05.07
Radar Observations of Near-Earth Asteroid 2063 Bacchus
L. A. M. Benner, S. J. Ostro, D. Choate, C. R. Franck, R. Frye, J. D. Giorgini, S. D. Howard, R. F. Jurgens, D. Kelley, R. Rose, K. D. Rosema, M. A. Slade, R. Winkler, D. K. Yeomans (JPL/Caltech), D. L. Mitchell (U. C. Berkeley), R. S. Hudson (Washington State Univ.)
We report Doppler-only (cw) and delay-Doppler radar observations of 2063
Bacchus (1977 HB) obtained at Goldstone at a transmitter frequency of
8510 MHz (3.5 cm) on 1996 March 22, 24, and 29. Weighted, optimally
filtered sums of cw and delay-Doppler echoes achieve signal-to-noise
ratios of 
and 
respectively, and cover about 
of rotation
phase (period = 14.90 hr; P. Pravec, pers. comm.). Our cw observations
place up to four 2-Hz-resolution cells on Bacchus at echo powers greater
than two standard deviations of the noise. Delay-Doppler observations
typically place about ten 75 m x 1 Hz cells on Bacchus above the same
threshold. A weighted sum of all cw spectra gives an OC radar cross
section of 
km 
and a circular polarization ratio of
The dispersion of the echoes in time delay indicates a
lower bound on Bacchus' maximum pole-on breadth of 0.7 km that is
consistent with the echo bandwidth of 
Hz and Pravec's rotation
period. Echo cw spectra on March 22 and delay-Doppler images on all
three days show a central deficit of echo power that indicates a
bifurcation in the shape. If we assume the projected area of Bacchus is the
same as that of a 0.7-km-diameter sphere, then its radar cross section
and absolute magnitude of 16.9 correspond to upper limits on the radar
and optical albedos that do not exclude Bacchus from any taxonomic
class. The circular polarization ratio is indicative of a near-surface
that is less rough at centimenter-to-decimeter spatial scales than the
average radar-detected near-Earth asteroid.