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J. Wm. Parker, S. A. Stern, W. J. Merline, M. C. Festou (SwRI), P. C. Thomas (Cornell U.), R. P. Binzel (MIT), B. Flynn (CEA), L. A. Lebofsky (LPL)
We present an analysis of Hubble Space Telescope/Faint Object Camera observations of the asteroid 1 Ceres. We observed Ceres for over 2 hours of total exposure time with far-ultraviolet (FUV: 1730 Å), near-ultraviolet (NUV: 2750 Å), and U (3400 Å) filters. Eight images (four FUV, two NUV, and two U) were taken during a single 9 hour rotation period. The NUV and U images have sufficient signal to allow study of the disk of Ceres, and data from all three bands were used to obtain integrated fluxes. With these data, we have been able to examine the shape of Ceres and its surface properties.
Ceres is well resolved, with approximately 30 FOC pixels across the diameter. Surface variations can clearly be seen. We determined the diameter and apparent shape parameters of Ceres by performing ellipse fits to each of the good-signal HST images. We obtain values for Ceres' projected semi-major and semi-minor axes of a=485 ±5 km and b=466 ±6 km, respectively. Our results are in good agreement with the sizes obtained from the occultation and adaptive optics observations. With our data, we also are able to create surface maps of Ceres, to analyze its albedo and color. We find albedo values that vary from 0.03--0.06 from the FUV to U bands in our dataset.
We will present out results at this meeting, and will discuss their implications to the homogeneity of Ceres and the various different pole positions estimated from other studies.