Detailed Hubble Images of the Surface of Asteroid Ceres

By Keith Cowing
October 12, 2001
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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

Analysis of the First Disk-Resolved Images of Ceres from Ultraviolet Observations with the Hubble
Space Telescope

Authors: Joel Wm. Parker, S. Alan Stern, Peter C. Thomas, Michel C. Festou, William J. Merline, Elliot F. Young, Richard P. Binzel, Larry A. Lebofsky

To appear in the
Astronomical Journal (January 2002). Also available at this http URL


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.

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