Spectral properties of near-Earth and Mars-crossing asteroids using Sloan photometry

5.3 MB·

by  Benoit Carry

Spectral properties of near-Earth and Mars-crossing asteroids using Sloan photometry
B.Carrya,b,E.Solanoc,d,S.Eggla,F.E.DeMeoe,f
aIMCCE,ObservatoiredeParis,PSLResearchUniversity,CNRS,SorbonneUni-versites,UPMCUnivParis06,Univ.Lille
bEuropeanSpaceAstronomyCentre,ESA,P.O.Box78,28691VillanuevadelaCan˜ada,Madrid,Spain
cCentrodeAstrobiologia(INTA-CSIC),DepartamentodeAstrofisica.P.O.Box78,E-28691VillanuevadelaCan˜ada,Madrid,Spain
dSpanishVirtualObservatory
eDepartmentofEarth,AtmosphericandPlanetarySciences,MIT,77MassachusettsAvenue,Cambridge,MA,02139,USA
fHarvard-SmithsonianCenterforAstrophysics,60GardenStreet,MS-16,Cambridge,MA,02138,USA
6
1
Abstract
0
2Thenatureandoriginoftheasteroidsorbitinginnear-Earthspace,includingthoseonapotentiallyhazardoustrajectory,isofboth

nscientific interestandpracticalimportance. We aimhereatdeterminingthetaxonomyofalargesampleofnear-EarthandMars-
acrosser asteroids and analyze the distribution of these classes with orbit. We use this distribution to identify the source regions
J
of near-Earthobjects and to study the strength of planetaryencountersto refresh asteroid surfaces. We measure the photometry

9ofthese asteroidsoverfourfiltersatvisible wavelengthsonimagestakenbytheSloanDigitalSkySurvey(SDSS).Thesecolors

are used to classify the asteroids into a taxonomy consistent with the widely used Bus-DeMeo taxonomy (DeMeo et al., Icarus
]
P202,2009)basedonvisibleandnear-infraredspectroscopy. Wereporthereonthetaxonomicclassificationof206near-Earthand
E776 Mars-crosser asteroids determinedfrom SDSS photometry,representingan increase of 40% and 663% of knowntaxonomy
.classificationsinthesepopulations. UsingthesourceregionmapperbyGreenstreetetal. (Icarus,217,2012),wecompareforthe
h
pfirst time the taxonomic distribution among near-Earth and main-beltasteroids of similar diameters. Both distributions agree at
-the few percentlevelforthe innerpartof the Main Belt andwe confirmthis regionas a main sourceof near-Earthobjects. The
o
effectofplanetaryencountersonasteroidsurfacesarealsostudiedbydevelopingasimplemodelofforcesactingonasurfacegrain
r
tduringplanetaryencounter,whichprovidesthe minimumdistance at whicha close approachshouldoccurto trigger resurfacing
s
aevents.Byintegratingnumericallytheorbitofthe519S-typeand46Q-typeasteroidsinoursamplebackintimefor500,000years
[andmonitoringtheirencounterdistancewithVenus,Earth,Mars,andJupiter,weseektounderstandtheconditionsforresurfacing

events. ThepopulationofQ-typeisfoundtopresentstatisticallymoreencounterswithVenusandtheEarththanS-types,although
1
vbothS-andQ-typespresentthesameamountofencounterswithMars.
7
8Keywords: Near-Earthobjects,Asteroids,composition,Photometry
0
2
01. Introduction bits cross that of the telluric planets, form a transient popu-
.
1 lation. Their typical lifetime is of a few million years only
0 Asteroids are the leftovers of the building blocks that ac- (Bottkeetal.2002;Morbidellietal.2002)beforebeingejected
6cretedtoformtheplanetsintheearlySolarSystem. Theyare from the Solar System, falling into the Sun, or impacting a
1alsotheprogenitorsoftheconstantinfluxofmeteoritesfalling
planet. Thesepopulationsarethereforeconstantlyreplenished
:
von the planets, including the Earth. Apart from the tiny sam- byasteroidsfromthemainasteroidbelt,thelargestreservoirof
iple of rock from asteroid (25143) Itokawa brought back by
X knownsmallbodies,betweenMarsandJupiter.
the Hayabusa spacecraft (Nakamuraetal. 2011), these mete-
The resonances between the orbits of asteroids and that of
r
aoritesrepresentoursolepossibilitytostudyindetailsthecom- Jupiterhavebeenlongthought(Wetherill1979;Wisdom1983)
position of asteroids. Identifying their source regions is cru-
toprovidethekickineccentricitynecessarytoplaceasteroids
cial to determine the physical conditions and abundances in on planet-crossing orbits. It was later found that the secular
elements that reigned in the protoplanetarynebula around the
resonance ν , delimiting the inner edge of the main belt, and
6
youngSun(see,e.g.,McSweenetal.2006). Fromtheanalysis
the 3:1 mean-motionresonance (MMR) with Jupiter, separat-
of abolidetrajectory,itis possibletoreconstructits heliocen- ing the inner from the middle belt, were the most effective,
tricorbitandtofinditsparentbody(e.g.,Gounelleetal.2006),
compared to the 5:2 resonance, for instance, which tends to
butsuchdeterminationshavebeenlimitedtoafewobjectsonly
ejectasteroidsfromthesolarsystem(seeMorbidellietal.2002,
(Rudawskaetal.2012).
for a review). The major role playedby the ν resonancewas
6
Amongthedifferentdynamicalclassesofasteroids,thenear-
confirmed by the comparison between the reflectance spectra
EarthandMars-crosserasteroids(NEAsandMCs), whoseor-
ofthemostcommonmeteorites,theordinarychondrites(OCs,
80%ofallmeteoritefalls),thedominantclassinthenear-Earth
space, the S-type asteroids (about65% of the observedpopu-
Emailaddress:[email protected](B.Carry)
PreprintsubmittedtoIcarus January12,2016
lation,Binzeletal.2004),andthedynamicalfamilyofS-types 2. VisiblephotometryfortheSloanDigitalSkySurvey
asteroidslinkedwith(8)Floraintheinnerbelt(Vernazzaetal.
