Provenance 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 D similar to 50 m or smaller. Approximately half of these objects lie in a region where few large near Earth asteroids are found; they have the following orbital parameters: perihelia (q) and aphelia (Q) near 1 AU, e < 0.35, and i from 00 degrees to similar to 30 degrees. Given these size and dynamical constraints, we assess the possible origins for these objects by tracking the orbital evolution of test bodies from several possible source regions using an Opik-type Monte-Carlo dynamical evolution code. We have modified this code to include (a) impact disruption, based on a map in orbital 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. We find that Amor asteroid fragments evolving from low eccentricity Mars-crossing orbits beyond the q = 1 AU line provide the best 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 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. Planetary ejecta from the Earth-Moon system and Venus, are only consistent with low-inclination S-SEA orbits.