Radiation-induced torques on spheroids

Radiation-induced torques on ellipsoids of revolution are discussed. Exact formulae for the thermal YORP torques are given in terms of elliptic integrals. It is demonstrated that in the absence of thermal inertia, the average values of dynamically significant projections of these torques are zero if a spheroid rotates around the axis of maximum inertia and if there are no resonances between rotation and orbital motion. The thermal lag leads to a systematic drift in the obliquity, but it does not affect the rotation period. The direct radiation pressure torques on spheroids are shown to be zero.