Pristine fragments of asteroid breakup. (Astronomy).

Planetary scientists have for the first time precisely dated a collision that smashed an asteroid into fragments. The breakup is so recent--just 5.8 million years ago--that the pieces probably haven't been altered by contact with one another, other space debris, or cosmic rays.

These pristine fragments could reveal how easily an asteroid breaks apart and whether a space rock on a collision course with Earth could be destroyed by a nuclear blast. The fresh collision may also shed light on planet formation, notes David Nesvorny of the Southwest Research Institute in Boulder, Colo.

He and his colleagues searched for clusters of asteroids that have similar orbits and surface reflectivity. The most intriguing duster, dubbed Karin, is composed of 39 rocks ranging in diameter from 2 to 19 kilometers and lies in the outer part of the asteroid belt.

The chunks belong to a huge group of asteroids that were probably generated by the collision of two large bodies a few hundred million years ago. But the 39 fragments are more alike and much more tightly clustered than other members of the family, Nesvorny says. He and his Southwest colleagues describe their analysis in the June 13 Nature.

From the orbits of 13 of the fragments, the team inferred that the cluster was originally part of a 25-kilometer-wide asteroid that broke apart. Researchers may be able to determine whether the relatively unweathered Karin-cluster members are solid fragments or rubble piles. That knowledge could prove vital for protecting Earth from future asteroid threats because a rubble structure would be harder to destroy with a nuclear blast (SN: 7/28/01, p. 61).--R.C.