Size Frequency Distribution of Near--Earth Objects and of Lunar Craters

Kurzfassung

The threat of destructive impact events on Earth is mirrored in more than 160 proven impact structures and is even more evident on the Moon's surface. To assess the threat posed by near--Earth asteroid (NEA) impacts on the Earth, it is necessary to know either the size frequency distribution (SFD) of objects in Earth--approaching orbits and the evolution of their orbits, or the crater production rate derived from the crater SFD of the Moon, recorded over the lifetime of their surfaces. In this work we estimated the complete NEA population up to an absolute magnitude H <= 20 based on the known NEA population at the end of 1999. Therefore we deduced re--detection rates for the observations of the LINEAR search program listed by the MPC (Minor Planet Center) and compared this results with re--detection rates derived from a list of detection in 1999 provided by E. Bowell. We tested the plausibility of our estimations of the total population of NEAs combined with estimations based on NEAT and Spacewatch (Rabinowitz et al., 2000) as a function of H with a projectile SFD derived from lunar crater SFD (Neukum, 1983; Neukum and Ivanov, 1994). A very good fit is obtained in the projectile diameter range 0.04 km -- 20 km. We principally reconfirmed the results of Rabinowitz et al. (2000) for NEAs with diameter D >= 1 km, but with higher accuracy and from a different data set: N(D >= 1 km) = 725 +/- 25. These results imply that the shape of the SFD of NEAs seen as a snapshot of the average flux of impactors onto the Moon has not changed since the final stage of the late heavy bombardment period.