IMPACT EJECTS ROTATIONAL BURSTING AS A MECHANISM FOR PRODUCING STABLEIDA-DACTYL SYSTEMS

The existence of Dactyl, the small satellite of asteroid 243 Ida, pres ents an intriguing paradox: if exposed to the same projectile bombardm ent as Ida, it should have been disrupted long ago. To solve this para dox, it has been proposed that either Ida (and the entire Koronis fami ly) is relatively young (approximate to 100 Myr) or Dactyl has reaccre ted many times from its own debris after having been disrupted. Here w e propose a third alternative, that is that Dactyl is much younger tha n Ida and it was formed by rotational bursting of a precursor fragment ejected from Ida after an impact. We discuss some recent experimental results showing that sizable fragments from shattered targets do unde rgo rotational bursting and are fissioned after traveling over a lengt h of several target diameters; the relative speed between the fission remnants is comparable to the initial ejection velocity. Then we have performed a number of numerical integrations of the orbits of fictitio us particles resulting from an assumed rotational bursting event in th e gravitational held of Ida; the results show that, depending on the i nitial conditions, up to several percent of the particles get trapped into stable satellite-like orbits resembling that of Dactyl. We conclu de that this mechanism may have formed Dactyl in the last approximate to 10% of Ida's lifetime, either after an energetic cratering impact o r (more probably) after a collision which shattered Ida without disper sing most of its fragments ''to infinity.'' (C) 1998 Academic Press.