The Puzzle of Asteroid Families

Abstract

Seventy years after their discovery by K. Hirayama, asteroid families still provide a number of puzzling problems to solar system dynamicists, being at the same time a potential source of valuable information on the internal structure and the collisional history of bodies representing an intermediate stage in the growth process from planetesimals to major planets. Hirayama (1918) first pointed out the existence of several groups of asteroids whose orbital parameters were clustered at a statistically significant level, provided they had been properly corrected to account for planetary gravitational perturbations (the so-called ‘proper elements’ being the outcome of this delicate correction procedure). He also suggested that the origin of the asteroids belonging to the same family could be traced back to an unique event, namely a primordial explosive breakup of a parent body, ejecting fragments into independent heliocentric orbits with relative velocities much lower than their orbital speed. In the last few decades, physical studies have shown that the members of the most populous families bear some similarity in their surface composition (Zeliner et al., 1985) and their rotational behaviour (Binzel, 1987), supporting the hypothesis of a common origin and opening up for the first time the opportunity of directly probing via astronomical observations into the inner structure of the parent bodies. Moreover, collisional fragmentation has been proven to represent a plausible formation mechanism (Fujiwara, 1982; Farinella et al., 1982), both from the point of view of the probability of occurrence during the solar system’s lifetime, and with respect to the expected outcomes of such catastrophic events, which have been simulated in the laboratory by small-scale experiments on high velocity impacts against rocky bodies (Fujiwara, 1986).