DPS 2001 meeting, November 2001
Session 29. Rings I
Oral, Chair: L. Spiker, Thursday, November 29, 2001, 11:40am-12:40pm, Regency GH

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[29.05] The Renegade Roamings of Prometheus and Pandora

R. G. French, C. A. McGhee (Wellesley Coll.), L. Dones (SwRI), J. J. Lissauer (NASA Ames)

Prometheus and Pandora, the wayward small satellites orbiting on either side of Saturn's F ring, continue to surprise us with their deviant wanderings. Observations with the Hubble Space Telescope (HST) WFPC2 obtained from 1994--2000 revealed that both satellites were lagging behind their predicted orbital longitudes based on the Synnott et al. ([1983] Icarus 53, 156) Voyager ephemerides (French et al. [2000] B.A.A.S. 32, 1090, and references cited therein). However, the Pandora orbital elements had been derived from Voyager 1 and 2 observations without taking into account the perturbation in Pandora's motion (clearly seen in the HST observations) due to the nearby 3:2 corotation resonance with Mimas. Murray et al. ([2000] B.A.A.S. 32, 1090) and Evans ([2001] Ph.~D.~Thesis, Queen Mary Coll.) analyzed an expanded set of Voyager images and derived new orbital elements for both satellites, taking proper account of the 3:2 resonance for Pandora. From the HST observations, we find that Prometheus' semimajor axis is now 0.29 km larger, and Pandora's 0.22 km smaller, than during the Voyager epoch. As of September 2001, Prometheus is ~25\circ behind, and Pandora is ~28\circ ahead, of the new Voyager based predictions. These most recent HST observations reveal a new twist in the satellites' meanderings: Prometheus is lagging by 0.8\circ and Pandora is leading by 0.8\circ, relative to their expected locations based on the 1994--2000 HST measurements. If this pattern persists in upcoming HST observations in 2001-2002, we could be witnessing a change in their semimajor axes similar to what took place during the unobserved period between 1981 and 1994. Furthermore, if the perigrinations of Prometheus and Pandora are caused by exchange of energy between these two moons, then the observed deviations imply that the ratio of their masses is closer to unity than is the ratio of their volumes derived from Voyager images.

This work was supported in part by NASA Planetary Geology and Geophysics Program grants NAG5-4046 and 344-30-50-01, and by STSCI grant GO-06806.01-95A.

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