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SEARCHING FOR COMET CORES AMONG APOLLO/AMOR ASTEROIDS. C. A. Wood, SN4INASA Johnson Space Center, Houston, TX 77058

Other than occasional comets, Apollo and hor asteroids (AIA) approach the Earth more closely than any other celestial bodies, and hence are likely contributors to the flux of meteorites. Be- cause dynamic lifetimes of A/A objects are only lo7-lo8 years they must be continually resupplied from a longer-lived source. Orbital dynamical considerations appear to dictate that comets mst supply the majority of A/A1s, but, assuming that meteorites do come from A/A1s, current models of meteorite origins preclude this possibility. This standoff, between theories and models, has con- tinued for nearly 30 years, but sufficient data on A/A objects themselves are now available to consider the likelihood of a canetary or asteroidal origin for individual Earth-approaching objects. How to Identify Cometary Cores: Based upon previous discussions, extinct comets in A/A orbits are likely to be distinguishable from objects which escaped from the main asteroid belt by both physical and orbital characteristics (e.g., 1). Shape: Anders (2) argued that small objects such as Icarus (dia -1 km) could not maintain spherical shapes against collisional destruction if they had spent most of the last 4.5 b.y. in the asteroid belt. Collisional fragmentation would be much less likely in the Oort cloud, thus spheric- ity can be interpreted as evidence for a cometary origin of A/A objects. This criteria ignores the splitting of comets, which has only been observed in three shortperiod comets (13). Asteroid sphe- ricity is indicated by a flat light curve; light curve ranges are listed in Tedesco (3). Associated Meteor Streams: Sekanina (4) discovered that the orbits ofil the Apollo asteroid Adonis and the Sigma Capricornids meteor stream are so similar as to imply a likely evolutionary re1ationship." Because meteor streams are known to be derived from comets, Sekanina' s discovery strongly implies a canetary origin for A/Ais with associated meteor streams. Indeed the Apollo object Hephaistos has an orbit similar to that of comet Encke! Sekanina (4) and Drummond (5) have found that ten Apollos and three Amors can be associated with meteor showers; since most Amors do not cross Earth's orbit, detection of Amor-related meteor streams is difficult. Geocentric Velocity: Anders and Arnold (61, noting that Apollo asteroids fell into two groups differing in geocentric velocity, proposed that the low-velocity group was asteroidal and the high- velocity group was canetary. With more than three times as many Apollos now known compared to 1965, the bimodal grouping still exists; based on a minimum in the velocity histogram I propose that a geocentric velocity of 0.8 marks the division. Hi h e i Q: Because most main belt asteroids have low to moderate values of orbital eccentricity dhkffiic~ination(11, higher values are often considered (e.g., 14 to imply a cometary object. I consider i > 20' or e > 0.5 to suggest a cmetary origin. Similarly, Marsden (7) believes that the single most important factor for distinguishing asteroidal and cmetary orbits is the aphe- 1ian distance (Q). Only 95 asteroids (out of 2500) have Q > 3.9 AU, and no known comet has Q < 4.1 AU; thus Q > 3.9 AU is taken to indicate a possible cometary object. Reduction of Aphelion: As comets sublimate their icy mantles they can experience a nongravita- tional acceleration that reduces their aphel ion, Q. Sekanina (8) has applied a nongravi tational model to A/A objects, finding that six may have had their orbits changed by this process and hence may be cometary nuclei. Constant Eccentricity: Taking into account secular perturbations of orbital elements, Kozai (9) found that of all the asteroids only 11 (all A/A1s) have nearly constant values of e, and hence their aphelian and perihelion distances hardly vary. This is also a property of short period comets and hence Kozai proposes that these 11 are the remains of cometary nuclei. Discussion None of the eight characteristics described above is uniquely diagnostic of a cometary origin (except perhaps associated meteor streams), but each is unexpected for an asteroidal origin. In Table 1 A/A objects are ranked by the percentage of their known characteristics that are indicative of a cometary origin. Note that certain characteristics have not been determined for objects discovered since pub1 ication of particular research papers (e.g., 4,9), and that only orbital infona- tion is usually known for most recent discoveries. A remarkable result of this simple study is that 51 of the 64 known A/A objects have one or more cometary characteristics, and 17 (26%) are cmetary in half or more of their properties. Thus, we can conclude that comets probably contribute significantly to the A/A population, and hence, perhaps to our meteorite collections. The spectral types of the most canet-like AIA's(~~O%CC in Table 1)are not definite but include one possible ordinary chondrite, three S types, two C's, a diogenite, three U'S, and eleven unknowns. If all of these objects are derived from canets, then traditional concepts of comets as core-less dirty snowballs are incorrect. Likewise, it would seem impossible for such a diverse group of inferred chemical compositions to have fonned in the outer solar system, reinforcing ideas (10,11,12) that many bodies which formed in the inner solar system became cometary cores. Finally, there appears to be a difference between Apollos and Amors: 35% of Apollos have more than 50% cometary characteristics, whereas Amors are only 17% cometary. This implies that Apollos and Amors may not be a continuum artifically divided by the Earth's orbital radius. If so, derivation of Apollos from the asteroid belt by way of Amor orbits may be very inefficient and perhaps the majority of Apollos are extinct comets. References: (1) Kresak, L. (1977) In Comets, Asteroids, Meteors. Univ. Toledo, 313-321. (2) An- ders, t. (197n In "Phys. Studies Minor Planets" NASA SP-ZbI. 429-446. (3) Tedesco. E. F. (19771 In Asteroids, Univ. Arizona, 1098-1078. (4) Sekanina, Z. (1971 1- In "Ph s. Studies in or Planets" NASA -261, 423-428. (5) Drumnond, J. D. (1982) Icarus 49, 143-153. &I Anders, E. and J. R. Arnold (1965) -- Science 149, 1494-1496. (7) Marsden, B.-o Tn "Phys. Studies Minor Planets" NASA SP-267,

