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R.G. French, C.A. McGhee (Wellesley C.), L. Dones (SwRI), J.J. Lissauer (NASA Ames)
Using archival HST data and a large series of targeted WFPC2 observations between 1996 and 2002, we have determined highly accurate sky plane positions for Prometheus, Pandora, and nine other satellites. We compare the Prometheus and Pandora measurements to the predictions of substantially revised and improved ephemerides based on an extensive analysis of a large set of Voyager images (Murray et al. [2000] B.A.A.S. 32, 1090; Evans [2001] Ph.D. thesis, Queen Mary Coll.). Prometheus's orbital longitude lag is increasing by -0.71\circ yr-1 relative to the new Voyager ephemeris. In contrast, Pandora is ahead of the revised Voyager prediction. Its longitude offset is increasing by +0.44\circ yr-1, showing in addition a ~585 day oscillatory component with amplitude \Delta\lambda{\rm CR}_0=0.65±0.07\circ whose phase matches the expected perturbation due to the nearby 3:2 corotation resonance with Mimas, modulated by the 72 yr libration of Mimas due to its 4:2 resonance with Tethys. We determine orbital elements from fits to the 1994-2000 HST observations, and find that Prometheus's semimajor axis was 0.31 km larger, and Pandora's was 0.20 km smaller, than during the Voyager epoch. Observations in 2001-2002 reveal a new twist: Prometheus's mean motion slowed suddenly by an additional -0.77\circyr-1, equivalent to a further increase in semimajor axis of 0.33 km, at the same time that Pandora's mean motion increased by +0.92\circyr-1, corresponding to a decrease of -0.42 km in its semimajor axis. There is a striking anticorrelation of the motions of these two moons seen in the 2001-2002 observations as well as over the 20 year interval since the Voyager epoch. This suggests a common origin for their wanderings, perhaps through direct exchange of energy between the satellites as the result of resonances, possibly involving the F ring. This work was supported by the STSCI and NASA PGG. .
The author(s) of this abstract have provided an email address for comments about the abstract: rfrench@wellesley.edu
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Bulletin of the American Astronomical Society, 34, #3
© 2002. The American Astronomical Society.