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R. G. French, C. McGhee (Wellesley College), L. Dones (SWRI), J. Lissauer (NASA Ames)
Saturn's narrow F ring is flanked by two nearby small satellites, Prometheus and Pandora, discovered in Voyager images taken in 1980/81 (Synnott et al.,  Icarus 53, 156). Observations with the HST during the ring plane crossings (RPX) of 1995 led to the unexpected finding that Prometheus was ~19\circ behind its predicted orbital longitude, based on the Voyager ephemeris (Bosh and Rivkin  Science 272, 518; Nicholson et al.,  Science 272, 509). While Pandora was at its predicted location in Aug.~1995, McGhee ( Ph.D.~thesis, Cornell U.) found from the May and Nov.~1995 RPX data that Pandora also deviates from the Voyager ephemeris. Using archival HST data from 1994, previously unexamined RPX images, and a large series of targeted WFPC2 observations between 1996 and 2000, we have determined highly accurate sky plane positions (±00 km for non--RPX PC images) for 126 images of Prometheus and 117 of Pandora. Prometheus's orbital longitude continues to lag by about -0.57\circ~yr-1 relative to the Voyager ephemeris, while Pandora's lags by -1.27\circ~yr-1, showing in addition a ~600~d oscillatory component that matches in amplitude (± 0.78\circ) and phase the expected perturbation due to the nearby 3:2 corotation resonance with Mimas, modulated by the 70~yr libration in Mimas's longitude from its 4:2 resonance with Tethys. We determined the following orbital elements for freely precessing equatorial elliptical orbits from fits to the observations:
|Object||a (km)||n (\circd-1)||\lambda0||e||\varpi0|
The reference epoch is Aug 10.5 1995=JD 2449940.0, and the rms residuals are 183 km for Prometheus and 177 km for Pandora (~0.5 px in a typical PC image). The quoted errors are three times the formal errors.