TANGENTIAL COMPONENT OF THE YORP EFFECT

This Letter discusses how re-emission of absorbed solar light by centimeter- to decimeter-sized structures on the surface of an asteroid can create a component of the recoil force that is parallel to the surface. Under certain conditions, the western sides of stones appears to be on average slightly warmer than their eastern sides, thus experiencing stronger recoil force and increasing the rotation rate of the asteroid. We study this effect, called the tangential YORP effect, in a toy model, replacing stones with walls and simulating heat conductivity in them. We discuss general trends of the effect, estimate its magnitude, and find it to be comparable to the normal YORP effect determined by gross-scale asymmetry of the asteroid. The existence of this effect would modify the predictions of the YORP acceleration of asteroids. In some cases, equilibrium between tangential and normal components of YORP is possible, resulting in a large number of asteroids expected to have no net YORP acceleration, which can be the case for Itokawa.