Where do the Apollo objects come from?

Abstract

Monte Carlo calculations have been carried out to trace the orbital evolution of ∼1- to 45-km-diameter bodies produced by collisions in the asteroid belt interior to 2.6 AU. It is found that about 3% of the asteroidal fragments ejected at 100 m/sec in this size range are perturbed into Earth-crossing orbits as a consequence of the 3:1 Jovian commensurability resonance at 2.5 AU and the ν dot 6 secular resonance in the innermost asteroid belt. For ejection velocities of 100 m/sec the calculated steady-state number of Apollo objects (454) and Amor objects (1600) brighter than absolute V magnitude 18 is in the general range of the estimated number based on observations. Considering the uncertainties in both the theoretical results and the observational data, this should be considered to be good agreement. Even though the initial Earth-crossing orbits of these bodies are concentrated near 1 AU, their steady-state orbital distribution is widely dispersed. There is strongly suggestive, but not yet compelling, evidence for an additional component of Apollo-Amor objects that are inactive comets. It is found that a modest production rate (∼1 in 6 × 10 4 years) of extinct comets with V(1,0) < 18 in Encke-like orbits is required to maintain a steady-state population of cometary Apollos equal in number to the asteroidal Apollos.