Obliquities of "Top-Shaped" Asteroids May Not Imply Reshaping by YORP Spin-up

Abstract

The timescales over which the YORP effect alters the rotation period and the obliquity of a small asteroid can be very different, because the corresponding torques couple to different aspects of the object's shape. For nearly axisymmetric, "top-shaped" near-Earth asteroids such as 101955 Bennu, spin timescales are an order of magnitude or more longer than obliquity timescales, which are of order 10^5 to 10^6 yr. The observed low obliquities (near 0 or 180 degrees) of top-shaped asteroids do not constitute evidence that they acquired their present shapes and spins through YORP spin-up, because low obliquities are expected regardless of the spin-up or reshaping mechanism.