Radar Observations of Asteroid 2063 Bacchus

Abstract

We report Doppler-only (cw) and delay-Doppler radar observations of Bacchus 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 ∼80 and ∼25, respectively, and cover about 180° of rotation phase (period=14.90 h; Pravec et al. 1998). 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 0.5-μs (75-m)×1-Hz cells on Bacchus above the same threshold. A weighted sum of all cw spectra gives an OC radar cross section of 0.12+0.06−0.02 km2 and a circular polarization ratio of 0.21±0.01. The dispersion of the echoes in time delay indicates a lower bound on Bacchus' maximum pole-on breadth of 0.6 km that is consistent with the echo bandwidth (6±2 Hz) and rotation period. Echo spectra on March 22 and delay-Doppler images on all three days show a central deficit of echo power that provides strong evidence for a bifurcation in the shape. Inversion of delay-Doppler images, cw spectra, and optical lightcurves obtained at Ondr̆ejov Observatory yields single-lobe and two-lobe models that define lower and upper bounds on the degree of bifurcation. Both shape models have a prominent central concavity, modestly asymmetric shapes, and similar physical dimensions, spin vectors, and radar and optical geometric albedos. We adopt the more conservative single-lobe shape model as our working model and explore its implications. It has a radar-derived sidereal rotation period Psid=15.0±0.2 h and a north pole within a few tens of degrees of ecliptic longitude λ=24° and ecliptic latitude β=−26°; retrograde rotation is likely. It has dimensions of 1.11×0.53×0.50 km, an effective diameter (the diameter of a sphere with the same volume as the model) Deff=0.63+0.13−0.06 km, and radar and optical geometric albedos σ=0.33+0.25−0.11 and pv=0.56+0.12−0.18, respectively, that are larger than most estimated for other asteroids. Bacchus' low circular polarization ratio and high radar albedo are consistent with nearly regolith-free ordinary chondrite and basaltic achondrite compositions, but its high optical geometric albedo seems inconsistent with an ordinary chondrite composition and may favor a V-class composition. Bacchus has less structural complexity at centimeter-to-decimeter spatial scales and its near-surface is more dense (either more metal, lower porosity, or both) than the average radar-detected near-Earth asteroid.