Planetary scientists have long realized that many asteroids are just pieces of their former selves. But now astronomers have identified fragments from an asteroid-belt smashup that happened only 5.8 million years ago. Apparently, a 25-kilometer main-belt asteroid was shattered by a much smaller body, estimated to be 3 km wide, hitting it at a speed of perhaps 5 km per second. A team led by David Nesvorný (Southwest Research Institute, Boulder, Colorado) has identified 39 pieces of debris: a group of asteroids in similar orbits. The largest, 832 Karin, is 19 kilometers wide.

Nesvorný and his colleagues describe in the June 13th Nature how they found the cluster by carefully sifting through tens of thousands of known asteroid orbits and looking for similarities. The 39 bodies wouldn’t show any clustering in a snapshot of the asteroid belt taken today. But in a 3D graph plotting their orbits' semimajor axes, eccentricities, and inclinations, the family clearly stands out.

By extrapolating the slowly changing orientation of 13 of the orbits backward in time, the team found that they were exactly aligned with each other 5.8 plus or minus 0.2 million years ago. Clearly that’s when the group formed by the collisional breakup of a parent body.

At least 20 other asteroid families are known, all of which probably resulted from breakups. In fact, the Karin cluster is part of the much larger Koronis family. But this is the first time a collision has been firmly dated. In the case of the other, much older families, the orbits have been changed too much by subsequent collisions, gravitational disturbances, and radiation pressure to trace them back to a convergence.

The Karin-cluster asteroids must have large, fresh areas on their surfaces that are only 5.8 million years old. Spectroscopic studies could provide what amount to views inside an asteroid in these areas, for comparison with older, more space-weathered surfaces. And the cluster may tell a lot about collision dynamics, an important issue for studies of planet formation. “This new cluster will no doubt be the focus of attention for the asteroid community for some time,” writes Derek Richardson (University of Maryland) in the same issue of Nature.