Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000dps....32.4912f&link_type=abstract
American Astronomical Society, DPS Meeting #32, #49.12; Bulletin of the American Astronomical Society, Vol. 32, p.1090
Astronomy and Astrophysics
Astronomy
Scientific paper
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 ~19o 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 (+/-100 km for non--RPX PC images) for 126 images of Prometheus and 117 of Pandora. Prometheus's orbital longitude continues to lag by about -0.57o yr-1 relative to the Voyager ephemeris, while Pandora's lags by -1.27o yr-1, showing in addition a ~600 d oscillatory component that matches in amplitude (+/- 0.78o) 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 (od-1) & λ 0 & e & \\varpi0 & 3σ (a) & 3σ (n) & 3σ (λ 0) & 3σ (e) & 3σ (\\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.
Dones Luke
French Richard G.
Lissauer Jack . J.
McGhee Colleen
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