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

Abstract

Context. The Yarkovsky effect, which causes orbital drift, and the YORP effect, which causes changes in rotation rate and pole orientation, play important roles in the dynamical and physical evolution of asteroids. Near-Earth asteroid (1862) Apollo has strong detections of both orbital semimajor axis drift and rotational acceleration. Aims. 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. Methods. We use light-curve shape inversion techniques and the advanced thermophysical model (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. Results. 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 • YORP asymptotic state at a rate of 1.5 +0.3 −0.5 degrees per 10 5 yr. Conclusions. 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. The YORP effect is acting on a much faster timescale than the Yarkovsky effect and will dominate Apollo's long term evolution. The ATPM can readily be applied to other asteroids with similar observational data sets.

Physical characterisation of near-Earth asteroid (1620) Geographos

“…Like (1862) Apollo in Rozitis et al (2013), Geographos has three measurements of Yarkovsky semimajor axis drift: -17.7 ± 5.9 m yr −1 by Chesley et al (2008), -37.4 ± 9.0 m yr −1 by Nugent et al (2012), and -27.2 ± 9.0 m yr −1 by Fig. 3.…”

“…The methodology used here is exactly the same as that presented in Rozitis et al (2013). To determine the asteroid thermal emission, the ATPM is used to compute the surface temperature variation for each shape model facet during a rotation by solving the 1D heat conduction equation with a surface boundary condition that includes direct and multiple scattered solar radiation, shadowing, and re-absorbed thermal radiation from interfacing facets.…”

“…Durech et al (2008b) has measured Geographos's YORP rotational acceleration to be (1.5 ± 0.2) ×10 −3 rad yr −2 , which we use for model comparison. As in Rozitis et al (2013), the overall YORP rotational acceleration, dω/dt(D, f R , ρ), can be predicted using…”

“…The Yarkovsky effect results in a drift in the semimajor axis of an asteroid's orbit, and the YORP effect changes its rotation period and the direction of its spin axis. Both effects are of fundamental importance for the dynamical and physical evolution of small asteroids in the solar system (see review by Bottke et al 2006 and introduction of Rozitis et al 2013). Furthermore, asteroid bulk densities can be determined from model-to-measurement comparisons of the Yarkovsky semimajor axis drift.…”

“…These observations complete Geographos's data set, and allow a full thermophysical analysis of its Yarkovsky and YORP effects to be performed using the same methodology as that presented in Rozitis et al (2013) for (1862) Apollo. In the following sections, we present results from application of the advanced thermophysical model (ATPM; Rozitis & Green 2011, 2012, 2013a, which explicitly incorporates 1D heat conduction, shadowing, multiple scattering of sunlight, global self-heating, and rough surface thermalinfrared beaming, to this complete data set in order to constrain Geographos's thermophysical properties and bulk density.…”