L. A. M. Benner, M. W. Busch, S. J. Ostro, J. D. Giorgini, A. A. Hine, J. K. Harmon, M. C. Nolan, R. Rose, R. F. Jurgens, J. S. Jao, C. Magri, and J. L. Margot
We report Goldstone (8560-MHz, 3.5-cm) and Arecibo (2380-MHz, 13-cm) delay-Doppler radar images of 1992 UY4 that were obtained during August 1-10, 2005. The images achieve resolutions as fine as 7.5 m/pixel, place thousands of pixels on the object, and reveal a lumpy, modestly asymmetric, 2-km-diameter object whose shape resembles a meatball. The surface is characterized by gently undulating topography with many modest concavities. Numerous fine-scale, radar-bright features are evident at the trailing edges and limbs; one of the most prominent has a visible extent of about 100 m and juts out abruptly from the approaching limb, suggesting a large block similar to those seen on 25143 Itokawa by the Hayabusa spacecraft. The progression of features in the images is consistent with the photometrically-derived rotation period of 12.91 h and asymmetric lightcurve shape obtained by Warner et al. (2006, Minor Planet Bulletin 33, 20-21). If we assume that the diameter is twice the visible range extent, which would be true for a sphere, then the absolute magnitude of 17.6 and the 2-km-diameter correspond to an optical albedo of only ~0.04, strongly suggesting that 1992 UY4 is an optically-dark object. The radar data cover more than one-half of a rotation and the combined radar + photometric datasets should yield a detailed 3-D shape estimate.