Physical Properties Of Near-Earth Objects

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Binzel et al.: Physical Properties of Near-Earth Objects255Physical Properties of Near-Earth ObjectsRichard P. BinzelMassachusetts Institute of TechnologyDmitrij F. LupishkoInstitute of Astronomy of Kharkov National UniversityMario Di MartinoAstronomico Osservatorio di TorinoRobert J. WhiteleyUniversity of ArizonaGerhard J. HahnDLR Institute of Space Sensor Technologyand Planetary ExplorationThe population of near-Earth objects (NEOs) contains asteroids, comets, and the precursorbodies for meteorites. The challenge for our understanding of NEOs is to reveal the proportionsand relationships between these categories of solar-system small bodies and their source(s) ofresupply. Even accounting for strong bias factors in the discovery and characterization of higheralbedo objects, NEOs having S-type spectra are proportionally more abundant than within themain asteroid belt as a whole. Thus, an inner asteroid belt origin (where S-type objects dominate) is implied for most NEOs. The identification of a cometary contribution within the NEOpopulation remains one of a case-by-case examination of unusual objects, and the sum of evidence suggests that comets contribute at most only a few percent of the total. With decreasingsize and younger surfaces (due to presumably shorter collisional lifetimes for smaller objects),NEOs show a transition in spectral properties toward resembling the most common meteorites,the ordinary chondrites. Ordinary chondritelike objects are no longer rare among the NEOs,and at least qualitatively it is becoming understandable why these objects comprise a high proportion of meteorite falls. Comparisons that can be performed between asteroidal NEOs andtheir main-belt counterparts suggest that the physical properties (e.g., rotation states, configurations, spectral colors, surface scattering) of NEOs may be representative of main-belt asteroids(MBAs) at similar (but presently unobservable) sizes.1.INTRODUCTIONPlanetary science investigations of asteroids, meteorites,and comets all have a common intersection in the study ofnear-Earth objects (NEOs), represented schematically inFig. 1. (Here we define a NEO as an object having a perihelion distance of 1.3 AU.) Dynamical calculations (seeMorbidelli et al., 2002; Bottke et al., 2002a) show that lifespans for NEOs are typically a few million years, eventuallymeeting their doom by crashing into the Sun, being ejectedfrom the solar system, or impacting a terrestrial world. Withsuch short lifetimes, NEOs observed today cannot be residual bodies that have remained orbiting among the innerplanets since the beginning of the solar system. Instead, theNEO population must have some source of resupply. Understanding the source(s) and mechanism(s) of their resupplyis one of the fundamental scientific goals for NEO studies.Key questions include the following: What fraction comesfrom the asteroid belt? What fraction of the NEOs that donot display a coma or a tail are in fact extinct or dormantcomet nuclei? Pinpointing the source regions of NEOs isalso a matter of high scientific priority for fully utilizingthe wealth of information available from laboratory studies of meteorites (e.g., Kerridge and Matthews, 1988). Theimmediate precursor bodies for meteorites are, by definition of proximity, NEOs objects. Thus, the scientific goal ofunderstanding the source(s) for NEOs is identical to the goalof finding the origin locations for meteorites. A key component in tracing meteorite origins is discovering links between the telescopically measured spectral (compositional)properties of asteroids with those measured in the laboratoryfor meteorites (see Burbine et al., 2002).The proximity of NEOs also makes them worlds forwhich we have substantial practical interest. Those having255

