An improved model of the seasonal Yarkovsky force for regolith-covered asteroid fragments

We derive a new analytical solution for the seasonal Yarkovsky effect, the mean-motion frequency mode of the recoil force due to reradiated sunlight, on a spherical asteroid fragment. The body is assumed to have a thin low-co nductivity (regolith-like) surface layer, covering a much more thermally co nductive core. If the penetration depth of the seasonal thermal wave in the low-conductivity surface material is larger than the regolith's geometrica l thickness, the previous simplified solution assuming a homogeneous interi or of the body might lead to wrong estimates on the intensity of the pertur bing force. Our approach removes this problem and the results indicate: (i) an increased seasonal mobility of 10-m sized and larger fragments with an insulating surface layer, and (ii) a decreased seasonal mobility of meter-s ized fragments with the same structure. These results may affect the accura cy of simulations of meteorite and NEA transport to the Earth, as well as t he dynamical evolution of some real small asteroids (e.g. 1566 Icarus).