Radar Images of Binary Asteroid 2003 YT1

On 2004 May 1--4 we observed near-Earth asteroid 2003 YT1 with the Arecibo Observatory Planetary Radar system. The images show the object to be a binary system. Based upon range extents in the delay-Doppler images, the primary is 1100 ± 200 m diameter and the secondary is 210 ± 60 m diameter.

On 2003 May 2, the centers of mass of the two bodies were at their maximum observed separation, 2700m apart, an estimate of the orbit scale. The orbit is eccentric, with a period of about 30h, but not easily determined in detail from the radar data alone.

The primary shows a generally round shape, but with large surface features, several hundred meters across. Its rotation rate is rapid: no longer than 2.6h, and consistent with the 2.3h period determined by Pravec et al. The secondary has a rotation rate no longer than 6h, inconsistent with synchronous rotation.

The timescale for damping to synchronous rotation is of order 10⁸ years, due to the small size of the secondary. The secondary could not easily have achieved the observed orbit by tidal spindown from a spin-induced fission. Yet the primary is rotating near the breakup rate.