The impact origin of Eunomia and Themis families

Abstract

Criteria for finding asteroid families (Zappala et al. 1995) are applied to a large (205,770 member) data set of asteroid orbital elements. The cases of the Eunomia and Themis families are considered as examples. This is combined with the cratering criteria for catastrophic disruption of small bodies in the solar system (Leliwa-Kopystyński et al. 2008). We find that the Eunomia parent body itself was not catastrophically disrupted in the family-generating impact event; after impact, the current body contains as much as 70% of its primordial mass. However, by contrast with Eunomia, the present mass of 24 Themis is only about 21% of that of its primordial body. Limits are placed on the sizes of the impactors in both examples, and for the case of Eunomia, the radius of the just sub-critical crater (which may be present on 15 Eunomia) is predicted as <58 km.