Co-orbital Asteroids of Earth as Candidates for Asymmetric Impactors on the Moon

The observed leading/trailing asymmetry of rayed lunar craters may be owed to impactors having lower average encounter velocity than that of the current model of Near Earth Objects [NEOs] (Ito and Malhotra 2010). We are investigating the possibility that currently undetected co-orbital asteroids of Earth may partially account for the missing low velocity impactors. First, we obtain the dynamical lifetimes of asteroids in the co-orbital region of Earth's orbit. We adopt the realistic model of the eight major planets of the solar system and examine a wide range of initial orbital parameters of Earth co-orbital asteroids: semi-major axis, eccentricity, inclination, and relative longitude; we compare our results with those of a previous study (Tabachnik and Evans 2000, Evans and Tabachnik 2002). Then, we trace the escaping objects from the co-orbital region and determine their impact velocity with the Moon. We confirm that these objects have typical impact velocities much lower than the average impact velocity of NEOs. We estimate the current co-orbital population that may account for the leading/trailing asymmetry of lunar rayed craters. We discuss mechanisms for the possible origin and re-supply of co-orbital asteroids and the role of hypothetical primordial co-orbitals on the lunar crater asymmetry.

This research is supported by NSF grant AST-0806828.