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L. A. M. Benner, S. J. Ostro, K. D. Rosema, J. D. Giorgini, D. Choate, R. F. Jurgens, R. Rose, M. A. Slade, M. L. Thomas, R. Winkler, D. K. Yeomans (JPL/Caltech)
We report Doppler-only (cw) radar observations of 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 echoes achieve signal-to-noise ratios in excess of 300 per day that thoroughly cover the asteroid in rotation phase (synodic period = 2.39 h, obtained photometrically by Pravec et al., submitted). A weighted sum of all cw spectra gives an OC radar cross section of 0.24±0.08 km2 and a circular polarization ratio of 0.28±0.001. Our observations place up to fifty 0.98-Hz resolution cells on 1991 CS at echo powers greater than two standard deviations of the noise. Variations of ~10 Hz in the echo's 2-sigma bandwidth are evident on each day and are consistent with the rotation period. Inversion of echo edge frequencies yields convex hulls of the pole-on silhouette for each day. The hulls have a mean elongation and rms dispersion of 1.18±0.02 and place a lower bound on the maximum pole-on dimension of 1.3 km/cos(delta), where delta is the angle between the radar line-of-sight and the asteroid's pole direction (which is unknown). The hulls suggest that 1991 CS has the least elongated pole-on silhouette of any of the eight near-Earth 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 that 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' radar cross section and absolute magnitude of 17.5 correspond to upper limits 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 radar albedo do not vary substantially as a function of rotation phase, suggesting that the near-surface of 1991 CS is relatively uniform. Among the twenty S-class asteroid radar detections previously reported, the circular polarization ratio of 1991 CS is greater than eleven, comparable to four, and less than five.