256Asteroids IIIFig. 1. Cartoon illustration of the many different groups of objects found within near-Earth space. One of the principal objectives for studying NEOs is to understand how these groups maybe related. Thus the regions of intersection denote key researchareas. As surveys increase their capabilities, human-made spaceflight hardware (“space junk”) is also being increasingly found.low-inclination and low-eccentricity orbits closest to Earthare among the most accessible spacecraft destinations in thesolar system. In terms of the propulsion energy required,more than 20% of the NEOs are known to be more accessible than the Moon for long-duration sample-return missions (Lewis and Hutson, 1993). The fact that many NEOsremain well within the inner solar system during their orbits further simplifies thermal-design and power-generationconsiderations for exploratory spacecraft (Perozzi et al.,2001). The proximity of NEOs also makes them prime targets for radar experiments designed to measure surface properties and achieve image reconstructions. Ostro et al. (2002)highlight the spectacular success of this technique and describe results for specific objects. Most renowned of thepractical importance of the NEOs is the small, but nonzero,probability of a major impact that could threaten civilization. The hazard issue is addressed in Morrison et al. (2002),and the physical properties of NEOs as they pertain to thehazard have been reviewed by Chapman et al. (1994) andHuebner et al. (2001).The purpose of this review chapter is to serve as a focalpoint for what we know about the physical properties ofNEOs and how these data serve to illuminate the myriadinterrelationships between asteroids, comets, and meteorites.Thus, in a way, we hope this chapter will serve as a “central node” in guiding the reader toward the interconnectionsthat NEOs have to a broad range of planetary-science topics(and chapters within this volume). In particular, we wish totake advantage of the scientific insights that can be achievedby virtue of their proximity: NEOs are the smallest individually observable bodies in our solar system. Thus, theseobjects, which reside at the crossroads of many differentareas of study, are also an end member to the size distribu-tion of measurable planetary worlds. Here we draw upon,build upon, and update previous reviews by McFadden etal. (1989) and Lupishko and Di Martino (1998).The terms used to refer to objects in the vicinity of Earthhave gone through a rapid convergence as interest in themhas increased over the past decade. When speaking broadlyof the population, “near-Earth objects” (NEOs) has becomethe most widely used term, since this inclusive label doesnot presuppose an origin or nature as an asteroid or a comet.When speaking about objects in the vicinity of the Earththat are presupposed to have an asteroidal origin, the term“near-Earth asteroids” (NEAs) is commonly used. In thischapter we attempt not to make any general suppositionsabout the origins of these bodies and therefore mostlyemploy the term “NEO.” Objects that appear “asteroidal”(starlike with no apparent coma or tail that would give themthe label “comet”) dominate the NEO population, with thecurrently known number having reasonably well-determinedorbits approaching 2000 (see Stokes et al., 2002). Onlyabout 50 short-period comets (Marsden and Williams, 1999)satisfy the NEO definition of having a perihelion distance 1.3 AU.Asteroidal NEOs are traditionally subdivided into groupsbased on their orbital characteristics a, q, Q (semimajor axis,perihelion distance, aphelion distance) with respect toEarth’s and are called “Amor,” “Apollo,” and “Aten” asteroids (Shoemaker et al., 1979). Amor objects are defined asbodies residing just outside the orbit of Earth (a 1 AU),having 1.017 q 1.3 AU. Objects having a semimajoraxis 1 AU and q 1.017 AU are known as Apollos. Relatively equal numbers of Amor and Apollo asteroids are currently known; combined they account for 90% of all currently known NEOs. Atens have orbits substantially insidethat of Earth (a 1 AU, Q 0.983 AU), and represent about8% of the known NEO population. (Short-period cometsaccount for the remaining 2%.) By these definitions, Atenand Apollo objects cross the orbit of Earth while Amor objects do not. However, orbital precession, periodic variations in orbital elements, and planetary perturbations overtimescales of centuries are sufficient for objects straddlingthe boundaries between groupings to change their affiliation. Milani et al. (1989) performed an orbital-evolutionanalysis involving 89 NEOs over a timespan of 200,000 yr.Based on these results, they propose six dynamical classes,named after the best-known and most representative objectin each class: Geographos, Toro, Alinda, Kozai, Oljato, andEros. This classification is indicative of long-term behaviorand, of course, differs from the Amor-Apollo-Aten nomenclature, which is based only on the osculating orbital elements. The name “Apohele” (the Hawai‘ian transliterationfor orbit) has been proposed (Tholen and Whiteley, 1998)for one additional group of objects whose orbits resideentirely inside that of Earth (Q 0.983 AU). At present only1998 DK36 (Tholen and Whiteley, 1998) has been discovered as a potential member of this class, although this resultis controversial due to uncertainties in the values of its or-