2.1. TheSloanDigitalSkySurvey
2008).
The NEAs also represent ideal targets for space explo- TheSloanDigitalSkySurvey(SDSS)isa wide-fieldimag-
ration owing to their close distance from Earth. This prox- ingsurveydedicatedtoobservinggalaxiesandquasarsatdiffer-
imity is quantified by the energy required to set a space- entwavelengths. From1998to2009,thesurveycoveredover
craft on a rendezvous trajectory and is often expressed as ∆v 14,500 square degrees in 5 filters: u′, g′, r′, i′, z′ (centered
(in km/s), the required change in speed. This is the reason on 355.1, 468.6, 616.5, 748.1 and 893.1 nm), with estimated
why the first mission to an asteroid targeted the Amor (433) limitingmagnitudeof22.0,22.2,22.2,21.3,and20.5for95%
Eros(Veverkaetal.2000),whyallthetargetsofsample-return completeness(Ivezic´etal.2001).
missions were selected among NEAs: (25143) Itokawa for
JAXA Hayabusa (Fujiwaraetal. 2006), (101955) Bennu for 2.2. TheMovingObjectCatalog
NASAOSIRIS-REx(Origins-SpectralInterpretation-Resource In the course of the survey, 471,569 moving objects were
Identification-Security-RegolithExplorer,Laurettaetal.2011), identifiedintheimagesandlisted inthe MovingObjectCata-
(162173)RyuguforJAXAHayabusa2(Yanoetal.2010),and logue (SDSS MOC, currently in its 4th release, including ob-
(175706) 1996FG3 and (341843) 2008EV5 for the former servationsthroughMarch 2007). Amongthese, 220,101were
ESAM3/M4candidateMarcoPolo-R(Baruccietal.2012)and successfullylinkedto104,449uniqueobjectscorrespondingto
ARM (AsteroidRedirectMission,Abelletal. 2015), andwhy known asteroids (Ivezic´etal. 2001). The remaining 251,468
the recentpropositionfor a demonstrationprojectof an aster- moving objects listed in the MOC corresponded to unknown
oid deflection by ESA, AIDA (Asteroid Impact & Deflection asteroidsatthetimeoftherelease(August2008).
Assessment, Murdochetal. 2012), targets the NEA (65803) First, we keep objects assigned a number or a provisional
Didymos. This latter point, the protection from asteroid haz- designation only, i.e., those for which we can retrieve the or-
ard,iscertainlythemostfamousaspectoftheasteroidresearch bital elements. Among these, we select the near-Earth and
knowntothegeneralpublic,andhastriggeredmanyinitiatives Mars-crossers asteroids according to the limits on their semi-
leading to breakthroughsin NEA discovery and characteriza- majoraxis,perihelion,andaphelionlistedinTable1,resulting
tion of their surface and physical properties (see, e.g., Binzel in2071observationsof1315uniqueobjects. We thenremove
2000; Stokesetal. 2000; Ostroetal. 2002; Binzeletal. 2004; observations that are deemed unreliable: with any apparent
Jedickeetal. 2007; Mainzeretal. 2011a; Muelleretal. 2011, magnitudesgreaterthanthelimitingmagnitudesreportedabove
amongothers). (Section2.1),oranyphotometricuncertaintygreaterthan0.05.
InbothattemptingtolinkNEAsandMCstransientpopula- These constraintsremovea largeportionof the dataset(about
tionswith theirsourceregionsandmeteoritesanddesigninga 75%), primarily due to the larger typical error for the z′ fil-
protection strategy, the study of their composition is key. In- ter. Whilethereisonlyasmallsubsetofthesampleremaining,
deed, dynamicalstudiesallowstodeterminerelativeprobabil- weareassuredofthequalityofthedata(seeDeMeoandCarry
ities of the origin of asteroids belonging to those populations 2013,foradditionalinformationonthedefinitionofphotomet-
(e.g., Bottkeetal. 2002; Greenstreetetal. 2012). These links riccuts). Additionally,forhighererrors,theambiguityamong
are however not sufficient, and must be ascertained by com- taxonomicclassespossibleforanobjectbecomessolargethat
positionalsimilarities(Vernazzaetal.2008;Binzeletal.2015; theclassification(Section3)becomesessentiallymeaningless.
Reddyetal.2015). Moreover,differentcompositionsyielddif- Inthisselectionprocess,wekept588observationsof353indi-
ferent densities and internal structure/cohesion (Carry 2012), vidualasteroidsfromtheSDSSMOC4,aslistedinTable1.
andanasteroidonaimpacttrajectorywithEarthofagivensize
will requirea differentenergyto bedeflectedordestroyedac- 2.3. IdentifyingunknownobjectsintheMOC4
cordingtoitsnature(JutziandMichel2014).
