The near-earth object population

We examine the dynamics of a sample of 117 near-Earth objects (NEOs) over a time scale of 60 Myr. We find that while 10-20% end their lifetimes by striking a terrestrial planet (usually Venus or Earth), more than half end their lives in a Sun-grazing state, and about 15% are ejected from the Solar System. The median lifetime of our (biased) sample is about 10 Myr. We discuss the exchange of these objects between the various orbital classes and observe the creation of orbits entirely interior to that of Earth. A variety of resonant processes operating in the inner Solar System, while not dominant in determining the dynamical lifetimes, are crucial for understanding the orbital distribution. Several dynamical mechanisms exist which are capable of significantly increasing orbital eccentricities and inclinations. In particular, we exhibit important new routes to the Sun-grazing end-state, provided by the nu(5) and nu(2) secular resonances at high eccentricity between a = 1.3 and 1.9 AU. We find no dynamical reason to demand that any significant component of the NEO population must come from a cometary source, although such a contribution cannot be ruled out by this work. (C) 2000 Academic Press.