We have investigated numerically the evolution of near-Earth asteroids (375
3) Cruithne, (3362) Khufu, 1989 VA, (10563) 1993 WD, and 1994 TF2 that ente
r the coorbital regions of the Earth or Venus. We show that the coorbital l
ibration modes identified in F. Namouni (1999, Icarus 137, 293-314) and F.
Namouni et al. (1999, Phys. Rev. Lett. 83, 2506-2509) are strong enough to
survive tens of thousands of years under planetary perturbations in the Sol
ar System. We give a detailed analysis of the nature of libration modes as
functions of the argument of perihelion. The effects of close approaches wi
th the coorbiting planet as well as neighboring planets are characterized.
Based on these results, we argue for a steady-state flux of objects tempora
rily captured in coorbital configurations with the planets. (C) 2000 Academ
ic Press.