Asmentionedabove,morethanhalfoftheMOC4entrieshad
Here,weaimatclassifyingalargenumberofnear-Earthand
notbeenlinkedwithknownasteroids.Atthetimeoftherelease
Mars-crosserasteroidsintobroadcompositionalgroupsbyus-
(August 2008), about 460,000 asteroids had been discovered
ing imaging archivaldata. We presentin Section 2 the proce-
and 350,000were numbered (i.e., had well-constrained orbits
dureweusedtoretrievethephotometryatvisiblewavelengths
allowing easy cross-matching with SDSS detected sources).
fromthepubliclyavailableimagesoftheSloanDigitalSkySur-
Thecurrentnumberofdiscoveredasteroidshasnowrisenabove
vey (SDSS). We describe in Section 3 how we use the SDSS
700,000,withmorethan370,000numberedobjects. Wethere-
photometrytoclassifytheobjectsintothecommonly-usedBus-
foreusetheimprovedcurrentknowledgeontheasteroidpopu-
DeMeo taxonomyofasteroids(DeMeoetal. 2009), following
lationtolinkunknownMOCsourcestoknownobjects.
the work by DeMeoandCarry (2013). We present the results
We use the Virtual Observatory (VO) SkyBoT cone-search
oftheclassificationinSection4beforediscussingtheirimpli- service (Berthieretal. 2006), hostedat IMCCE1, for that pur-
cationsfor sourceregionsin Section5 and forsurface rejuve-
pose. SkyBoT pre-computes weekly the ephemeris of all
nationprocessesinSection6.
known Solar System objects for the period 1889-2060, and
1http://vo.imcce.fr/webservices/skybot/
2
Class a(au) q(au) Q(au) MOC4 SVO-MOC SVO SVO
griz gri
min. max. min. max. min.
Atens – 1.0 – Q♂ 0.983 – – 10 1
Apollos 1.0 – – 1.017 – 14 18 82 70
Amors – – 1.017 1.3 – 29 73 111 40
Mars-crosser – – – Q♂ – 310 383 622 567
Total – – – – – 353 474 825 678
Table 1: Definition ofthe dynamical classes ofnear-Earth andMars-crosser asteroids usedinpresent study, basedonthesemi-major axis (a), perihelion (q),
andaphelion(Q)oftheirorbit. AlltheobjectshaveaperihelioninwardofMarsaphelion(Q♂)at1.666au. ThenumberofobjectslistedintheSDSSMOC4
(Section2.2),identifiedinSDSSMOC4usingSkyBoT(SVO-MOC,Section2.3),andbytheSVONEAproject(Section2.4)arealsolisted. SeeFig.1forthe
distributionoftheseobjectsintheorbitalelementspace.
80 keepobjectswithahighprobabilitytobelinkedwiththeMOC
Main-belt
Near-Earth sources.
Atens
To furtherrestrictthe numberof false-positiveassociations,
60 Apollos
Amors we comparetheposition,apparentmotion,andapparentmag-
on () Mars-crosser nitude of the MOC sources to that predicted from ephemeris
atio 40 provided by SkyBoT, based on the database of orbital ele-
n ments AstOrb2. We consider successful association of SDSS
cli
n sources with SkyBoT entry if the positions are closer than
I
20 30′′,theapparentV-Johnsonmagnitudesdonotdifferbymore
than0.5, andthe apparentmotionsare co-linear(differencein
dαcos(δ)/dt and dδ/dt of less than 3′′/h). However, neither
0
SkyBoTnorMOC4provideestimatesontheuncertaintyinthe
0.8
q=1.0 apparent velocity. The only information is the uncertainty in
q=1.3 thevelocitycomponentsparallelandperpendiculartotheSDSS
0.6 scanning direction. The mean value of this error (both in the
y q=1.67 parallelandin the perpendiculardirection)is of 1′′/h. We are
cit takingthisvalueasonestandarddeviationtosetthecutabove.
ntri 0.4 Q Of the 251,468 unidentified MOC sources, SkyBoT provides
ce =
c 1 known asteroids within 30 arcseconds for 68,497 (27%), cor-
E .0
responding to 41,055 unique asteroids. We trim this value to
0.2
57,646(36,730asteroids)forwhichtheassociationcanbecon-
sideredcertain.Thevastmajorityofthesenow-identifiedaster-
0.0 oidshaveorbitswithinthemainbelt(35,404,correspondingto
0.5 1.0 1.5 2.0 2.5 3.0 3.5 96%),butsomeareNEAs(48,0.1%),orMCs(73,0.2%).Their
Semi-major axis (au)
respectivenumbersare reportedin Table 1. The complete list
ofMOCentriesassociatedtoknownasteroids(277,747entries
Figure1:DistributionoftheNEAsandMars-crossersstudiedhereasafunction associatedto141,388asteroids)isfreelyaccessible3.
of their osculating elements (semi-major axis, eccentricity, and inclination).
Theblackdotsrepresentthefirst10,000main-beltasteroids,thegreydotsthe
532NEAswithspectralclassificationfromtheliterature(seeSection3.2),and 2.4. TheSVONear-EarthAsteroidsRecoveryProgram
theblackcircles,redsquares,bluetriangle,andgreenstarstheAtens,Apollos, Inaddition,wesearchtheimagesoftheSDSSforNEAsand
Amors,andMars-crossersweclassifyfromSDSS.
