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

## [49.11] The Orbits of Prometheus, Pandora and Atlas in 1980 and 1981.

C.D. Murray, M.W. Evans (Queen Mary, University of London), C.C. Porco (LPL/UofA), M.R. Showalter (Stanford)

HST and ground-based observations of the F ring region of Saturn have shown that the satellite Prometheus lags ~9\circ behind its predicted longitude based on the published orbit derived from {\em Voyager\/} images by Synnott {\em et al.~}([1983] {\em Icarus} 53, 156--158). Observations of Pandora appeared to show it at its predicted location; there have been no confirmed detections of Atlas since 1981. The anomalous position of Prometheus and the suggestion that its orbit can change have led us to carry out a re-examination of the original {\em Voyager\/} data for all three satellites.

We have identified 57 {\em Voyager 1} and 66 {\em Voyager 2} images of Prometheus suitable for orbit determination and calculated the orbital elements. We obtain mean motions of 587.3010 ±0.0028\circ{\rm d}-1 and 587.2927 ± 0.0011\circ{\rm d}-1 at the {\em Voyager 1\/} and {\em 2\/} epochs respectively. This strongly suggests that Prometheus' orbit changed in the nine months between the two encounters. Combining the data we obtain a mean motion of 587.28933 ±0.00008\circ{\rm d}-1. This compares favourably with the value of 587.2890±.0005\circ{\rm d}-1 quoted by Synnott {\em et al.~}(1983) but differs significantly from the HST value of 587.287751±.000045\circ{\rm d}-1 obtained by French {\em et al.~}([1998] {\em B.A.A.S.~}30, 1141). We also find evidence for (i) an eccentricity lower than the published value of 0.0024 and (ii) an inclination of 0.027±.006\circ.

Using 87 images of Pandora and 23 images of Atlas our preliminary results are consistent with previously published orbits provided the effect of the 3:2 near-resonance between Pandora and Mimas (Dones {\em et al.~}[1999] {\em B.A.A.S.~}31, 1228) is not included. In neither case did we find a significant difference in the orbits between the {\em Voyager\/} encounters.