Thermal Inertia of Near-Earth Asteroids and Magnitude of the Yarkovsky Effect

Abstract

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. Measurements of thermal inertia provide information on the presence or absence of loose surface material, such as thermally insulating regolith or dust. Such information is not only important for scientific studies of asteroid surface properties, but it is also vital for the design of lander missions and in the development of technology for the deflection of hazardous asteroids. At present, very little is known about the thermal inertia of asteroids in the km size range. Here we report on a method that has allowed us to derive a mean value for the thermal inertia of near-Earth asteroids on the basis of multi-wavelength thermal-infrared observations. We obtain a mean value of 200 ± 50 J m^-2s^-0.5K^-1 corresponding to a surface thermal conductivity of 0.03 ± 0.01 W m^-1K^-1. 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.