For the first 700 million years of their existence, the moon and Earth and the other rocky planers took a beating. Space debris hammered these bodies so fiercely that their surfaces were stripped away. Moon samples brought back by the Apollo missions in the early 1970s confirmed that this violent era ended about 3.85 billion years ago. But researchers haven't known the form of that early debris.The rocks came from the asteroid belt, which lies between the orbits of Mars and Jupiter. They ranged in diameter from about 100 meters to tens of kilometers, assert Robert G. Strom of the University of Arizona in Tucson and his colleagues in the Sept. 16 Science.

From the sizes of the craters, Strom's team determined the diameters of the projectiles that created them. The researchers then analyzed a compilation of recent asteroid surveys that provide the first detailed information on asteroids as small as 1 km in diameter. The scientists then compared the sizes of existing asteroids with the sizes of the projectiles that cratered the moon, Earth, Mars, Venus, and Mercury eons ago.

To the team's delight, the mix of sizes was identical to that of asteroids that now reside in the belt. The researchers conclude that an ancient population of belt asteroids did the damage the cratering record in the inner solar system changed dramatically after the initial violent era and indicates that most of the impactors since then have been near-Earth asteroids, less than 20 km across. These asteroids came from the main belt but were long ago nudged into Earth-crossing orbits.--R.C.