Planetary impact rates from ecliptic comets

We have reevaluated the impact rates for the planets from ecliptic comets using the integrations in H. Levison and M. Duncan (1997, Icarus 127, 13-32; LD97), We fmd that the current impact rates on the giant planets are actually about four times smaller than LD97's values due to differences in methods of calculating the relevant timescales. The newly calculated impact rates are listed in Table I. However, if the objects leaving the Kuiper belt were primarily on high inclination orbits, then the impact rates on the giant planets are larger than those in Table I by a factor less than or similar to 2. We discuss the dynamics of objects hitting the giant planets in detail, including measurements of the impact velocities. We find that 21% of the objects that hit Jupiter in our simulations were bound to the planet before the impact. The fraction of bound impactors for Saturn is much lower, Jupiter, Saturn, and Uranus have a significant apex-antapex asymmetry for the unbound impactors. (C) 2000 Academic Press.