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D. Pascu, J.R. Rohde, P.K. Seidelmann (USNO), E.N. Wells, J.L. Hershey (CSC/STScI), B.H. Zellner (GaSou), A.D. Storrs (STScI), D.G. Currie (UMd), A.S. Bosh (Lowell)
We obtained 39 HST/WFPC2 frames of the inner satellites of Neptune for astrometric, dynamical and photometric studies. These frames were taken in 3 HST orbits of 13 exposures each; 3 in the F439W filter, and 5 each in the F555W and F791W filters. Four of the six inner moons, discovered by Voyager 2, were detected. Images of Proteus and Triton were on all PC1 frames. In our photometric program, our strategy was to link the photometry of Proteus to that of Triton. In the first and second sets taken on 3 July 1997, Triton was near western elongation, while Proteus was near eastern elongation. In the third set, taken 3 days later, Triton was near eastern elongation, while Proteus was near western. By comparing similar exposures in the three sets, we found that Triton is 0.07 +/-0.02 mag brighter at western elongation (leading side) than at eastern elongation, with no change in color in (B-V) and (V-I). Our mean V-mag results for Proteus, V(1,0) = +5.6, agrees precisely with the Voyager results. However, we found that Proteus is more than 0.3 mag brighter in V on the leading side than its trailing side, resulting in a leading-side albedo of 0.07, and a trailing-side albedo of 0.05. Provisional relative magnitudes were also measured for the fainter moons. Comparing Proteus to Triton in (B-V), our preliminary finding is that Proteus is 0.2 mag bluer than Triton on its leading side, and 0.1 mag bluer on the trailing side. In (V-I), we found that Proteus is 0.1 mag bluer than Triton on its leading side, but slightly redder on its trailing side. Either Triton is at least 0.1 mag redder in (B-V) than its ground-based observations indicate, or Proteus, like Miranda, is an extraordinarily blue object.