Direct Detection of the Asteroidal YORP Effect

The Yarkovsky-O'Keefe-Radzievskii-Paddack effect is believed to alter the spin states of small bodies in the solar system. However, evidence for the effect has so far been indirect. Here we report precise optical photometric and radar observations acquired over four years of a small near-Earth asteroid (54509) 2000 PH5. We find the asteroid has been continuously increasing its rotation rate omega over this period by domega/dt = (2.0+/- 0.2) x 10-4 deg/day2. We simulated the close Earth approaches from 2001-2005, showing that gravitational torques cannot explain the observed spin-rate increase. Dynamical simulations also suggest that 2000 PH5 may reach a rotation period of ~20 seconds towards the end of its expected lifetime