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E. Karkoschka (U. Ariz.)
Sizes of the nine innermost satellites of Uranus have not been established. Thomas et al. (1989, Icarus 81, 92) estimated sizes based on assumed albedos and concluded that only one of these satellites in a single image should have been resolved by Voyager 2. The measurement of this detection indicated that the satellite could be 40 % larger than estimated, but the uncertainty of this measurement was large.
I searched the Voyager data set and found a total of some 30 resolved images for seven of these nine satellites. The measured sizes are 40-150 % larger than the previously estimated ones. Mass estimates for these satellites, important for gravitational ring-satellite interactions, need probably to be tripled or quadrupled.
While all these satellites have been assumed to be spherical so far, the measurements indicate prolatenesses of 0.2, 0.4, and 0.6 for Portia, Juliet, and Belinda, respectively. These three satellites show orbital light curves of 0.1, 0.2, and 0.3 magnitude, respectively, in Hubble Space Telescope images taken by Tomasko and Karkoschka in 1997. Both independent observations are perfectly consistent with each other. There is also evidence for non-spherical shape of several of the other satellites.
The 15 regular satellites known before 1999 show a good correlation between diameter and orbital radius. The scatter in sizes from this trend is only about 10 % which may even be partially due to uncertainties in the sizes. However, the satellite discovered in 1999 by Karkoschka (IAUC 7171) is four times smaller than predicted from the observed trend. Is this an outsider or the largest member of another population of satellites?
This work is supported by STScI grant GO-07429.01-96A.