New photometric observations of asteroids (1862) Apollo and (25143) Itokawa – an analysis of YORP effect

Aims. Asteroid (1862) Apollo is one of two asteroids in which the YORP effect was detected. We carried out new photometric observations of Apollo in April 2007 to enlarge the time line and to derive a more precise shape and spin state model. We also observed another YORP-candidate, asteroid (25143) Itokawa, in December 2006 and January 2007 to obtain a longer time line. An estimation of the YORP strength on Itokawa based on its precise shape model from the Hayabusa mission predicted the deceleration to be already observable during the 2007 apparition. Methods. We used the lightcurve inversion method to model the shape and spin state of Apollo. For Itokawa, the shape and pole direction are known to a high degree of accuracy from the Hayabusa mission, so we used a modified version of lightcurve inversion with only two free parameters – the rotation period and its linear change in time. Results. The new model of Apollo confirms earlier results. The observed acceleration of Apollo's rotation rate is $(5.5 \pm 1.2) \times 10^{-8}\,\mathrm{rad\,d}^{-2}$, which is in agreement with the theoretically predicted value. For Itokawa, the theoretical YORP value is sensitive to the resolution of the shape model and lies in the range from -2 to $-3\times 10^{-7}\,\mathrm{rad\,d}^{-2}$. This is inconsistent with results of lightcurve inversion that place an upper limit to the change of Itokawa's rotation rate ∼$1.5 \times 10^{-7}\,\mathrm{rad}\,\mathrm{d}^{-2}$.