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M. C. Nolan, E. S. Howell (Arecibo Obs.), G. Miranda (U. PR Mayagüez)
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.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.