Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2008t&link_type=abstract
American Astronomical Society, DPS meeting #42, #20.08; Bulletin of the American Astronomical Society, Vol. 42, p.984
Astronomy and Astrophysics
Astronomy
Scientific paper
New astrometry for the Pluto system has been extracted from HST/WFPC2 images taken during Cycle 15. By combining these results with the prediscovery astrometry extracted from the ACS/HRC images taken during Cycle 11, we have improved the orbit determinations for the three satellites, Charon, Nix, and Hydra, as well as having improved the mass determinations for all four bodies. Of particular interest is the mass for Hydra, whose previous determination had a large enough uncertainty to allow for zero mass, even though the nominal GM value was 0.022 km3 sec-2. The new GM determination is 0.028 km3 sec-2, with a one-sigma lower limit of about 0.008 and one-sigma upper limit of about 0.060 km3 sec-2, a considerable improvement on the previous result. The new GM determination for Nix is 0.068 km3 sec-2, compared to the earlier value of 0.039 km3 sec-2. Both results imply somewhat larger satellites than previously indicated and therefore lower albedos, making it more difficult to conclude that Nix and Hydra have surface properties similar to Charon. The Charon/Pluto mass ratio is 0.1163, and the system GM is 971.713 km3 sec-2, based on the adopted image scale that produces a semimajor axis of 19570 km for Charon's Plutocentric orbit. However, recent stellar occultation results favor the image scale derived from the Cycle 2 observations of Pluto and Charon, which indicated a semimajor axis of 19636 km. Resolving this discrepancy remains an open issue. Further improvement in the mass and orbit determinations can be expected from new WFC3 images being taken during Cycle 17.
Buie Marc William
Grundy Will M.
Tholen David J.
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