 Ida originated in the breakup of the roughly 120 km ( 75 mi ) diameter Koronis parent body . The progenitor asteroid had partially differentiated , with heavier metals migrating to the core . Ida carried away insignificant amounts of this core material . It is uncertain how long ago the disruption event occurred . According to an analysis of Ida 's cratering processes , its surface is more than a billion years old . However , this is inconsistent with the estimated age of the Ida – Dactyl system of less than 100 million years ; it is unlikely that Dactyl , due to its small size , could have escaped being destroyed in a major collision for longer . The difference in age estimates may be explained by an increased rate of cratering from the debris of the Koronis parent body 's destruction . 
