When an asteroid hundreds
of kilometers in diameter undergoes a collision with another body, can
it generate fragments that are both sizeable and travel at sufficiently
high velocities to become distinct? Patrick Michel (Cassini Laboratory
– CNRS – Observatoire de la Côte d’Azur) and his colleagues from
the Universities of Berne (Switzerland) and Maryland (USA) have just simulated,
for the first time, the process of a collision between large asteroids,
bringing into play both fragmentation and gravitation. The researchers
have developed simulations that reproduce the formation of families of
observed asteroids, and explain the presence of satellites around some
of them. This work, which should be continued, already constitutes a major
step forward in the understanding of processes of collision at such scales.
It plays an essential part in estimating the energy of the impact which
should be necessary, depending on its size, to deviate the course of an
asteroid on a path towards Earth. The results of this work are published
in the November 23, 2001 issue of Science.

By simulating the impacts of asteroids with diameters of several hundred
kilometers at typical speeds of 5 km/s, Patrick Michel and his colleagues
have shown that the asteroid is initially totally shattered. Subsequently,
gravitational attraction between the pieces leads to reaccumulations,
which finally form an entire family of large and small objects. The team’s
results show that all the large fragments must be aggregates of reaccumulated
small fragments, and that the collisions naturally produce satellites
around some of these fragments.

More than 20 families of asteroids have been observed in the region between
the orbits of Mars and Jupiter. Each one corresponds to concentrated groups
of small bodies that share identical physical properties. This means that
these families are made up of objects from the same parent body that was
destroyed on collision with another asteroid.

Fragmentation simulations have been developed using sophisticated numerical
codes, capable of reproducing experiments on centimeter scales. Until
now, they did not provide explanations of the dynamic properties and the
distribution of the sizes (or the mass) of members of asteroid families
simultaneously, which hindered our understanding of the collision process
at these scales. Moreover, a simple extrapolation of these experiments
to kilometer scale leads to a paradox. For a collision to generate bodies
as large as those of the members of asteroid families, the experiments
result in such low ejection velocities that no fragments could escape
from their initial position. In other words, although the parent body
is fragmented, the fragments do not disperse and no asteroid families
should exist. Conversely, for the fragments to attain sufficiently high
ejection velocities to form a dispersed group, the energy of impact has
to be so high that no large fragment could be produced. This is contrary
to what the families demonstrate.

The collisional origin of families of asteroids thus implies not only
that the parent body several hundred kilometers in diameter fragments
through crack propagation, but also that the fragments produced in this
way escape from the parent and then reaccumulate elsewhere to form aggregates
that will make up the largest members of the families.

The work of Patrick Michel and his team involved an explicit simulation
of the fragmentation of a large asteroid simultaneously with the gravitational
evolution of the debris generated . To do this, the researchers used sophisticated
numerical codes that they developed to calculate the fragmentation of
a rock and then the gravitational attraction between the hundreds of thousands
of fragments over several days. This allowed them to study two types of
event, from the weakest to the most catastrophic impact, that lie at the
origin of two actual, clearly identified families of asteroids. Simulating
the impacts successfully reproduced the expected properties and showed
that the parent body is, first of all, typically totally shattered into
small fragments. After this, gravitational interaction between the fragments
causes reaccumulations. This leads to the formation of a family of clearly
distinct small and large objects that form aggregates. In addition, satellite
systems form around certain asteroids. This work shows that satellite
formation is a frequent, natural phenomenon when a collision occurs, and
it explains the existence of the satellites we observe and why new discoveries
are on the increase.

According to this research, the majority of asteroids larger than one
kilometer cannot be purely solid monoliths, but are, rather, aggregates
of rocky blocks, because most of them originate from larger bodies that
were destroyed by past collisions. Although these simulations are still
based on hypotheses and on many parameters which the team of researchers
will continue to explore, this breakthrough in the general understanding
of the collisional phenomenon will already help to refine the evolutionary
models of populations of small bodies. It should enable estimations to
be made of the energy of the impact required to deviate asteroids on a
path towards Earth.


Collisions and Gravitational Reaccumulation: Forming Asteroid Families
and Satellites, Science, Novembre 23, 2001.


Patrick MICHEL
Laboratoire Cassini (CNRS – Observatoire de la Côte d’Azur)
Tel: + 33 4 92 00 30 55
E-mail: michel@obs-nice.fr

CNRS-INSU Contact:

Philippe CHAUVIN
Tel: + 33 1 44 96 43 36 ;
E-mail: Philippe.Chauvin@cnrs-dir.fr

Press contact:

Martine Hasler
Tel : +33 1 44 96 46 35
e-mail : martine.hasler@cnrs-dir.fr