MCs that were either not identified as moving objects by the
automaticSDSSpipeline,rejectedbytheMOCdataselection4,
orimagedafterthelatestcompilationoftheSDSSMOC4(i.e.,
stores their heliocentricpositions with a time step of 10 days, observedafterMarch2007). Indeed,onlymovingobjectswith
allowingfastcomputationofpositionsatanytime. Thecone- an apparent motion between 0.05 and 0.050 deg/day were in-
searchtoolallowstorequestthelistofknownobjectswithina cludedintheMOC,leavingasignificantfractionofNEAsun-
fieldofviewatanygivenepochasseenfromEarthintypically cataloged(Solanoetal.2013).
less than 10s. We send 251,468 requests to SkyBoT, corre- We use the resources of the citizen-science project “Near-
sponding to the 251,468 unknownobjects in the MOC4, cen- EarthAsteroidsRecoveryProgram”oftheSpanishVirtualOb-
teredontheMOC4object’scoordinates,atthereportedepoch servatory (SVO) which was originally designed for this very
ofobservation,withinacircularfieldofviewof30arcseconds.
Although many asteroids among the 700,000knownhave po-
2http://asteroid.lowell.edu/
sition uncertainty larger than this value (as derived from their 3http://svo2.cab.inta-csic.es/vocats/svomoc
orbital parameter uncertainty), this cut ensures that we only 4http://www.astro.washington.edu/users/ivezic/sdssmoc/sdssmoc.html
3
purpose: to identify and measure the astrometry of NEAs in
g r
archival imaging data (Solanoetal. 2013). For each Aten, 22
N = 71 N = 531
griz griz
Amor, Apollo, and Mars-crosser listed by the Minor Planet
N = 57 N = 469
Center5 (MPC),itsephemerisarecomputedovertheperiodof gri gri
20
operationoftheSDSSimagingsurvey(1998to2009)andcom-
paredtothefootprintsoftheimagesofthesurvey. Theimages -0.25 0.25 -0.25 0.25
18
possibly containingan objectbrighter than the SDSS limiting
40 40
magnitude (V=22) are then proposed to the public for iden-
e
tification through a web portal6. Since the beginning of the ud 16 20 20
projectin2011,over2,500astrometrymeasurementsofabout gnit
a
600NEAsnotidentifiedintheMOChavebeenreportedtothe SS m 22 i z
MPC(seeSolanoetal.2013,fordetailsontheproject). D N = 399 N = 93
S griz griz
To compute the photometry of the NEAs measured by the N = 413
gri
users we first searched in the photometric catalog of the 8th 20
SDSS Data Release7. If no photometry associated with the
-0.25 0.25 -0.25 0.25
NEA was found, we ran SExtractor on the correspondingim-
18
ages and calibrated the SExtractor magnitudes by comparing
40 40
themwiththeSDSSmagnitudesofthesourcesidentifiedinthe
16 20 20
image.
Owingtothemorestringentlimitingmagnitudeinthez′ fil-
16 18 2 0 22 16 18 2 0 22
ter,manyasteroidsareidentifiedoverthreebands(g′r′i′)only.
SVO magnitude
We also report these objects here, although deriving a taxo-
nomicclassificationisofcourselessaccurate.Overall,wecol- Figure2:Comparisonoftheapparentmagnitudemeasuredbyoursystem(SVO
lect1194fourbands(g′r′i′z′)photometrymeasurementsof825 magnitude)withthatreportedintheSDSSMOC4(SDSSmagnitude)forthe
uniqueasteroidsand976three-bands(g′r′i′)photometrymea- fourg′,r′,i′,andz′filters.Foreach,thenumberofcommonobjectsisreported,
forbothfour-bands (black circles) andthree-bands (greysquares) sets. The
surements of 678 distinct asteroids (Table 1). We present in
insertedhistogramsshowthedistributionofthemagnitudedifference(MOC-
Fig.2acomparisonofourmeasurementswiththemagnitudes SVO)
reportedintheSDSSMOC4forthecommonasteroidsinboth
sets,showingexcellentagreement(valuesagreewithastandard
deviationof0.05mag). unityinfilter g′. Second,wecomputetheslopeofthecontin-
uum over the g′, r′, and i′ filters (hereaftergri-slope), and the
z′−i′ color(hereafterzi-color),representingthebanddepthof
3. Taxonomicclassification apotential1µmband,becausetheyarethemostcharacteristic
spectraldistinguishersinallmajortaxonomies(beginningwith
TheSDSSphotometryhasbeenusedtoclassifyasteroidsac- Chapmanetal. 1975). The classification into the taxonomyis
cordingtotheircolorsbymanyauthors(e.g.,Ivezic´etal.2002; thenbasedonthesetwoparameters.
Nesvorny´ etal. 2005; Parkeretal. 2008; Carvanoetal. 2010). As a results of the limited spectral resolution and range
One key advantage of the survey was the almost simultane- of SDSS photometry, we group together certain classes into
ousacquisitionofallfilters(5minintotal),hencelimitingthe broadercomplexes(seecorrespondencesinTable2).Foraster-
impact of geometry-relatedlightcurveon the apparentmagni- oidswithmultipleobservationsthatfallundermultipleclassifi-
tude. Here we follow the work by DeMeoandCarry (2013, cations,weusethetree-likeselectiontoassignafinalclass(see
2014) in whichthe class definitionsareset to be as consistent DeMeoandCarry(2013)fordetailsandDeMeoetal.(2014a)
aspossiblewithpreviousspectraltaxonomiesbasedonhigher foranexampleofaspectroscopicconfirmationcampaignofthe
spectral resolution and larger wavelength coverage data sets, SDSS classification used here). We successfully classify 982
specifically Bus and Bus-DeMeo taxonomies (BusandBinzel asteroidsfromthesampleof1015near-EarthandMars-crosser
2002;DeMeoetal.2009). Wepresentconciselytheclassifica- asteroidswithfour-bandsphotometry(i.e.,97%ofthesample).
tionschemebelowandrefertoDeMeoandCarry(2013)fora For objects with three-bands photometry only, we set their
completedescription. z′ magnitude to the limiting magnitude of 20.5 (Ivezic´etal.
