Spin vector alignment of Koronis family asteroids

Studies of asteroid families-groups of asteroids that formed from the fragmentation of larger bodies-are of broad interest to solar system researchers because they can provide insights into collisional processes, as well as the interior structures, strengths, and compositions of asteroids. It is generally accepted that members of the Koronis family were created by collisional disruption of a homogeneous parent body 1 and therefore share the same formation age and subsequent collisional history. The temporal variations in observed brightnesses of the Koronis family members (a consequence of their rotation) are, however, larger than expected(2). Preferential alignment of spin vectors had been proposed 2 as a possible explanation, but recent modelling 3 predicted that family formation yields random spin vectors among the resulting fragments. Both hypotheses have been untested by observations. Here I show that the actual distribution of spin vectors among the largest members of the Koronis family falls within markedly nonrandom 'spin clusters'. Reconciling models of family formation and evolution with the unexpected alignments of spin obliquities and correlations with spin rates presents a new challenge in understanding asteroid collisional processes.