Formation and Destruction of Small Binary Asteroids

Abstract

Our understanding of binary asteroids is currently undergoing major revisions. Until recently, all known small binaries (primary diameters ~=1 km) were near-Earth asteroids (NEAs), making it plausible that they originate in close encounters with terrestrial planets, and were not to be found among the main belt asteroids (MBAs). First simulations of tidal disruption have produced incidence of binaries that roughly matched the observed fraction of binary NEAs. Thermal-radiation effects, most notably YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack), were not needed to explain the life cycle of binary asteroids, despite some theoretical arguments in favor of significant YORP evolution. Recent discovery of abundant small MBA binaries and a more self-consistent reassessment of effects of planetary encounters have put the primacy of planetary encounters in doubt and shown a need for new theoretical explanations. Here I show that the mass loss from the primary's surface due to YORP could explain the formation of kilometer-sized binary NEAs. I also suggest that the small-separation NEA binaries' observed properties are consistent with evolution through ``binary YORP,'' rather than tidal stripping during planetary passages (when the pairs evolve beyond ~10 primary radii, encounters become important). Such a life cycle implies not only the extreme youth of small binary asteroids (<105 yr) but also short timescales for large-scale mass redistribution on most kilometer-sized asteroids, leading to a near-complete lack of craters on their surfaces.