2001) as an upper limit for their brightness. We then classify
3.1. FromSDSStoBus-DeMeotaxonomy theseasteroidsusingtheschemepresentedabove. Becausethe
magnitudeof20.5inz′isanupperlimit,theactualzi-colorfor
First, we convertthe photometryinto reflectance (using so-
these asteroids may be overestimated. The classification can
lar colors from Holmbergetal. 2006) and normalize them to
thereforebe degenerated,all the classes with similar gri-slope
andlowerzi-colorbeingpossible. Weassigntentativeclassifi-
cationto254asteroidsfromthesampleof678near-Earthand
5http://minorplanetcenter.org/
6http://www.laeff.cab.inta-csic.es/projects/near/main/ Mars-crosserasteroidswiththree-bandsphotometry(i.e.,37%
7http://cdsarc.u-strasbg.fr/viz-bin/Cat?II/306 ofthesample).
4
SDSS&Literature Bus-DeMeo Diameter: 10.0 1.0 0.1 (km)
110044
A,AR,AS A
B B 110033
C,C:,Cb,Cg,Ch C ersers
D,DT D crosscross 110022
K,K: K ars-ars-
MM
L,Ld L 110011
Q,Q/R,R,RQ Q
11
O,Q/R/S,R,RS S
MPC
S,S:,S(I→V),Sa,Sk,Sl,Sq,Sq:,Sr S oidsoids 110033 Spectra-only
V,V: V erer Common spectra/SDSS
E,M,P,X,X:,XT,Xc,Xe,Xk X h Asth Ast 110022 SDSS
U,C(u),R(u),S(u),ST,STD,QX U artart
EE
ar-ar- 110011
ee
Table2: Correspondencebetweentheclassesfromdifferenttaxonomies(e.g., NN
TholenandBarucci1989;BusandBinzel2002)foundintheliterature,usedto 11
classifytheSDSSphotometry(inbold),andtheirequivalence inthereduced 1100 1155 2200 2255
versionofthetaxonomybyDeMeoetal.(2009)adoptedhere, followingthe AAbbssoolluuttee mmaaggnniittuuddee ((HH))
work by DeMeoandCarry (2013). We strive to preserve the most extreme
classes (like A,B)andweconvert thetentative classification (e.g., RS)into Figure3:Distributionoftheknowntaxonomyfornear-EarthandMars-crosser
theirbroader,safer,complex(S).Wealsoconsiderasunknown(U)anydataof asteroids as a function of their absolute magnitude H. The white histogram
insufficientquality. correspondstothetotalnumberofdiscoveries(listedinAstOrb),thegreytothe
taxonomicclassificationfoundintheliterature,thebluetotheoverlapbetween
literatureandpresentstudy,andtheredthefour-bandsset. Anapproximative
Inallcases,wemarkobjectswithpeculiarspectralbehavior conversion todiameter is alsoreported, using D = 1329× pv−0.5 ×10−0.2H,
with the historicalnotation“U” (forunclassified), anddiscard withthegeometricalbedo pvtakenas0.2(themajority,≈60%,oftheobjects
classifiedherebeingS-types).
them fromthe analysis. Thereare 33and424asteroidsin the
four-bands and three-bands photometry samples respectively
for which we cannot assign a class. These figures highlight photometry.The254asteroidswiththree-bandsphotometryare
theambiguityraisedbythelackofinformationonthepresence listedseparatelyinTableB.2,becausetheirtaxonomicclassifi-
orabsenceofanabsorptionbandaroundonemicron,towhich cationislessrobust. Inmanycases,theupperlimitof20.5for
thez′filterissensitive. their z′ magnitude provides a weak constraint on their taxon-
omy,andclasseswithhighzi-color(mainlyV-types)aremore
3.2. Gatheringclassificationsfrompaststudies easilyidentified.Thissamplebasedonthree-bandsphotometry
only is therefore biased, but it can be used as a guideline for
Many different authors have reported on the taxonomic
selectingtargetsforspectroscopicfollow-ups. We concentrate
classification of NEAs. We gather here the results of
belowonthesamplebasedonfour-bandsphotometry.
Dandyetal. (2003), Binzeletal. (2004), Lazzarinetal.
The206NEAspresentedherehaveabsolutemagnitudesbe-
(2005), deLeo´netal. (2006), deLeo´netal. (2010),
tween12and23.Oursamplefullyoverlapswiththesizerange
ThomasandBinzel (2010), Popescuetal. (2011), Ye (2011),
of the 523 NEAs characterized by visible/near-infrared spec-
Reddyetal. (2011), Polishooketal. (2012), Sanchezetal.
troscopypublishedtodateandrepresentsanincreaseof≈40%
(2013), and DeMeoetal. (2014b). These authors used differ-
ofthecurrentsamplesize(Fig.3). Asignificantfraction(46%)
ent taxonomic schemes to classify their observations, using
of the NEA population with H<16 (about 2km diameter for
either broad-bandfilter photometryor spectroscopy, at visible
an albedoof0.20)hasa taxonomicclassification. Forsmaller
wavelengths only or also in the near infrared. We therefore
diameters,thefractiondropsquicklyto10%andless. Thesub-
transposetheclassesofthesedifferentschemes(Tholen1984;
kilometerpopulationof NEAs is thereforestill poorlycatego-
TholenandBarucci 1989; BusandBinzel 2002; DeMeoetal.
rized.
