ABSTRACT
We consider an exact solution of 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 low--conductivity (``regolith--like'') surface layer covering a more conductive core. The penetration depth of the seasonal thermal wave in the low--conductivity surface material is typically larger than its geometrical thickness, a fact that has not been taken into account consistently by the previous simplified solutions. Our approach removes this drawback and the results indicate (i) a significantly increased seasonal mobility of the dekameter--sized (and larger) fragments with an insulating surface layer and (ii) a significantly decreased seasonal mobility of the meter--sized fragments with this structure. These results may affect the accuracy of simulations of the meteorite and NEA delivery to the Earth.