The formation and collisional/dynamical evolution of the Ida/Dactyl system as a part of the Koronis family

The Ida/Dactyl system is the first confirmed asteroid/satellite pair as well as the first family asteroid to be studied in detail by a spacecraft. We explore consequences for Ida of formation by disruption of the Koronis parent body. A significant flux of projectiles onto Ida in a few years following the formation of the family is found, based on models of the disruption of the Koronis parent body by Marzari et al. (Marzari, F., D. Davis, and V. Vanzani 1995. Icarus 113, 168-187). This flux generated craters at a rate much higher than the current crater production rate; however, the flux decreases rapidly as fragment orbits become randomized. Also, we compare the figure of Ida with figures of equilibrium fluid bodies and find that the interior of Ida is nearly stress-free and that Ida could be a rubble pile. Finally, collisional models predict that Dactyl would have been shattered several times in the past 2 billion years, the lower bound on Ida's age derived from its cratered surface. However, some of the ejecta from such disruptions would be trapped in orbit about Ida, subsequently re-accumulating into a satellite. This process could explain the rather regular shape of Dactyl as well as explaining how Dactyl now exists, given its short (compared with Ida's age) collisional disruption lifetime.