Binzel et al.: Physical Properties of Near-Earth Objects257bital elements. Michel et al. (2000) refer to these as “inner-Earth objects” (IEOs) and estimate that Atens and IEOstogether could constitute 20% of the multikilometer-sizedEarth-crossing population.Harris and Lagerros (2002), and Ostro et al. (2002). Ongoing updates to Table 1, as well as citations for the references to the individual entries, may be found at TABULATION OF NEAR-EARTH-OBJECTPHYSICAL PROPERTIES3. ANALYSISOver the past decade the growth in measurements ofNEO physical properties has increased at a pace nearlycommensurate with the increase in their interest and discovery rate. Physical parameters (such as spectroscopic androtation properties) were known for only a few dozen NEOsat the time of publication of Asteroids II (McFadden et al.,1989). An extension of this work is presented by Chapmanet al. (1994), and a more thorough review of NEO physicalproperties by Lupishko and Di Martino (1998) summarizesresults for about 100 objects, where the growth during thistime period can largely be credited to the work of WieslawWisniewski (Wisniewski et al., 1997). Since the Lupishkoand Di Martino review, a significant amount of new workhas pushed the number of NEOs having (at least some)physical characterization up to more than 300 objects (e.g.,Binzel, 1998, 2001; Erikson et al., 2000; Hammergren, 1998;Pravec et al., 2000a; Rabinowitz, 1998; Hicks et al., 1998,2000; Whiteley and Tholen, 1999; Whiteley, 2001).Table 1 presents an extensive summary of the currentlyknown physical parameters (derived primarily by spectroscopic and photometric techniques) for asteroidal NEOs.Objects are designated as belonging to the Amor (Am),Apollo (Ap), and Aten (At) groups. Mars-crossing (MC)objects of special interest are also included: 9969 Braille,encountered by the Deep Space 1 mission in 1999 (Oberstet al., 2001), and (5407) 1992 AX, a likely binary (Pravecet al., 2000b). For most objects, only approximate estimates(guesses) can be made for albedos and diameters. Therefore, analyses and conclusions based on these parametersmust be made with considerable caution. Taxonomic classesare from the system defined by Tholen (1984) and extendedto include the additional designations developed by Bus(1999; Bus and Binzel, 2002; Bus et al., 2002). When NIRspectral data are available such that the S-class subgroupsdescribed by Gaffey et al. (1993) are determined, taxonomicdesignations are given in this system. Rotational periods (inhours) are given if known, along with the range of lightcurve amplitudes represented by these measurements. Thefinal columns present U-B and B-V colors, when available.Physical measurements of NEOs are certainly not limitedto those parameters in Table 1, with the most exhaustiveadditional tabulations available in Lupishko and Di Martino(1998). These additional tabulations include information onindividual measurements of pole coordinates, senses ofrotation and asteroid triaxial shapes, photometric and polarimetric parameters, radiometric albedos and diameters, andradar parameters. More thorough information on some ofthese latter parameters is presented in Pravec et al. (2002),In this analysis of the currently known physical properties of NEOs, we focus on those properties that give the bestindication of origin. We particularly focus on the extent towhich asteroidal NEOs may be similar to or different frommain-belt asteroids in the same size range. Key differencesmay distinguish the relative importance of asteroid or cometorigins for the population. Size dependences in the spectral properties, for example, may also illuminate links forasteroid-meteorite connections.3.1.Taxonomy of Near-Earth ObjectsFigure 2 shows the relative abundance of various taxonomic classes of NEOs, as analyzed from the data in Table 1.Note that there are subtle differences between the asteroidtaxonomies derived by Tholen (1984) and Bus (1999), andthese differences affect some of the identifications in Table 1.(Taxonomic designations given are as cited in the publishedreference.)Almost all taxonomic classes of main-belt asteroids arerepresented among classified NEOs, including the P- andD-types most commonly found in the outer asteroid belt,among the Hilda and Trojan asteroids, or possibly amongcomet nuclei (see Barucci et al., 2002; Weissman et al.,2002). This broad representation of types, including thosefrom distant regions, suggests that the processes deliveringobjects to the inner solar system are broad in scope (seeBottke et al., 2002b; Morbidelli et al., 2002). A key question we appear to be on the verge of answering is this: Howsignificantly is the delivery of NEOs dominated by processes operating within the inner asteroid belt? S-type asteroids that dominate the inner asteroid belt also dominate thesampled NEO population by a ratio of 4:1 (Fig. 2). Thisratio, however, is subject to selection effects because S-typeasteroids have higher albedos than C-types, making theirdiscovery and observation more likely. (In a magnitude-limited survey, their higher reflectivity allows more S-asteroidsto be bright enough for detection.) Luu and Jewitt (1989)also point out that C-type asteroids fall off in their apparent brightness more rapidly with increasing phase anglethan do S-type asteroids (see Muinonen et al., 2002). SinceNEOs are typically discovered at larger phase angles, thecoupling of this phase-angle effect with the albedo effectcan create a strong bias in favor of S-type asteroids. Luuand Jewitt (1989) use a Monte Carlo model to estimate thisbias factor to be in the range of 5:1 to 6:1.While bias effects certainly are a major factor in creating the high proportion of S-types observed among NEOs,Lupishko and Di Martino (1998) argue that even after bias

