Asteroids and Their Collisional Disruption

Abstract

The collisional process between small bodies is one of the key processes in the formation and evolution of a planetary system. Asteroids are the remnants which kept the memory of collisional processes that took place in different regions of our Solar System during its past and present history. Telescopic observations have collected data of such records, such as the ones provided by asteroid dynamical families and their related dust bands. Meteorite meteorites, micrometeorites, and interplanetary dust particles (IDP IDPs), which are pieces of asteroids or comets collected on Earth, provide also information on the material properties of these small bodies, although they may only tell us about the strongest components capable of surviving the entry in Earth’s atmosphere. In order to understand the collisional process, impact experiments have been performed in laboratory, using as targets terrestrial rocks whose mechanical properties are similar to those of some meteorite meteorites. The results of experiments together with numerical simulations and theoretical considerations have led to the conclusion that most asteroids smaller than several tens of kilometers in size have experienced major impact events, during which they have been at least severely shattered so that cracks and voids could be formed in their interior. For those who underwent a catastrophic disruption catastrophic disruption as a result of a collision at high impact energy, the outcome has been the formation of an asteroid family asteroid family, some of which are still identifiable in the main asteroid belt main asteroid belt, MB. During such an event, the largest fragments that originate from the parent body can be large enough to undergo gravitational re-accumulations, so that at the end of the process, the cluster of fragments larger than a few hundreds of meters resulting from such a disruption is mostly composed of gravitational aggregates or rubble piles. Spacecraft explorations of multi-kilometer asteroids, namely 951 Gaspra Asteroid:951 Gaspra, 243 Ida Asteroid:243 Ida, and 433 Eros Asteroid:433 Eros, who belong to the S taxonomic class taxonomic class – the dominant class in the inner Solar System – revealed that the surface of these bodies are shaped by impact processes, and that the bulk density density (2.6 and 2.67 g/cm3 for Ida Asteroid:243 Ida and Eros Asteroid:433 Eros, respectively) is generally lower than the supposed grain density density of their material. However, direct evidence of a rubble pile structure has not been obtained, as the only information on their internal structure are inferred mostly from their surface properties. Conversely, in spite of its small self-gravity, the sub-kilometer asteroid 25143 Itokawa Asteroid:25143 Itokawa explored by the JAXA Hayabusa Hayabusa spacecraft in 2005 is the first S-class asteroid whose porosity porosity is estimated to be as high as 40% (with a bulk density density of 1.9 g/cm3) and thus is considered to be a gravitational aggregate formed by reaccumulation of smaller pieces. The boulders on Itokawa Asteroid:25143 Itokawa have shapes and structures similar to those of laboratory rock fragments, suggesting some universal character of the disruption process. Since more and more asteroids are believed to have substantial porosity porosity, current studies on the collisional disruption of solid bodies are to be extended to porous bodies, taking into account microporisity effects which have been neglected so far. Such porous bodies are not only present in the asteroid populations (Near-Earth Objects, main belt, and Trojan asteroids) but they are also supposed to constitute the populations evolving in the outer Solar System (Kuiper belt objects Kuiper belt objects) and beyond (long-period comets). Thus, understanding the collisional process for different kinds of material appears crucial to determine its influence in the history of different populations of small bodies.