Directions and magnitudes of the averaged radiation forces of surface elements on sphere model and YORP model. A thermal conductivity K₁ = 0.0015 W m⁻¹ K⁻¹ (correspondingly Θ = 0.736) is adopted for both models. The direction and magnitude have been projected in the orbital (equatorial) plane here (see text). (a) The magnitudes of radiation forces of surface elements versus the delay angle (upper panel), and the frequency of delay angles of all surface elements (lower panel). The delay angle of a surface element is the angle between the radiation force averaged over a rotation period and the corresponding averaged force of the same surface element but with K = 0. Blue and orange indicate the results for sphere and YORP model, respectively. (b) The direction and magnitude of the radiation force at different latitudes on the surface of a sphere. The longitude 0° is at noon. The longitude of the black dot at certain latitude indicates that the radiation force at this latitude points to the direction given by the longitude. The size of black dots represents the magnitude of the radiation force (in N/m², see text) projected in the orbital plane. The yellow circle indicates the direction and magnitude (F/4πR²) of the total radiation force. The background colour gives the surface temperature.