A thermophysical analysis of the (1862) Apollo Yarkovsky and YORP effects

Abstract: Near-Earth asteroid (1862) Apollo has strong detections of both orbital semimajor axis drift and rotational acceleration. We produce a unified model that can accurately match both observed effects using a single set of thermophysical properties derived from ground-based observations, and we determine Apollo's long term evolution. We use light-curve shape inversion techniques and the ATPM on published light-curve, thermal-infrared, and radar observations to constrain Apollo's thermophysical properties. The derived properties are used to make detailed predictions of Apollo's Yarkovsky and YORP effects, which are then compared with published measurements of orbital drift and rotational acceleration. The ATPM explicitly incorporates 1D heat conduction, shadowing, multiple scattering of sunlight, global self-heating, and rough surface thermal-infrared beaming in the model predictions. We find that ATPM can accurately reproduce the light-curve, thermal-infrared, and radar observations of Apollo, and simultaneously match the observed orbital drift and rotational acceleration using: a shape model with axis ratios of 1.94:1.65:1.00, an effective diameter of 1.55 +/- 0.07 km, a geometric albedo of 0.20 +/- 0.02, a thermal inertia of 140 +140/-100 J m-2 K-1 s-1/2, a highly rough surface, and a bulk density of 2850 +480/-680 kg m-3. Using these properties we predict that Apollo's obliquity is increasing towards the 180 degree YORP asymptotic state at a rate of 1.5 +0.3/-0.5 degrees per 10^5 yr. The derived thermal inertia suggests that Apollo has loose regolith material resting on its surface, which is consistent with Apollo undergoing a recent resurfacing event based on its observed Q-type spectrum. The inferred bulk density is consistent with those determined for other S-type asteroids, and suggests that Apollo has a fractured interior.