2009) into the single, consistent, set of 10 classes we already
Theabsolutemagnitudeofthe776MCsreportedhereranges
use for the SDSS data. Here also, we attribute the historical
from11to19. Oursamplerepresentsthefirstclassificationof
“U” designation for objects with apparently contradictory
sub-kilometricMars-crossers,andasixfoldincreasetothesam-
classifications (e.g., QX or STD in Ye 2011). These patho-
pleof117MCsfromspectroscopy(Fig.3).SimilarlytoNEAs,
logical cases represent15% of the objects with multiple class
about40%oftheMCpopulationwithH<14(about5kmdiam-
determinations. Intotal,wegather1022classificationsfor648
eterforanalbedoof0.20)nowhasataxonomicclassification,
objectslistedintheliterature.
andthefractiondropsquicklyto10%forsmallerdiameters.
4.1. Taxonomyandorbitalclasses
4. Results
We present in Fig. 4 how the different classes distribute
We list in Table B.1 the photometry and the taxonomy of among the orbital populations. As already reported by
all982near-EarthandMars-crossersasteroidswithfour-bands Binzeletal. (2004), the broad S- (including Q-types), C-,
5
X-complexes, and V-types dominate the NEA population, objects must be injected in the NEA space to explain the cur-
the minor classes (A, B, D, K, L) accounting for a few rent observed population. We use our sample of 982 NEAs
percentsonly,similarlytowhatisfoundintheinnermainbelt andMCs to identifypossiblesourceregions. For that, we use
(DeMeoandCarry 2013, 2014). We find that the S-complex the source region mapper8 by Greenstreetetal. (2012), built
encompassestwiceasmanyobjectsastheC-andX-complexes, on the result of numericalsimulationsof the orbitalevolution
compared to the threefold difference reported by Binzeletal. of test particules in the five regions defined by Bottkeetal.
(2004). (2002): ν secularresonance,3:1mean-motionresonancewith
6
ThedistributionoftaxonomicclassesamongMCsissimilar Jupiter,Mars-crossers(MC),OuterBelt(OB),andJupiterFam-
to that of Apollos and Amors (with only 8 Atens, our sample ilyComet(JFC).
suffers from low-number statistics). Our findings of V-types For each object, we compute its probability P to originate
i
accounting for roughly 5% of all MCs may therefore seem from the ith source region. We then normalize all P for each
i
puzzling considering the lack of V-types among the ≈100 sourceregion. ThesumofthenormalizedP overagiventax-
i
classified MCs highlighted by Binzeletal. (2004). It is, onomic class therefore represents the fraction of objects (by
however,anobservationselectioneffect.Indeed,Mars-crossers number) of this class in the source region (Table 4). Uncer-
are inner-main belt (IMB) asteroids which eccentricity has tainties are computed from the source region mapper uncer-
been increased by numerous weak mean-motion resonances tainties, quadraticallyadded with the margin of error (at 95%
(“chaotic diffusion”, see MorbidelliandNesvorny´ 1999). The confidencelevel)to accountforthe sample size. We can then
IMBhostingthelargestreservoirofV-typesinthesolarsystem compare these predicted fractions to the observed distribution
(Vestoids,BinzelandXu1993),V-typeswereexpectedinMC oftaxonomicclassesforeachsourceregion.
space. ThevastmajorityofNEAswithtaxonomicclassificationsus-
The size distribution of known Vestoids however peaks taindiametersoflessthan5km(absolutemagnitudeabove14).
at H=16 (about 1.5km diameter), and the largest members Because thedistributionoftaxonomicclassesin the mainbelt
have an absolute magnitudeof 14 (we use the list of Vestoids varieswithdiameter(DeMeoandCarry2014),weneedtocom-
fromNesvorny2012). All the V-typesidentifiedhere havean parethepredictedfractionofTable4withobjectswithH>14
absolutemagnitudeabove14,andsodoes(31415)1999AK23 in each source region. For the first time, thanks to the large
(H=14.4), the first V-type among MCs reported recently by dataset of asteroidsprovidedby the Sloan Digital Sky Survey
Ribeiroetal.(2014).Only33MCshadbeencharacterizedwith (SDSS), suchinformationis availableforthe innerpartof the
thisabsolutemagnitudeorhighertodate,andthepreviouslack mainbelt(DeMeoandCarry2013,2014)andisreportedinTa-
ofreportofV-typesamongMCsisconsistentwithourfindings. ble4(3to5kmdiameterrange).
The comparison of the ν and 3:1 source regions (delim-
6
iting the inner belt) with the observations in Fig. 5 shows a
4.2. Low-δvasspacemissiontargets very good match of the distributions (correlation coefficient
of 0.99, maximum difference of 5%). This validates the
Withinthe206NEAserendipitouslyobservedbytheSDSS,
dynamical path from the inner belt to the near-Earth space
we identify 36 potential targets for space missions based on
as described in the model by Greenstreetetal. (2012). Al-
their accessibility. We select all the NEA with a δv below
though the inner belt is widely accepted as a major source
6.5km/s. As a matter of comparison, the requiredδv to reach
forNEAsandmeteorites(e.g.,Bottkeetal.2002;Binzeletal.
the Moon and Mars are of 6.0 and 6.3km/s (e.g., Abelletal.
