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Centre National de la Recherche Scientifique
Paris, France

Researcher contact: 
Patrick Michel
Observatoire de la Côte d'Azur
e-mail: michel{at}obs-nice.fr
Tel: +33 4 92 00 30 55

Contact INSU:
Philippe Chauvin
e-mail: philippe.chauvin{at}cnrs-dir.fr
Tel: +33 1 44 96 43 36

Press contact:
Martine Hasler
Tel: +33 1 44 96 46 35
e-mail: martine.hasler{at}cnrs-dir.fr

February 6, 2003

Simulations of collisions shed light 
on the internal structure of asteroids
======================================

An international team of researchers led by Patrick Michel
(Observatoire de la Côte dAzur  CNRS, Nice) have carried out 
simulations of asteroid collisions. For the first time, such 
simulations have made it possible to provide information about the 
internal structure of asteroids and, in particular, have shown that 
the parent bodies from which asteroid families have originated must 
have been fragmented (and non-monolithic) bodies or stacked rocks.
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 Côte 
dAzur  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.

Reference:
P. Michel, W. Benz & D.C. Richardson, Disruption of
fragmented parent bodies as the origin of asteroid
families, Nature Vol. 421, 608-611, 2003. 

For more information about asteroid collisions, see: 
Press release dated November 22, 2001
   http://www.cnrs.fr/cw/en/pres/compress/collisionsAsteroides.htm

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