Radar observations of asteroid 7822 (1991 CS)

We report Doppler-only (cw) radar observations of near-Earth asteroid 1991 CS obtained at Goldstone at a transmitter frequency of 8510 MHz (3.5 cm) on 1996 August 26, 27, 28, and 29. Weighted, optimally filtered sums of cw ec hoes achieve signal-to-noise ratios in excess of 300 per day and thoroughly cover the asteroid in rotation phase (synodic period = 2.39 h, obtained ph otometrically by P. Pravec et al. 1998, Icarus 136, 124-153). A weighted su m of all cw spectra gives an OC radar cross section of 0.24 +/- 0.08 km(2) and a circular polarization ratio of 0.28 +/- 0.001. Our observations place up to 50 0,98-Hz resolution cells on 1991 CS at echo powers greater than 2 standard deviations of the noise. Variations of similar to 10 Hz in the ec ho's 2-sigma bandwidth are evident on each day and are consistent with the rotation period. Inversion of echo edge frequencies yields convex bulls of the pole-on silhouette for each day. The hulls have a mean elongation and r ms dispersion of 1.18 +/- 0.02 and place a lower bound on the maximum pole- on dimension of 1.3 km/cos delta, where d is the angle between the radar li ne-of-sight and the asteroid's apparent equator The hulls suggest that 1991 CS has the least elongated pole-on silhouette of any of the nine near-Eart h asteroids for which similar shape information from radar observations is available. If we assume that the projected area of 1991 CS is the same as t hat of a sphere with a diameter of 1.1 km, equal to the lower bound on the minimum breadth of the pole-on silhouette on August 28 and 29, then 1991 CS 's radar cross section and absolute magnitude of 17.5 correspond to upper l imits on the radar and optical geometric albedos of 0.25 and 0.14 that are consistent with its S-class taxonomy. The circular polarization ratio and r adar albedo do not vary substantially as a function of rotation phase, sugg esting that the near-surface of 1991 CS is relatively uniform. Among the 20 S-dass asteroid radar detections previously reported, the circular polariz ation ratio of 1991 CS is greater than 11, comparable to 4, and less than 5 . (C) 1999 Academic Press.