Press release


Collisions between asteroids: simulation makes the process clear

Paris, November 22, 2001


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.

Researcher contact:
Patrick MICHEL
Laboratoire Cassini (CNRS - Observatoire de la Côte d’Azur)
Tel: + 33 4 92 00 30 55

CNRS-INSU Contact:
Philippe CHAUVIN
Tel: + 33 1 44 96 43 36 ;

Press contact :
Martine Hasler
Tel : +33 1 44 96 46 35
e-mail :