34th Meeting of the AAS Division on Dynamical Astronomy, May 2003
2 Small Bodies
Oral, Monday, May 5, 2003, 10:40am-12:10pm,

## [2.04] The Impact Frequency of Near-Earth Asteroids

A. W. Harris (Space Science Inst.)

The impact probability of a given asteroid with the Earth depends strongly on the details of its orbit, i.e. orbits which intersect the Earth's orbit at low inclination or nearly tangent at perihelion or aphelion have much higher impact probabilities than more steeply crossing orbits. Since the Earth may clear out objects in such orbits faster than they are supplied, the actual distribution of such orbits may be highly non-random, and thus the collision frequency from the actual equilibrium population is difficult to model theoretically. I have taken a direct approach to estimating the current impact flux by using the sample of all discovered NEAs with absolute magnitude less than 18.0 (approximately 1 km in diameter or larger). This provides a large enough sample, more than 600 objects, to provide good statistics of impact frequency, and yet is relatively unbiased since it is expected to be more than half complete of all existing NEAs that large. Of this sample, approximately half are Earth-crossing asteroids (have perihelia less than 1.0 AU and thus can impact the Earth at some time in the normal precessional cycle of their orbits. I compute the circumstances of passages of all these objects within 0.1 AU of the Earth over a period of one century (in the future to remove any bias from the discovery apparition). The frequency of passages within 0.1 AU can be easily scaled to obtain the frequency of passages within the Earth's radius, with allowance for gravitational focusing, and thereby obtain an estimate of the impact frequency of the actual observed population of NEAs. The result is that the ''per object'' impact frequency of all NEAs (q < 1.3 AU) is 1.7 x 10-9 y-1. An associated result is the RMS impact velocity, which is about 20 km/sec.

This research will hopefully be funded by the Planetary Geology and Geophysics Program of NASA, when 2003 funding finally gets released.