) son 1985).Thecosmic-rayexposureagesoftheabundant IVA ironsand650+100X10yrfortheIIIAB(Was- transport timefromthemainbelttoEarth-approachingor- 744 E.F.TEDESCOANDJ.GRADIE:MCLASSNEAR-EARTHASTEROIDS tions ofanEarth-approachingasteroid, hencethesuggestion of themeantimebetweencatastrophiccollisionaldisrup- der ofafewmillionyears.Theselatteragesareontheorder chondritic meteoritesaremuchshorter,generallyontheor- asteroidal, thenitispossibletospeculatethattheendprod- irons andthechondritesisdue mostlytothedifferencein that thedifferenceincosmic-ray Exposureagesbetweenthe lic objectssurviveintacttensto hundredsoftimeslonger. are destroyedinafewmillionyears, whereasstrongermetal- strength ofthematerials:weaker stonyobjectsofmetersize class material,thecoreofacometmaybesofriablethatit cores. Alternatively,asinthecaseofsurvivabilityC streams, i.e.,themajorityofcometsmaylackvolatile-free uct ofatleastsomecometsissimplydustandmeteor cannot surviveintactaslongasteroidalmaterials. (1979) onthebasisofdynamicalarguments.Thestill-exis- The resultsofthisstudyareshowninTableIX.num- The grosssimilaritiesbetweenthecompositionalstruc- Cosmic-ray exposureagesofironmeteoritesdetermined © American Astronomical Society • Provided by the NASA Astrophysics Data System Wetherill andWilliams(1968) andWetherill(1976) Taxonomic Observed4:13:15:22:1 Other 718+61550535 class numberObservedCorrected2.062.502.823.28(AU) M 26+45010 C 513±540106560 S 2463±84035205 Table IX.DistributionoftaxonomicclassesamongObservedAten,Apollo,andAmorAsteroids. Earth-approaching 9 were unabletoidentifyanymetallicironanalogamongthe McFadden etal.(1985)supportthisideabynotingthatthey there canstrongmeter-sizedobjectssurvivefor10yr. with meanlifetimeslongerthanthoseoftheAten,Apollo, argue thatthesourcesofironmeteoritesmustbeinorbits have usedthelongcosmic-rayexposureagesofironsto iron incomposition,suggeststhatanear-Earthsourcemay Earth-approaching asteroidpopulation. population butmustbeinthemainasteroidbeltsinceonly suggest thatthesourcecannotbeinnear-Earthasteroid and Amorpopulations.GreenbergChapman(1983) the Mclassnear-Earthasteroidsmaynotbesourcebut exist foratleastsomeoftheironmeteorites.Alternatively, in theasteroidbelt(neararesonance)anddeliveredto meteoritic materialejectedfroma20kmdiameterironbody model fortheproductionofmeteoritesinfersthatyield meteorites arederived.GreenbergandChapman’s(1983) rather apartofthesamepopulationfromwhichiron but notforsmallironbodies.Irons,though,arefivetimes based uponnumericalsimulationsofcrateringprocesseson lifetime againstcollisionaldestruction,andpassagethrough weak (stony)objectofthesamediameterwhenfactorssuch surface oftheEarthshouldbeabouteighttimesthatfroma more likelytosurvivepassagethroughtheatmosphere. production fromsmallbodies,whichGreenbergandChap- value isstronglyinfluencedbythedependenceofmeteorite for producingejectathatescapethegravitationalfield.This the atmosphereareconsidered. as ejectaproduction,communicationwithresonances,mean in anEarth-approachingorbitis15to20timesmoreeffec- collisional destructionbyothermeteoroids(althoughnot Also, ironobjectsapparentlyhavemeanlifetimesagainst man (1983)estimatetobeefficientforsmallstonybodies ter ironobjectisnolessthan1/3asefficientastonybody longer thanthoseforstonymaterials.Hence,anironobject against collisionsorejectionsbyplanets)thataretentimes small bodiesbyGreenbergetal.(1978),thata2kmdiame- with theresonance.GreenbergandChapmanestimate, consider thedegreetowhichejectawillcommunicate tive atproducingmeteoritesthanastonyobject,leastac- listed inTableVIIIareareasonablecrosssectionofthe calculation basedupontheassumptionthatasteroids cording totheGreenbergandChapmanmodel.Asimple Relative populationsinpercent Earth-approaching population, andassumingthatthemete- produce about20%ofthemeteorites producedfromthe squared (area),indicatesthat1986 DAand1986EBshould orite production(ejecta)rateis proportionaltothediameter cording totheGreenberg-Chapman model,onlyabout5% other observedEarth-approaching asteroids.However,ac- of allmeteoriticmaterialfalling ontotheEarthisproduced The discoveryoftwoMclassobjects,probablymetallic For objectsinEarth-approachingorbits,oneneednot 744 1987AJ 93. . 738T 8 68 8 6 745 E.F.TEDESCOANDJ.GRADIE:MCLASSNEAR-EARTHASTEROIDS Wasson (1985),isthatthesmallnumberofironmeteorites cosmic-ray exposureagesbringsintoquestionthevalidityof number ofironmeteoritesfoundandtheirobservedlong predict thatthesetwoobjectsaloneshouldproduceabout case, thencurrentmodelsformeteoriteproductionbasedon and Amorpopulations.TheinferredcompositionoftheM first ofthisclasstobeidentifiedamongtheAten,Apollo, hundreds ofmillionsyears. tal bias.Inanycase,thefactthatsomeironshavecosmic-ray ages lessthan10yr.Apossibleexplanation,suggestedby that abouthalftheironmeteoritesshouldhaveexposure lifetimes (10-10yr)oftheseasteroids. by near-Earthasteroidsand,hence,onlyabout1%ofthe strength-related collisionalprocessesonasteroidalsurfaces class asteroidsisprimarymetalliciron.Ifthisindeedthe are foundtobelongtheMclassofasteroids.These ly canonlybeexplainedbysourcesinorbitsstableformany exposure agesgreaterthan10yrissignificantandapparent- with ageslessthan200X10yrmayresultfromexperimen- sure agesissurprisingsinceoursimplecalculationsshow have cosmic-rayexposureagesnearthatofthedynamical DA or1986EB,thenabouthalftheironmeteoritesshould are irons(Anonymous1986).Ifhalfofthesefrom1986 these twoasteroids.Inactualfact,about2%ofallmeteorites meteorites inourcollectioncanbeexpectedtocomefrom Bowell, E.,Gehrels,T.,andZellner,B.(1979).\nAsteroids,editedbyT. 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