The formation of an asteroid family results from the break-up of such a body, which creates hundreds of thousands of fragments, certain of which could become dangerous asteroids and meteorites.

These findings also show that the impact energy during a collision is highly dependent upon the internal structure of the target; this information is very useful for the development of a strategy of defense against the threat of an impact with the Earth.

The researchers' results are published in the February 6, 2003, issue of Nature and are featured on the journal's cover.

In the asteroid belt, which is located between Mars and Jupiter, asteroid families are concentrated groups of small bodies that share the same spectral properties.

More than 20 families have been identified, each family believed to be fragments resulting from the break-up of a large parent body in a regime where gravity, more than the material strength of the rock, is the key factor.

The actual size and velocity distributions of the family members provide the main constraint for testing our understanding of the break-up process in this gravitational context. A new asteroid family, which bears the name of its largest member, Karin, was recently identified and studied.

It is the youngest family discovered to date, and appears to have resulted from a collision around 5 million years ago. This family provides a unique opportunity to study a collisional outcome that is relatively unaffected by phenomena such as collisional erosion and the dynamic diffusion of fragments, which, over time, alter the properties resulting directly from the collision.

Patrick Michel of the Cassini Laboratory (Observatoire de la C�te d'Azur � CNRS) and two of his colleagues from the Universities of Bern (Switzerland) and Maryland (USA), have developed numerical simulations of collisions with the aim of determining the classes of events that make it possible to reproduce the main characteristics of the Karin family.

As the results depend to a large degree on the internal structure of the parent body, they were able to show that this family must have resulted from the break-up of a body that was originally full of fracture and/or empty zones, rather than a purely monolithic body.

Their findings moreover indicate that all the members of this family are aggregates formed by the gravitational re-accumulation of smaller fragments, and that certain of them could have been ejected on trajectories that cross the Earth's trajectory. Since those families that are already known and the oldest families share similar properties, the authors suggest that they are likely to have had a similar history.

This information concerning the internal structure of large asteroids also has consequences for the impact energy that would destroy them. This is useful not only to estimate the lifetime of these objects in the asteroid belt, but also in order to develop strategies that aim to redirect such a potentially dangerous asteroid.

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