Thermal Inertia of near-Earth Asteroids and Strength of the Yarkovsky Effect

Thermal inertia is the physical parameter that controls the temperature distribution over the surface of an asteroid. It affects the strength of the Yarkovsky effect, which causes orbital drift of km-sized asteroids and is invoked to explain the delivery of near-Earth asteroids (NEAs) from the main belt. Moreover, measurements of thermal inertia provide information on the presence or absence of loose surface material, such as thermally insulating regolith or dust. At present, very little is known about the thermal inertia of asteroids in the km size range. Using an extensive dataset of thermal infrared observations obtained at the Keck 1, the ESO 3.6m and the IRTF telescopes, we find that the mean thermal inertia of near-Earth asteroids in the km-size range is 200 ± 50 J m-2 s-0.5 K-1 corresponding to a surface thermal conductivity of 0.03 ± 0.01 W m-1K-1. Combining this result with published values of asteroid thermal inertias, we also identify a trend of increasing thermal inertia with decreasing asteroid size. As a consequence, the dependence of the Yarkovsky-induced semimajor axis drift rate on object diameter, D, departs from the 1/D dependence commonly assumed in models of the dynamical evolution of asteroids