A trio of new studies is helping to fill in astronomers' view of how asteroids near Earth orbit the sun and how they ended up there in the first place. The picture emerging is one in which asteroids in the belt between Mars and Jupiter collide, shatter and then clump together into families, migrate into regions of space where the gravity of Jupiter can jar them loose from their orbits and finally take up residence close to Earth. "It's pretty clear that [near-Earth asteroids] come from the main asteroid belt between Mars and Jupiter," explains Joseph Stuart of the Massachusetts Institute of Technology, an author of one of the reports. These new studies, all published in the current issue of the journal Science, are "refining the details of how that might happen."

Stuart, who compiled data from the Lincoln Near-Earth Asteroid Research (LINEAR) project, found more than 1,200 kilometer-size rocks orbiting the sunmore than recent counts have detected. Their orbits are also more highly angled relative to the plane that Earth sweeps out as it circles the sun than previous surveys had revealed. It's key for astronomers to know how many such objects there are and how they revolve around the sun in order to assess the risk that one might collide with Earth, he notes. While the greater angle between Earth's orbit and the asteroids' could translate to a lower collision risk, Stuart says he thinks the odds probably won't deviate much from earlier estimates. "There are a couple of different factors that pull in opposite directions and probably balance out," he remarks.

To get at the origins of Earth's rocky neighbors, Patrick Michel of the Observatoire de la Cte d'Azur in France and colleagues simulated what would happen if a large asteroid shattered. The researchers found that such an object would fragment into a range of smaller pieces but that gravity would pull the larger chunks back together, leaving one big asteroid surrounded by a family of smaller rocks. Until now, the authors write, scientists had assumed that collisions were responsible for asteroid families, but they didn't have a good grasp of the physics underlying these events.

Still, this violent formation doesn't account for all the properties of asteroid families, according to William Bottke of the Southwest Research Institute in Boulder and his co-workers. They point out that some clusters are nearly split into two separate groups composed of different-sized pieces and that some members sit on the cusp of resonancesareas where a planet's gravity periodically tugs the rocks as they orbit. Such orbits can only last a few million years, which is nothing compared with an asteroid's billion-year age. The team's calculations indicate that after the initial collision, the asteroid fragments will enter a slow and steady drift whose speed depends on each piece's size. The rocks get a gentle push from sunlight they absorb and then emit back into space. Resonances will capture some of these family members and eject them from their orbit, causing them to eventually hit the sun or a planet, or leave the solar system entirely. Stuart says this result provides good evidence that such drifting, rather than the collision that generates a cluster, is responsible for moving asteroids into a resonance.