Radar Observations of Asteroid 7822 (1991 CS)
Abstract
We report Doppleronly (cw) radar observations of nearEarth 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 echoes achieve signaltonoise ratios in excess of 300 per day and thoroughly cover the asteroid in rotation phase (synodic period=2.39 h, obtained photometrically by P. Pravecet al.1998,Icarus136, 124153). A weighted sum 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 0.98Hz resolution cells on 1991 CS at echo powers greater than 2 standard deviations of the noise. Variations of ∼10 Hz in the echo's 2σ bandwidth are evident on each day and are consistent with the rotation period. Inversion of echo edge frequencies yields convex hulls of the poleon 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 poleon dimension of 1.3 km/cosδ, where δ is the angle between the radar lineofsight and the asteroid's apparent equator. The hulls suggest that 1991 CS has the least elongated poleon silhouette of any of the nine nearEarth 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 poleon silhouette on August 28 and 29, then 1991 CS's 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 Sclass taxonomy. The circular polarization ratio and radar albedo do not vary substantially as a function of rotation phase, suggesting that the nearsurface of 1991 CS is relatively uniform. Among the 20 Sclass asteroid radar detections previously reported, the circular polarization ratio of 1991 CS is greater than 11, comparable to 4, and less than 5.
 Publication:
 Icarus
 Pub Date:
 February 1999
 DOI:
 10.1006/icar.1999.6044
 Bibcode:
 1999Icar..137..247B
 Keywords:

 asteroids