DPS Pasadena Meeting 2000, 23-27 October 2000
Session 49. Rings Posters
Displayed, 1:00pm, Monday - 1:00pm, Friday, Highlighted Tuesday and Thursday, 3:30-6:30pm, C101-C105, C211

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[49.12] The Peripatetics of Prometheus and Pandora

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., [1983] 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 [1996] Science 272, 518; Nicholson et al., [1996] Science 272, 509). While Pandora was at its predicted location in Aug.~1995, McGhee ([2000] 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
3\sigma(a) 3\sigma(n) 3\sigma(\lambda0) 3\sigma(e) 3\sigma(\varpi0)
Prometheus 139377.6107 587.287555 339.12 0.00169 248.5
0.0078 0.000048 0.05 0.00039 12.9
Pandora 141713.1119 572.785574 96.03 0.00453 359.8
0.0084 0.000051 0.05 0.00036 6.3

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.

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