2004;Vernazzaetal.2008;Binzeletal.2015),therelativecon-
2012). We listinTable3thebasiccharacteristicsofthesepo-
tributionofthedifferentsourceregionsparticularlywithrespect
tential targets, together with the targets already, or planned to
toasteroidsizeisstillamatterofdebate. Forthefirsttime,we
be,visitedbyspacecraft.
compareherepopulationsofthesamesizerange.
Among the list of low-δv objects, we find a large majority
Unfortunately,thereisnosimilardatasetfortheothersource
ofS-types,followingtheirdominanceinthesamplepresented
regions (MC, OB, and JFC) to be compared with our predic-
here of about60%. We, however,note the presenceof poten-
tion.Wecanstillnotetheoveralltrendofincreasingfractionof
tialD-,L-,andK-types. Todate,ofthe24taxonomicclasses,
C/D/X-typesin OBandJFC comparedwiththe innerregions,
onlyC-(Mathilde,Ceres),S-(Ida,Eros,Gaspra,Itokawa,and
as expected. The fraction of K-types peaks in the outer belt,
Toutatis),Xe-(Steins),Xk-(Lutetia),andV-types(Vesta)have
place of the Eos family, also as expected, although a steady
beenvisitedbyspacecraft.ThesepotentialD-,L-,andK-types
distributionofK-typesacrosssourceregionsisalsoconsistent
targetsmayrepresentgoodopportunitiesforexploration. Data
withinuncertainties.Wealsonoteastrongcorrelationbetween
in the visible can only suggest the presence of an absorption
MCandsmallinner-beltasteroidpopulations(correlationcoef-
bandat1µm,andnear-infrareddataisrequiredtoconfirmthese
ficient0.97),whichisnotrelatedtotheoriginofNEAs,butof
potentialclassifications.
MC themselves, via chaotic diffusion from the inner belt (see
MorbidelliandNesvorny´ 1999;Micheletal.2000).
5. Sourceregions ApeculiarfeatureofTable4isthehighfractionofS-typein
The population of NEAs being eroded on short timescale 8UpdatedbyS.Greenstreetfromtheoriginalmappertoincludetheproba-
(<10My)byplanetarycollisionsanddynamicalejections,new bilityofthesourceregionsoftheMCsthemselves.
6
all source regions. Based on presentset of NEAs and the dy-
namicalmodelbyGreenstreetetal.(2012),oneasteroidbelow
5kmdiameteroutoffourshouldbeanS-typeintheouterbelt
and the same applies to Jupiter family comets. Although the
census of composition in this size range (3–5km) is far from
being completein these regions, there is a biastoward detect- 4
50
Aten: 8
ing S-typesdue to their high albedo(≈0.20)comparedto that
oftheC/P/D-typesfoundthere(around0.05,seeMainzeretal. 40 Ngri 3
2011b; DeMeoandCarry 2013, for albedo averages over tax- N
griz
30
onomic classes) S-types are minor contributors to the outer
belt for diameters above 5km (DeMeoandCarry 2014) and
20
searchesforS-typematerialhavebeenunsuccessfulamongCy-
1
beles, Hildas, andTrojans(seeEmeryandBrown2003,2004; 10
Emeryetal.2011;Fornasieretal.2004,2007;YangandJewitt
0
2007, 2011; Roigetal. 2008; Gil-HuttonandBrunini 2008;
A B C D K L Q S V X
Marssetetal.2014). However,only4%ofthesample(42ob-
jects) are predicted to originate from the OB and JFC source 50 Apollo: 106
regions. The results for these regionsis, thus, based on small
40 Ngri
numberstatistics. ThelargefractionofS-typesinthesesource
N
regions suggests nevertheless that dynamical models may re- 30 32 griz 30
quirefurtherrefinements.
20 18
)
% 11
( 10
n 4 4 5
o 1 1
i 0
t
la A B C D K L Q S V X
u
p
po 50 Amor: 116 51
he 40 Ngri
t
f N
o griz
n 30
o
i
ct 20 21
a
r 15
F
10 9
5 5
3 3 3
1
0
A B C D K L Q S V X
434
50
MC: 873
40 Ngri
N
griz
30
20 160
116
10
23 24 22 31 26 34
0 3
A B C D K L Q S V X
Taxonomic classes
Figure4:Distributionofthetaxonomicclassesforeachdynamicalgroup:Aten,
Apollo,Amor,andMars-crosser.Thenumberofobjectwithfour-bandphotom-
etryineachclassisreported.Emptyandfilledbarsstandsforthethreeandfour
bandphotometrysamplesrespectively.
