Saturn's wayward shepherds: Pandora and Prometheus

Details Saturn's wayward shepherds: Pandora and Prometheus Saturn's wayward shepherds: Pandora and Prometheus

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999dda....31.0901f&link_type=abstract

American Astronomical Society, DDA meeting #31, #09.01

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Saturn's irregular F Ring is flanked by the small satellites Prometheus and Pandora, discovered in Voyager images taken in 1980/81 (Synnott et al., [1983] Icarus 53, 156). Observations with HST during the ring plane crossings in 1995 led to the surprising discovery that Prometheus lagged behind its predicted position by some 19 deg in longitude (Bosh & Rivkin [1996] Science 272, 518; Nicholson et al., [1996] Ibid 272, 509). Subsequent HST observations in 1996, 1997 and most recently October 1998 showed that this lag is steadily increasing by approximately 0.6 deg yr(-1) (French et al., [1998] B.A.A.S. 30, 1141). This lag remains unexplained. A reexamination of the 1995 HST data now shows that Pandora also deviates from the Voyager ephemeris (McGhee et al., in preparation). Although within one degree of its predicted longitude in 1995, Pandora's mean motion was ~ 4 deg/yr less than expected, corresponding to an increase in the semimajor axis of 1.8 km. Archival HST data from 1994 and additional observations in 1996-1998 reveal a periodic signature in Pandora's mean longitude with an amplitude of about 0.8 deg and a period of ~ 580 days, superimposed on a slow drift of -1.25 deg/yr. While this drift, like that of Prometheus, is unexplained, the oscillatory component matches both in amplitude and phase the expected perturbation due to the nearby 3:2 corotation resonance with Mimas (Dones et al., this meeting). An improved fit is obtained by taking into account the 70 yr. libration in Mimas' longitude due to the 4:2 resonance with Tethys.

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