The orbit of Charon .1. New Hubble space telescope observations

The orbit of Charon has been determined from 60 images of the Pluto-Charon system acquired with the Hubble Space Telescope Wide Field and Planetary Camera between 1992 May 21 and 1993 August 18. The semimajor axis was found to be 19,636 +/- 8 km, in good agreement with an older determination upon which mutual-event-based radius computations have relied, but significantly larger than two other recent determinations, Contrary to expectations based on the tidal evolution of the system, a surprisingly large eccentricity of 0.0076 +/- 0.0005 was found, with the line of apsides aligned nearly along the line of sight from Earth. Approximately half of this apparent eccentricity could be due to offsets in the center of light from the center of body arising from surface albedo features on Pluto and Charon, which leaves a statistically significant portion unexplained. We propose that the remaining eccentricity is the result of a recent energetic impact. The remaining elements are generally consistent with previously determined values, though in the case of the inclination, the various determinations appear to have underestimated error bars. The system GM derived from our data is 981.5 +/- 1.1 km(3) sec(-2). Some information on the Charon/Pluto mass ratio is present in the data, though our determination of 0.110(-0.056)(+0.063) does not improve on previous determinations, or resolve the discrepancy between them. (C) 1997 Academic Press.