258Asteroids IIITABLE 1. Physical parameters of NEOs (readers utilizing individual entries are reminded to cite theoriginal source for each datum; original source references for each datum listed here, as well ascurrent updates to this table, may be found at* McAuliffeOrpheusKhufuVereniaDon yapunovTaranisProvisionalDesignationGroupH (mag)†Albedo‡Diameter(km)§1898 DQ1911 MT1918 DB1924 TD1932 EA11949 MA1950 KA1951 RA1929 SH1948 OA1932 HA1948 EA1971 FA1971 UA1972 XA1953 EA1953 RA1968 AA1973 EC1950 LA1973 EA1960 UA1976 AA1977 HB1978 RA1975 YA1947 XC1978 SB1976 UA1977 RA1978 DA1981 QA1982 BB1981 ET31982 RA1983 TB1982 DV1981 CW1981 VA1982 HR1984 QA1983 RD1983 SA1986 EB1984 KD1984 KD1982 FT1985 PA1986 TO1982 XB1986 WA1980 PA1986 LA1979 VA1985 DO21989 AC1959 LM1982 TA1987 KF1989 WM1982 DB1980 WF1989 PB1988 TJ11990 MU1990 SQ1990 XJ1990 BG1991 VL1987 SL1990 DA1986 RA1990 160.530.020.170.1623. 2 0.410.07–0.09 0.1 .72––0.300.11–0.26 .10–0.3815.8 .810.87

Binzel et al.: Physical Properties of Near-Earth ObjectsTABLE 1.AsteroidNumber* 92 AX1992 AX1990 OA1991 FE1990 TR1992 WD51974 MA1993 EA1992 AC1980 AA1993 MF1983 RB1988 EG1991 TB11993 BW31984 KB1986 DA1992 HE1991 JX1991 OA1993 MO1993 VW1993 QA1992 LC1987 PA1989 JA1989 RS11991 VK1995 YA31992 TC1994 PC1994 PC11988 XB1991 CS1993 UC1994 LX1977 QQ51990 KA1992 LR1991 WA1994 AH21996 EN1987 OA1994 TW11991 EE1992 KD1992 SK1995 BL21989 ML1993 WD1992 CC11993 XN21998 YP111999 CV31989 UR1991 BB1999 GK41997 BR1991 DB1986 JK1994 QC1999 RH271993 QP1993 UB1997 WU221998 QS522000 LC161992 QN1999 YN41997 GH31994 LW1998 BZ71998 FX21998 YN11998 WT259(continued).GroupH . .25 .120.10–0.170.53–0.76 SBSSVSSQOUBSrSQLXQSSqSSrSBCXCSrSSqXSSQSqQ0.470.84

260Asteroids IIITABLE 1.AsteroidNumber* ).ProvisionalDesignationGroupH e(mag)1994 EF21998 EC31998 SF361998 DV91950 DA1998 PG1998 PG1998 PB11998 WT241991 VH1991 VH1998 BG91999 ND431999 TX161999 GU31999 GU31977 VA1978 CA1988 TA1989 DA1989 UP1989 UQ1989 VA1989 VB1990 HA1990 SA1990 UA1990 UP1991 AQ1991 VA1991 XB1992 BF1992 NA1992 UB1993 BX31993 TQ21994 AB1B1994 AW1B1994 AW11994 CB1994 GY1994 TF21995 BC21995 CR1995 EK11995 FJ1995 FX1995 HM1995 WL81996 BZ3B1996 FG3B1996 FG31996 FQ31996 JA11997 AC111997 AQ181997 BQ1997 GL31997 MW11997 NC11997 QK11997 RT1997 SE51997 TT251997 UH91997 US91997 VM41998 BB101998 BT131998 FM51998 HD141998 HE31998 KU21998 KY261998 ME31998 Mmhmhmm0. 55.6114.499.03d 248.580.121.160.27 0.15–0.4 0.32 0.096.25?20. 920.4220.4630.912.51922.408.6762.55530.3 .080.255.2270.39–0.89.05830.23 53.580.150.26.351.00.1780.3014.980.120.71

Binzel et al.: Physical Properties of Near-Earth ObjectsTABLE 1.AsteroidNumber* NameProvisionalDesignation1998 MQ1998 MT241998 MW51998 MX51998 QA11998 QC11998 QH21998 QK281998 QP1998 QR151998 QR521998 QV31998 SG2B1998 ST271998 ST491998 TU31998 UT181998 VD311998 VO1998 VO331998 VR1998 WB21998 WM1998 WP51998 WZ11998 WZ61998 XA51998 XS161999 CF91999 DJ41999 EE51999 FA1999 FB1999 GJ4B1999 HF1B1999 HF11999 JD61999 JE11999 JM81999 JO81999 JU31999 JV31999 JV6B1999 KW4B1999 KW41999 NC431999 PJ11999 RB321999 RQ361999 SE101999 SF101999 SK101999 SM51999 TA101999 TY21999 VM401999 VN61999 WK131999 XO351999 YB1999 YD1999 YF31999 YG31999 YK52000 AC62000 AG62000 AX932000 AE2052000 AH2052000 DO8B2000 DP107B2000 DP1072000 EB142000 EE142000 ES702000 ET70261(continued).GroupH 0.721.2QVXSqSSCSSqSqSkSqSXQSqSSkQSX1.70.94

