Is asteroid 951 Gaspra in a resonant obliquity state with its spin increasing due to YORP?

Abstract

Asteroid 951 Gaspra appears to be in an obliquity resonance with its spin increasing due to the Yarkovsky-O'Keefe-Radzievskii-Paddack effect (YORP effect, for short). Gaspra, an asteroid 5.8 km in radius, is a prograde rotator with a rotation period of 7.03 hours. A 3 × 106 year integration indicates that its orbit is stable over at least this time span. From its known shape and spin axis orientation and assuming a uniform density, Gaspra's axial precession period turns out to be nearly commensurate with its orbital precession period, which leads to a resonance condition with consequent huge variations in its obliquity. At the same time, its shape is such that the YORP effect is increasing its spin rate. YORP may be a reason for small asteroids entering resonances in the first place: They speed up or slow down and fall into resonances. The continued action of YORP probably ultimately causes asteroids to leave resonances, so that they are quasi-stable states.