ORIGIN OF THE SPACEWATCH SMALL EARTH-APPROACHING ASTEROIDS

Recent discoveries of small Earth-approaching asteroids by the 0.9 m S pacewatch telescope (referred to here as S-SEAs) reveal 16 objects whi ch have diameters similar to 50 m or smaller. Approximately half of th ese objects lie in a region where few large near-Earth asteroids are f ound, with perihelia (q) and aphelia (Q) near 1 AU, e < 0.35, and i fr om 0 degrees to similar to 30 degrees. Possible origins for these obje cts are examined by tracking the orbital evolution of test bodies from several possible source regions using an Opik-type Monte Carlo dynami cal evolution code, modified to include (a) impact disruption, based o n a map in orbital (a, e, i) space of collision probabilities and mean impact velocities determined using actual main-belt and near-Earth as teroid orbits, (b) fragmentation, and (c) observational selection effe cts. Amor asteroid fragments evolving from low eccentricity Mars-cross ing orbits beyond the q = 1 AU line provide a reasonable fit to S-SEA orbital data. Planetary ejecta from Mars is only consistent with low a nd moderately inclined S-SEA orbits. Asteroidal fragments from the mai n-belt via the 3:1 or nu(6) chaotic resonance zones rarely achieve low -e orbits before planetary impacts, comminution, or ejection remove th em from the system. This source could produce the observed moderate-to -high eccentricity S-SEAs. Plantary ejecta from the Earth-Moon system and Venus are only consistent with low-inclination S-SEA orbits. Moreo ver, constraints set by the planetary cratering record and the meteori te record suggest that the Earth, Moon, and Venus are unlikely to prov ide many S-SEAs. All of these results are predicated on the observatio nal bias computations (Rabinowitz, D.L. 1994. Icarus 111, 364-377) tha t provide the current definition of the S-SEA population. (C) 1996 Aca demic Press, Inc.