Radar Observations Of Asteroid 2004 Xp14: An Outlier In The Near-earth Population

We report Goldstone radar (8560 MHz, 3.5 cm) observations of 2004 XP14 on 2006 July 3, 4, and 6 during its encounter within 1.1 lunar distances of Earth, the closest approach by any asteroid detected by radar. The asteroid has a same-sense to opposite sense circular polarization ratio of about 0.9, which is among the highest observed for any asteroid and a value indicative an extremely rough near-surface at decimeter spatial scales. Delay-Doppler images with resolutions of 19 m x 0.004 Hz reveal a nonspheroidal object with a visible range extent of about 130 meters, establishing a lower bound on the asteroid's maximum dimension; if the total range extent is double the visible extent, as would be true for a sphere, then 2004 XP14 is about 260 meters in diameter. The echoes reveal an extremely narrow bandwidth that is consistently 0.05 Hz during an interval when the asteroid's sky motion exceeded 50 degrees, establishing that the view was not pole on, that the subradar latitude was not far from equatorial, and thus that narrow bandwidth is due to very slow rotation, consistent with a lack of rotational motion in two-hour image sequences. The bandwidth and diameter suggest a rotation period of roughly 500 hours, marking 2004 XP14 among the slowest known rotators in the near-Earth asteroid population. The images achieve the finest Doppler resolution obtained for any asteroid and yield Doppler astrometry with a fractional precision of about 1 part in 10**8. 2004 XP14 clearly is an outlier and its properties underscore enormous diversity within the "Potentially Hazardous Asteroid" population.