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 tex2html_wrap_inline12 and tex2html_wrap_inline14 respectively, and cover about tex2html_wrap_inline16 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 tex2html_wrap_inline18 km tex2html_wrap_inline20 and a circular polarization ratio of tex2html_wrap_inline22 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 tex2html_wrap_inline24 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.