7
60
Inner Main Belt 3-5 km
NEAs from ν
6
NEAs from 3:1 MMR
%)
Fraction of the population ( 2400
0
A B C D K L S V X
Figure5: Relativefractions(bynumber)oftaxonomicclassesforasteroidsin
theinnerpartofthemain-belt(2.0–2.5au)withdiameterbetween3and5km
(computed fromDeMeoandCarry2013,2014)compared withthepredicted
fractionsoriginatingfromν6and3:1resonances(seeSec.5)
8
Designation Type Class δv(km/s) H s(%/100nm) z-i a(au) e i(o)
2004EU22 X Apollo 4.420 23.00 0.78 0.044 1.175 0.162 5.3
1996XB27 D Amor 4.750 22.00 0.85 0.123 1.189 0.058 2.5
2000TL1 C Apollo 4.870 22.00 -0.09 0.002 1.338 0.300 3.6
2001QC34 Q Apollo 4.970 20.00 0.69 -0.228 1.128 0.187 6.2
1999FN19 S Apollo 5.020 22.00 0.96 -0.135 1.646 0.391 2.3
2000SL10 Q Apollo 5.080 21.00 0.86 -0.189 1.372 0.339 1.5
1994CN2 S Apollo 5.150 16.00 1.12 -0.073 1.573 0.395 1.4
2002LJ3 S Amor 5.280 18.00 1.07 -0.258 1.462 0.275 7.6
2004UR C Apollo 5.320 22.00 0.13 -0.025 1.559 0.406 2.4
2006UP S Amor 5.350 23.00 1.43 -0.074 1.586 0.301 2.3
1994CC S Apollo 5.370 17.00 0.99 -0.210 1.638 0.417 4.7
2010WY8 K Amor 5.670 21.00 0.92 -0.042 1.385 0.136 6.0
2002XP40 S Amor 5.720 19.00 1.63 -0.092 1.645 0.296 3.8
1993QA D Apollo 5.740 18.00 1.02 0.174 1.476 0.315 12.6
2004RK9 C Amor 5.760 21.00 0.14 -0.050 1.837 0.426 6.2
2001FC7 C Amor 5.780 18.00 0.16 -0.034 1.436 0.115 2.6
1977VA C Amor 5.940 19.20 0.34 -0.010 1.866 0.394 3.0
2001WL15 C Amor 6.000 18.00 0.34 -0.179 1.989 0.475 6.9
2000XK44 L Amor 6.080 18.00 1.24 0.020 1.724 0.385 11.2
2003BH V Apollo 6.090 20.00 1.79 -0.273 1.456 0.356 13.1
2000NG11 X Amor 6.130 17.00 0.27 0.089 1.881 0.368 0.8
2000RW37 C Apollo 6.150 20.00 0.45 -0.166 1.248 0.250 13.8
2001FD90 V Amor 6.200 19.00 0.75 -0.419 2.046 0.478 7.3
2002PG80 S Amor 6.210 18.00 1.09 -0.225 2.013 0.438 4.4
2004VB S Apollo 6.260 20.00 1.04 -0.200 1.458 0.409 10.9
1993DQ1 S Amor 6.270 16.00 1.20 -0.207 2.036 0.493 10.0
2000YG4 Q Amor 6.300 20.00 0.61 -0.155 2.211 0.503 2.6
2004KD1 C Amor 6.330 17.00 0.13 -0.110 1.720 0.331 10.1
2004RS25 C Amor 6.410 20.00 0.39 -0.057 2.128 0.479 6.7
2004QZ2 S Amor 6.470 18.00 -5.74 -0.227 2.260 0.495 1.0
2001FY S Amor 6.530 18.00 1.36 -0.132 1.886 0.327 4.7
2009OC S Amor 6.540 20.00 0.85 -0.153 2.137 0.446 4.6
2004XM35 S Amor 6.560 19.00 1.13 -0.111 1.837 0.301 5.4
2005QG88 K Apollo 6.560 20.00 0.99 -0.056 1.728 0.493 11.3
1999KX4 V Amor 6.580 16.00 1.60 -0.432 1.457 0.293 16.6
2002TY57 S Amor 6.600 19.00 0.86 -0.154 1.922 0.327 3.5
Itokawa S Apollo 4.632 19.20 1.324 0.280 1.6
Bennu C Apollo 5.087 20.81 1.126 0.204 6.0
Ryugu B Apollo 4.646 19.17 1.189 0.190 5.9
Didymos X Apollo 5.098 17.94 1.644 0.384 3.4
Table3:ListofNEAswithalowδv,hencepotentialtargetsforspacemissions.ForeachNEA,thetaxonomicclassdeterminedhereisreported,togetherwiththe
dynamicalclass,theabsolutemagnitudeandtheorbitalelements.Thetargetsoftheasteroid-deflectionmissionAIDA(Didymos)andofthereturn-samplemissions
Hayabusa(Itokawa),OSIRIS-REx(Bennu),andHayabusa2(Ryugu)arealsoincludedforcomparison.
9
Class Sourceregions IMB
3−5km
MC ν MMR OB JFC (#) (%)
6 3:1
A 0.4±5.9 0.5±6.3 0.3±5.5 0.0±3.0 0.0± 3.0 9 0.4
B 2.4±5.8 1.7±5.4 2.9±6.1 6.1±7.5 0.4± 4.3 34 1.5
C 15.7±5.2 20.5±5.4 22.7±5.6 38.6±6.6 47.3±10.6 589 25.7
D 2.5±5.7 2.3±5.6 2.5±5.8 5.8±7.2 9.7± 8.5 20 0.9
K 2.8±5.9 2.5±5.8 2.4±5.7 4.8±6.9 1.8± 7.3 57 2.5
L 3.8±5.9 3.4±5.7 4.4±6.1 1.9±5.1 0.4± 4.2 73 3.2
Q 3.6±5.6 4.3±5.8 6.3±6.4 3.8±5.8 2.4± 5.5 0 0.0
S 52.8±4.6 44.5±4.6 42.0±4.8 23.8±4.8 23.4± 6.9 1145 50.0
V 13.0±5.5 15.6±5.7 12.5±5.5 7.0±5.0 3.0± 4.7 247 10.8
X 3.2±5.5 4.7±6.0 4.0±5.8 8.1±7.2 11.7±10.3 117 5.1
Table4: Relativefractionofeachtaxonomicclass(bynumberofobjects)fromeachsourceregiondefinedbyBottkeetal.(2002)andGreenstreetetal.(2012),
comparedwiththedistributionoftaxonomictypesamong3to5kminnerMainBelt(IMB)asteroids(whereQ-typesaremergedwithS-types,seeDeMeoandCarry
2013).
10