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Wood, C.A.

413-422;- (8) Sekanina, Z. (1973) Icarus 18, 253-284. (9) Kozai, Y. (1980) Icarus 41, 89-95. (10) Oort, J. H. (1950) Bull. Astron. mem11, 91-110. (8) Wood, C. A. (mui.Am. Astron. Soc. 9, 506. (12) Uood, C. A. and MendelT,A. W. (1982) Lunar Planet. Sci. XIII, 811-878. (13) mpc, F. L. (1982) IAU Repts. XVIII (in press). (14) Gehrels, T. et al. (191(5) Astron. J. 75, 186-1 95.

TABLE 1: COMETARY CHARACTERISTICS (CC) OF APOLLOS AND AMORS CC ' s Spectral CC's Spectral Apol 1o Known % CC Type Amor Known % CC Type

1979XB 4 .88 Quetzalcoatl 6 .66 Diogenite 1973NA 6 .83 Ganymede 6 .66 S Hephaistos 5 .80 1963UA 6 .66 1981VA 4 .75 Cuyo 8 .50 Adonis 7 .72 1 981QB 4 .50 Icarus 8 .69 A1 bert 7 .43 1974MA 5 .60 Betul ia 7 .36 cyu Mi das 5 .60 A1 inda 7 .36 U Sisyphus 5 .60 1978DA 6 .33 cysyu Orthos 5 .60 Ivar 7 .29 S Daedal us 6 .50 1980WF 4 .25 1 97 9VA 4 .50 1982FT 4 .25 Ari staeus 5 .40 Amor 5 .20 Tantal us 5 .40 Pele 6 .33 PL-6344 5 .40 An za 7 .29 C Apol 1o 8 .37 Anteros 6 .17 S Ra-Shal om 6 .33 Tezcatl ipoca 6 .17 U Antinous 6 .33 Lick 6 .17 1947XCl1979XA 6 .33 Eros 8 .12 OC ,S Hermes 7 .29 1972RB 5 .10 1950DA 7 .29 1982DV 5 0 Hathor 7 .29 1977VA 5 0 U 1982BB 4. .25 1353RA 5 0 S Aten 6 .25 1982XB 4 0 Cerberus 5 .20 1980PA 4 0 1978CA 6 .17 1980AA 4 0 H 1959LM 7 .14 1981 CW 4 0 Toro 8 .I2 1981 QA 4 0 Geographos 8 .12 1977RA 4 0 U PL-6743 5 .10 1954XA 6 0 Bacchus 5 0 1982DB 4 0 1982HR 4 0

Example: For Quetzalcoatl a1 1 of the diagnostic cometary characteristics except shape and associated meteor shower have been determined, and 4 (high e, i, Q and reduction in Q) are consistent with a cometary origin; hence % CC = 416 = .66. Note that a score of 0 could occur even for a cometary object so this listing does not distinguish comet AIAs from asteroidal ones, but rather objects with many cometary characteri s- tics from objects with few such properties.

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