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L.H. Wasserman, C.B. Olkin, O.G. Franz (Lowell Observatory)
We will present the first resolved observations of Pluto and Charon with a well--calibrated interferometric instrument: Fine Guidance Sensor #3 (FGS3) on the Hubble Space Telescope. We made 5 observations (HST orbits) of Pluto/Charon over a 4.4 day interval. In the approximately 45 minute window during each orbit, we measured the positions of Pluto and Charon using FGS3 in fringe scanning (TRANS) mode and also the positions of five reference stars using FGS3 in fringe tracking (POS) mode. From this data set, we determined the Charon/Pluto mass ratio to be 0.122 ± 0.005 which implies a density of 1.8 to 2.0 g\ cm-3 for Pluto and 1.6 to 1.8 g\ cm-3 for Charon. The resulting rock-ice fraction is in the range expected for bodies that form in a solar nebula (such as Kuiper Belt Objects).
1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy Inc., under NASA Contract NAS5-26555.
Data analysis was carried out with support from the Space Telescope Science Institute under grant GO-07494.01-96A.
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