How Many Rubble Piles Are in the Asteroid Belt?

Abstract

We have developed a new version of the code built by Campo Bagatin et al. (1994a, Planet. Space Sci. 42, 1079–1092; 1994b, ibid., 42, 1099–1107) and Campo Bagatin (1998, Ph.D. thesis, University of Valencia) to model the collisional evolution of the asteroid size distribution. The new code distinguishes between “intact,” unfractured asteroids that did not undergo catastrophic collisions and asteroids converted by energetic collisions into reaccumulated bodies, or “rubble piles.” The distinction can also be made on a physical ground by assigning different collisional parameters to the two kinds of objects, with the objective of simulating the different responses to energetic impacts that rubble piles may have—due to their different structure—in comparison to unshattered bodies. Rubble–piles abundance when such targets are supposed to transfer less kinetic energy to the fragments turns out to be generally higher than monolithic asteroids. We have run a number of simulations of the collisional evolution process to assess the size range where reaccumulated bodies should be expected to be abundant in the main asteroid belt. We find that this diameter range goes from about 10 to 100 km, but may extend to smaller or larger bodies, depending on the prevailing collisional response parameters, such as the strength of the material, the strength scaling law, the fraction of kinetic energy of the impact transfered to the fragments, and the reaccumulation model. Both the size range and the resulting fraction of rubble piles vary widely, depending on the input parameters, which reflects the large uncertainties still present in the modelization of high-velocity impact outcomes. In particular, the simulations that take into account the derived “hydrocode” scaling laws (Davis et al. 1994, Planet. Space Sci. 42, 599–610) show that nearly 100% of the main belt asteroids larger than a few kilometers should be reaccumulated objects. On the other hand, the present code shows that the scaling law recently proposed by Durda et al. (1998, Icarus 135, 431–440) produces almost no rubble pile. This scaling law was proposed to match the actual population of asteroids, which it fails to do if collisional processes are accounted for in a self-consistent way.