Origin of the spacewatch small earth-approaching asteroids

Recent discoveries of small Earth-approaching asteroids by the 0.9 m Spacewatch telescope (referred to here as S-SEAs) reveal 16 objects which have diameters ∼50 m or smaller. Approximately half of these objects lie in a region where few large near-Earth asteroids are found, with perihelia (q) and aphelia (Q) near 1 AU, e < 0.35, and i from 0° to ∼30°. Possible origins for these objects are examined by tracking the orbital evolution of test bodies from several possible source regions using an Öpik-type Monte Carlo dynamical evolution code, modified to include (a) impact disruption, based on a map in orbital (a, e, i) space of collision probabilities and mean impact velocities determined using actual main-belt and near-Earth asteroid orbits, (b) fragmentation, and (c) observational selection effects. Amor asteroid fragments evolving from low eccentricity Mars-crossing 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 and moderately inclined S-SEA orbits. Asteroidal fragments from the main-belt via the 3:1 or v₆ chaotic resonance zones rarely achieve low-e orbits before planetary impacts, comminution, or ejection remove them 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. Moreover, constraints set by the planetary cratering record and the meteorite record suggest that the Earth, Moon, and Venus are unlikely to provide many S-SEAs. All of these results are predicated on the observational bias computations (Rabinowitz, D.L. 1994. Icarus 111, 364-377) that provide the current definition of the S-SEA population.