**DPS Pasadena Meeting 2000, 23-27 October 2000**

*Session 8. Asteroid Posters I - Physical Studies*

Displayed, 1:00pm, Monday - 1:00pm, Friday, Highlighted Tuesday and Thursday, 3:30-6:30pm, C101-C105, C211
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## [8.03] Asteroid depletion by collisions

*O.S. Barnouin-Jha, Andrew Cheng (The Johns Hopkins University Applied Physics Laboratory)*

A new numerical method is used to compute the distribution
of impacts on the surface of asteroids. This method traces
potential impactor orbits to asteroids and comets from
triangular elementary areas defined on a triaxial ellipsoid
representing a target asteroid. It then determines the
probability that the potential impactor collides with the
target surface area. Preliminary tests of this method show
on a spherical target asteroid: (1) expected changes in
impact flux as a function of the orbital semi-major axis,
(2) expected differences in trailing versus leading edge
impact flux as a function of position relative to the
asteroid belt, and (3) an expected sin(2*theta) impact angle
distribution over the entire asteroid.

The new numerical method also shows that impactors with low
orbital eccentricity and low inclination are far more likely
to collide with a target asteroid anywhere in the asteroid
belt. We performed a statistical analysis and determined
that relative to an assumed uniform random distribution of
asteroids, observed asteroid are in fact depleted at low
eccentricity and inclination.

We further examine the implications of this discovery using
our new computational method. We investigate the
distribution of impact velocities and crater production
rates on Ida, Mathilde and Eros. Preliminary results show
equilibrium crater densities are reached on Ida for D <
500 m within the lifetime of the asteroid belt, mainly by
very low impact velocities ranging from 0 to 1 km/s. Higher
average impact velocities on Ida are achievable if we
eliminate low eccentricity and inclination orbits from the
model. However, contrary to observation, Ida would not
achieve equilibrium crater densities for D < 500 m. We can
reconcile this discrepancy if low eccentricity and
inclination asteroids were present earlier in the history of
the solar system. If this scenario is correct, hypervelocity
impacts (> 5 km/s) on asteroids may be infrequent compared
to low velocity impacts of the distant past with
implications for the style of cratering, block formation and
fragmentation of asteroids.

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