34th Meeting of the AAS Division on Dynamical Astronomy, May 2003
7 Space Missions, Astrometry, and Observables
Oral, Tuesday, May 6, 2003, 8:30-10:50am,

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[7.07] The Mass Ratio of Charon to Pluto from Hubble Space Telescope Astrometry with the Fine Guidance Sensors

C. B. Olkin, L. H. Wasserman, O. G. Franz (Lowell Observatory)

The mass ratio of Charon to Pluto is a basic parameter describing the binary system and is necessary for determining the individual masses and densities of these two bodies. Previous measurements of the mass ratio have been made, but the solutions differ significantly (Null et al. 1993, Young et al. 1994, Null and Owen 1996, Foust et al. 1997, Tholen and Buie 1997). We present the first observations of Pluto and Charon with a well-calibrated astrometric instrument -- the Fine Guidance Sensors on the Hubble Space Telescope. We observed the motion of Pluto and Charon about the system barycenter over 4.4 days (69% of an orbital period) and determined the mass ratio to be 0.122 ± 0.008 which implies a density of 1.8 to 2.1 g cm-3 for Pluto and 1.6 to 1.8 g cm-3 for Charon. The resulting rock-mass fractions for Pluto and Charon are higher than expected for bodies formed in the outer solar nebula, possibly indicating significant post-accretion loss of volatiles.

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 NAS 5-26555. These observations are associated with proposal #7494.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.