262Asteroids IIITABLE 1.AsteroidNumber* NameProvisionalDesignation2000 EV702000 EW702000 GK1372000 HB242000 JG52000 JQ662000 NM2000 OG82000 PH52000 QW72000 RD532000 SM102000 SS164B2000 UG11B2000 UG112000 WH102000 WL1072000 YA2001 CB212001 CP362001 OE84B2001 SL9B2001 SL92002 BM26(continued).GroupH* “B” before an asteroid number indicates a possible binary asteroid. For such objects, a second line gives the orbital period (if known) and the lightcurve amplitude contribution of the binary.† “M” within this column indicates the value is from the Minor Planet Center (‡ When albedo is not estimated through physical measurements, an approximation is assigned based on the taxonomic class. These assumed albedos are coded as follows:d for “dark” (0.06), m for “medium” (0.15), mh for “medium high” (0.18), h for “high” (0.30). “m” is assigned in the case of no taxonomic information.§ When diameter is not directly measured or determined through physical measurements, as is the case for all objects assigned an albedo code, the diameter (D, in km) isestimated from the following relationship (Fowler and Chillemi, 1992): 2 log(D) 6.247 – 0.4 H – log (albedo).¶ Taxonomic class. See text in section 2 for the conventions used.60N (%)5040302010PDCTSQVRMAEXOUTaxonomic ClassFig. 2. Histogram of the relative proportions of measured taxonomic properties for more than 300 NEOs listed in Table 1. Almost all taxonomic classes seen among main-belt asteroids arerepresented within the NEO population. As detailed by Luu andJewitt (1989), strong selection effects favor the discovery and characterization of higher-albedo objects such as S-type (and possibly Q-type) asteroids. Within this histogram, the designation “C”includes both C-types and related subgroups (B, F, G). Those having unusual characteristics that do not fall into any present category, or classes (such as L, K) having 1% representation, arecombined within the designation “U.”corrections are accounted for, a clear signature for a dominant contribution from the inner asteroid belt remains.Benedix et al. (1992), Lupishko and Di Martino (1998), andWhiteley (2001) all find that after applying bias-correctionfactors to the observed NEO population, at any given sizethere are relatively equal proportions of C- and S-type objects within near-Earth space. However the main belt, in itsentirety, is dominated by C-types. [A bias-correction analysis of the main belt performed by Zellner (1979) suggeststhat C-types dominate among all main-belt asteroids by asmuch as 5:1.] The fact that C-types do not dominate theNEO population (even after strong bias correction) indicatesthat asteroidal NEOs are not being contributed equally byall regions of the main belt. Thus the inner regions of theasteroid belt, where S-types are most common (Gradie andTedesco, 1982; Gradie et al., 1989) must preferentially contribute to the NEO population. Benedix et al. (1992) pointout that the region of the 3:1 resonance has roughly equalpopulations of C- and S-type asteroids in its vicinity, making it a compatible source. Dynamical models (e.g., Migliorini et al., 1998; Morbidelli and Nesvorný, 1999; Vokrouhlický et al., 2000; Bottke et al., 2000, 2002a; Morbidelli etal., 2002) certainly support the view of the 3:1 resonanceand inner asteroid belt dominating the contributions to thenear-Earth population.General taxonomic and spectral links between the mainbelt and near-Earth populations have been proposed sincethe beginning of substantial studies of NEO properties (McFadden et al., 1984, 1985). Unique taxonomic classifications and mineralogic interpretations do show evidence forspecific ties to main-belt sources. Most notable among theseis the E-type object 3103 Eger, which appears both com-

Binzel et al.: Physical Properties of Near-Earth Objectspositionally and dynamically related to the Hungaria region(high-inclination objects) of the inner asteroid belt (Gaffeyet al., 1992). These authors also argue for a connection tothe enstatite achondri

covery rate. Physical parameters (such as spectroscopic and rotation properties) were known for only a few dozen NEOs at the time of publication of Asteroids II (McFadden et al., 1989). An extension of this work is presented by Chapman et al. (1994), and a more thorough review of NEO physical properties by Lupishko and Di Martino (1998) summarizes

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