Binary Asteroid Systems: Tidal End States and Estimates of Material Properties

TL;DR

This paper analyzes the tidal evolution and material properties of binary asteroid systems, revealing differences between main belt and near-Earth binaries and suggesting additional mechanisms influencing their separation.

Contribution

It provides new calculations of tidal end states, estimates of material strength, and discusses the evolution mechanisms of binary asteroid separations.

Findings

Main belt binaries are consistent with monolithic or fractured rock.

Near-Earth binaries must be mechanically weaker to evolve within their lifetime.

Inter-component separation may evolve via mechanisms like the binary YORP effect.

Abstract

The locations of the fully despun, double synchronous end states of tidal evolution are derived for spherical components. With the exception of nearly equal-mass binaries, binary asteroid systems are in the midst of lengthy tidal evolutions, far from their fully synchronous tidal end states. Calculations of material strength indicate that binaries in the main belt with 100-km-scale primary components are consistent with being made of monolithic or fractured rock as expected for binaries likely formed from sub-catastrophic impacts in the early solar system. To tidally evolve in their dynamical lifetime, near-Earth binaries with km-scale primaries or smaller must be much weaker mechanically than their main-belt counterparts even if formed in the main belt prior to injection into the near-Earth region. Small main-belt binaries with primary components less than 10 km in diameter, depending on their ages, could either be as strong as large main-belt binaries or as weak as near-Earth binaries because the inherent uncertainty in the age of a binary system can affect the calculation of material strength by orders of magnitude. Several other issues are considered, though these typically affect the calculation of material strength by no more than a factor of two. We also find indirect evidence within all three groups of binary asteroids that the inter-component separation may evolve via another mechanism(s) with the binary YORP effect being a likely candidate.