The Orbit of Charon from Hubble Space Telescope Observations
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\title{The Orbit of Charon from Hubble Space Telescope Observations}
\author{
David J. Tholen, Institute for Astronomy, University of Hawaii\\
Marc W. Buie, Lowell Observatory}
\date{}

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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 orbital semimajor axis was found
to be $19,636  \pm 8 km$, in good agreement with an older determination upon which
mutual event radii computations have been based, but significantly larger than
two other recent determinations.  A surprisingly large orbital eccentricity of
$0.0076 \pm  0.0005$ was found, with the line of apsides aligned nearly along the
line of sight from Earth, which is the orientation to which the mutual events
were least sensitive.  Although part of this apparent eccentricity could be due
to surface albedo features on Pluto, the most likely explanation for the
remainder of this unexpected result is that the system suffered an energetic
impact in recent time, which implies a large population of bodies in the
trans-Neptunian region of the Solar System.  The remaining elements are
generally consistent with previously determined values, though in the case of
the orbital inclination, the various determinations appear to have
underestimated error bars.  The system GM derived from our data is
$9.815 \pm  0.012 . 10^11 m^3 s^{-2}$.  Some information on the Pluto/Charon mass
ratio is present in the data, though our determination of 9.08 is sufficiently
crude that it does not improve on previous determinations, nor resolve the
discrepancy between them.

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