The asteroidal impact rate upon the terrestrial planets - an update

Up to May 1989, there were seven known Aten-type, 58 Amor-type, and 57 Apollo asteroids. For each of these asteroids, the probability of a collision with Mercury, Venus, earth, and Mars is calculated using the methods of Kessler (1981) and Steel and Baggaley (1985). In addition, the minimum and maximum possible encounter velocities are given. The results indicate that, for Mercury, there are eight possible impactors, with a mean collision probability of 1.30 x 10 to the -9th per year; for Venus, there are 32 planet-crossing asteroids, with a mean collision probability of 9.63 x 10 to the -9th per year; and for Mars, 116 possible impactors gave 3.41 x 10 to the -10th per year. The corresponding figures for are 65 and 6.93 x 10 to the -9th per year.