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 $D \sim 50$ 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 $0°$ to $\sim 30°$. 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 Öpik-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.