Near-Critical Rotation of Binary Asteroid Primaries

About 15% of km-sized asteroids are binaries, typically with a rapidly spinning primary. It is widely accepted that these pairs formed as radiation forces accelerated the primary’s rotation beyond gravitational stability, It is still unknown whether the secondary forms through a catastrophic breakup or a steady mass loss, and whether the component masses and orbits of observed binaries are currently evolving or are in a stable state. It has been proposed by Jacobson and Sheeres (2011) that most observed binaries are in equilibrium between outward tidal evolution and inward BYORP radiation-driven migration. Observations (Scheirich et al. 2015, 2021) suggest that some systems may be in equilibrium while the others have evolving mutual orbits. Here we examine how the proximity of the primary’s rotation to the critical limit (i.e. breakup) may affect the long-term dynamics of the binary. We find that classical tidal theory needs major revision for the case of near-breakup rotation; this applies to all gravity-dominated bodies, including km-sized asteroids thought to be “rubble piles”. Near-critical rotation can enhance tidal effects as the effective gravity on the equator can be very weak for near-critically-rotating bodies. This “spin-boosting” of tides depends sensitively on the proximity to the exact rate of critical rotation, which is not always possible to determine from observations. We assess the potential strength of spin boosting in observed binaries and we find that according to the results of Pravec et al. (2016), when satellites are not in synchronous rotation, primaries are closer to the breakup spin rate. We will discuss the implication of this trend, and the possibility that mass transfer may be more important in some of these systems than long-term tidal interactions.