Zero secular torque on asteroids from impinging solar photons in the YORP effect: A simple proof

YORP torques, where “YORP” stands for “Yarokovsky–O’Keefe–Radzievskii–Paddack,” arise mainly from sunlight reflected off a Solar System object and the infrared radiation emitted by it. We show here, through the most elementary demonstration that we can devise, that secular torques from impinging solar photons are generally negligible and thus cause little secular evolution of an asteroid’s obliquity or spin rate.