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M. R. Showalter (Stanford), L. Dones (SWRI), J. J. Lissauer (NASA Ames)
Prometheus is the larger and closer of the two ``shepherding'' moons straddling Saturn's F~Ring. It was found to be lagging its predicted orbital longitude by 19\circ in 1995, compared to extrapolations from Voyager data in 1980 and 1981. That lag has been increasing since. We have identified two related gravitational interactions with F~Ring material that will produce a pseudo-random walk in Prometheus' orbit, plausibly causing the lag observed.
(1) When Prometheus passes a sector of the ring, its gravitational perturbations generate periodic clumps. These clumps persist until the next passage, where Prometheus' gravity can systematically ``throw'' the ring to an orbit higher or lower by ~ 1 km. Because energy is conserved, Prometheus' must recoil slightly. Our numerical and analytic models reveal that if the F~Ring has a mass comparable to that of Prometheus, then the integrated effect of this recoil can explain the orbital lag.
(2) The F~Ring could easily contain 100--1000 as-yet undetected objects ~ 5 km in radius, which contain the bulk of the ring's mass. The net gravitational perturbations from these bodies could introduce an additional random component to Prometheus' motion.
This research is supported by NASA through RTOP 344-30-51-05.