DDA 36th Meeting, 10-14 April 2005
Session 11 Orbits and Orbit Evolution II: Moons
Oral, Wednesday, April 13, 2005, 9:35am-12:15pm

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[11.02] The Orbits of Saturn's Small Satellites

J. N. Spitale (SSI), R. A. Jacobson (JPL), C. C. Porco (SSI), W. M. Owen (JPL), S. Charnoz (Universite Paris 7 / CEA Saclay)

We report on the orbits of the small, inner Saturnian satellites, either recovered or newly-discovered in recent Cassini imaging observations (excluding Helene, Telesto and Calypso, which will be discussed by another group). Using combined Cassini and Voyager observations, the mean motions of Pan and Atlas have been refined by several orders of magnitude. The Atlas orbit is based on a numerical integration perturbed by all of the massive Saturnian satellites including Prometheus, Pandora, Janus, and Epimetheus. We find that the dominant perturber is Prometheus. Cassini, Voyager, HST, and Earth-based data have been used to refine the orbits of Janus, Epimetheus, Prometheus and Pandora. The orbits of the co-orbitals, Janus and Epimetheus, remain stable; their orbital swap does not occur until Februrary, 2006. The orbits of Prometheus and Pandora remain close to recent values (Jacobson and French 2004, Icarus, 172, 382). Six new objects have been discovered to date -- three (S/2004 S3, S4, S6) in close proximity to the F ring, two (S/2004 S1(Methone), S/2004 S2(Pallene)) between the orbits of Mimas and Enceladus, and one (S/2004 S5(Polydeuces)) co-orbital with Dione, trailing by ~60 deg (Porco et al., Science 307, 25 Feb 2005). One of the F-ring objects -- S/2004 S3 -- was seen over a 118-day interval, but none of those objects, including S/2004 S3, were subsequently recovered in an F-ring movie acquired on 15 November 2004 (29 days after the last sighting of S/2004 S3) with an image scale of ~4 km/pixel, in which all were expected to appear. Consequently, we are confident only that Methone, Pallene and Polydeuces are solid satellites; S/2004 S3, S4 and S6 may be transient clumps. Our orbital fits, both precessing ellipse models and orbital integrations, suggest that Pallene is the same object as S/1981 S14, imaged by Voyager 2 on 23 August 1981, contrary to our initial reports (IAU circular 8389). The orbital inclination and eccentricity of Methone are considerably less excited than expected, given its proximity to Mimas (Porco et al., Science; in press), but orbital integrations spanning 4 years show that significant longitudinal perturbations from Mimas explain the observations. Polydeuces' orbit has been integrated over 5 years and is seen to librate about Saturn's L5 point with a period of about 792 days and an amplitude of about 25.8 deg.

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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.