ASTEROID SIZE DISTRIBUTION IMPLICATIONS FOR THE ORIGIN OF STONY-IRON AND IRON METEORITES

The diameter-frequency distribution of large asteroids is analyzed, based on polarimetric and radiometric albedo data. The population of carbonaceous objects shows a high degree of fragmentation. But a class of apparently istony-iron asteroids shows a relative excess frequency between diameters of 100 to 200 km. It is proposed that the latter objects are predominantly metallic cores of a group of chemically differentiated planetesimals all of which (except Vesta) have suffered catastrophic collisions. Collision frequencies in the asteroid belt may have been much greater than previously supposed.