Detailed Hubble Images of the Surface of Asteroid Ceres

In a paper set to appear in the January 2002 issue of the Astronomical Journal, astronomers report Hubble Space Telescope images taken of asteroid 1 Ceres that show surface detail at a resolution of ~50 km. Among the surface features they have observed is a large, ~250km diameter surface feature for which they propose the name “Piazzi” (after the asteroid’s discoverer). It is presently uncertain if this feature is due to a crater, albedo variegation, or other effect.

° Link to paper in various formats

We present HST Faint Object Camera observations of the asteroid 1 Ceres at near-, mid-, and far-UV
wavelengths (l = 3636, 2795, and 1621 Angstroms, respectively) obtained on 1995 June 25. The disk of Ceres
is well-resolved for the first time, at a scale of ~ 50 km. We report the detection of a large, ~ 250 km
diameter surface feature for which we propose the name “Piazzi”; however it is presently uncertain if
this feature is due to a crater, albedo variegation, or other effect. From limb fits to the images, we obtain
semi-major and semi-minor axes of R₁ = 484.8 ± 5.1 km and R₂ = 466.4 ± 5.9 km, respectively, for
the illumination-corrected projected ellipsoid. Although albedo features are seen, they do not allow for
a definitive determination of the rotation or pole positions of Ceres, particularly because of the sparse
sampling (two epochs) of the 9 hour rotation period. From full-disk integrated albedo measurements,
we find that Ceres has a red spectral slope from the mid- to near-UV, and a significant blue slope
shortward of the mid-UV. In spite of the presence of Piazzi, we detect no significant global differences
in the integrated albedo as a function of rotational phase for the two epochs of data we obtained. From
Minnaert surface fits to the near- and mid-UV images, we find an unusually large Minnaert parameter
of k ~ 0:9, suggesting a more Lambertian than lunar-like surface.