WANDER among the asteroids and you'll see anything from tumbling, pitted mountains hundreds of kilometres across to groups of fast-spinning fragments. Now a new simulation suggests why: the larger ones are primordial "planetesimals", the original building blocks of the solar system, while their tiny cousins are chips flung out by collisions.
Bill Bottke at the Southwest Research Institute in Boulder, Colorado, and colleagues created a model of the history of collisions in the early solar system and compared it to the size, spin rate and orbits of many of the known asteroids. They also took into account the surface brightness of the bodies - a measure of how much they have been weathered by space dust over the aeons.
Their detective work indicates that there must have been a violent period of collisions in the belt about 4.6 billion years ago, as the Earth was forming. The model also suggests the asteroid belt contained around 250 times as much material then, but within just 10 million years the gravitational interference of the planets had slung most of it away. Only this combination exactly reproduces the asteroid belt we see today.
Bottke compares the work to forensic investigation. "Go to a crime scene and you can use the bone fragments and the blood spattered on the wall to piece together what happened," he says. "In this case the asteroids provide the clues that allow us to piece together how planet formation may have worked."
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