Dynamics of Binary Near-Earth Asteroid System (35107) 1991 VH

Near-Earth Asteroid (35107) 1991 VH was discovered to be a binary in March 1997, based on its light-curve (IAUC 6607). It made a very close approach to the Earth in August 2008 at a distance of 0.045 AU. We used this opportunity to secure an extensive radar data set with the Arecibo S-band (2380 MHz, 13 cm wavelength) planetary radar system, including range-Doppler images with spatial resolution as fine as 15 m. The images (spanning 14 days) reveal that the primary is roughly spheroidal with a radius of 650 m. The range extent of the secondary in these images varies from less than 100 m to more than 200 m indicating that it is highly elongated. The radar data provide an excellent determination of the mutual orbit: The orbital period is 32 hours, the eccentricity is 0.05, and the total system mass is 1.5e12 kg. Numerical simulations of the spin of the elongated secondary in this eccentric mutual orbit reveal a large region of chaos in the phase space, similar to that observed in Saturn’s moon Hyperion (Wisdom, Peale, Mignard 1984). The chaotic region surrounds the 1:2, 1:1, 3:2 and 2:1 spin-orbit resonances, but allows for islands of stability around the 1:2 and 1:1 spin-orbit states. The secondary’s echo bandwidths indicate that its spin rate indeed lies within or very close to this chaotic region. To date no acceptable fit to the sequence of secondary images has been found under the assumption of synchronous spin. Saturn’s moon Hyperion is the only solar system object known so far to have a chaotic spin state (Wisdom, Peale, Mignard 1984).