An international team of
researchers led by Patrick Michel (Observatoire de la Côte d’Azur
– 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
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.

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

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

Contact INSU:
Philippe Chauvin
e-mail: philippe.chauvin@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@cnrs-dir.fr