Delivery of meteorites from the asteroid belt.

The process of the delivery of meteorites to the surface of the Earth from plausible source regions such as the asteroid belt is currently understood in general terms, but important uncertainties and conflicts remain to be resolved. Stochastic effects of the rare disruptions of large asteroids on the population of meteorite-sized Earth-crossing asteroids can change the flux and the proportions of compositional types in the infalling meteorite population. These changes can be significant in magnitude over timescales of 10⁸ years. Changes of the order of 1% can be expected on timescales of 10⁵-10⁶ y, consistent with small differences between the Antarctic meteorites and modern falls. The magnitude of changes depends strongly on poorly-understood details of collisions. Asteroids 951 Gaspra and 243 Ida were recently imaged by the Galileo spacecraft. I use a numerical hydrocode model to examine the outcomes of various size impacts into targets the sizes of these asteroids. A shock wave fractures the asteroid in advance of crater excavation flow; thus, for impactors larger than 100 m, impacting at 5.3 km s⁻¹, tensile strength is unimportant in these bodies, whether they are initially intact or are "rubble piles". Because of the shock-induced fracture, impact results are controlled by gravity. Therefore these asteroids are much more resistant to catastrophic disruption than predicted by previous estimates, which had assumed that strength was controlling these processes for rock targets. Fracture of km-size asteroids is different from fracture in terrestrial experiments using few-cm targets. The composition distribution of delivered meteorites depends on the outcomes of such asteroid impacts