Session 17P. Rings I, II
Contributed Poster Session, Tuesday, October 13, 1998, 4:15-5:20pm, Hall of Ideas

## [17P.10] Hubble Space Telescope (HST) observations of Saturn's inner satellites during the 1995 ring--plane crossings

C. A. McGhee, P. D. Nicholson (Cornell University), R. G. French (Wellesley College), K. J. Hall (Swarthmore College)

Saturn's ring--plane crossings in May, August and November 1995 presented an opportunity to study the faint inner satellites of Saturn, usually overpowered by the glare of Saturn's normally bright rings. Orbit fits for these satellites have been published for each separate HST data set, (Nicholson {\it et\ al.}; Bosh and Rivkin 1996, {\it Science} {\bf 272}, 509; 518), but the combination of the three data sets, along with refined ring template subtraction, has allowed us to obtain greatly improved fits (see below). Values of \lambda\rm obs-\lambda\rm pred for Janus and Epimetheus are given relative to the ephemeris of Nicholson {\it et al.} 1992 ({\it Icarus} {\bf 100}; 464); those for Pandora and Prometheus are compared with the 1995 model of Jacobson ({\it BAAS} {\bf 27}; 1202), while Mimas is relative to the ephemeris of Harper and Taylor 1993 ({\it Astron.~Astrophys.}~{\bf 168}; 326). The Janus, Epimetheus and Pandora fits are within 0.4\deg of Jacobson's 1996 ({\it BAAS}~{\bf 28}; 1185) fit to all available satellite data. Prometheus' lag of ~18.8\deg behind its predicted position seems to remain constant throughout the ~180 days of the 1995 RPX observing period, but subsequent HST observations have shown that this lag is increasing with time (French {\it et al.} 1998, DDA meeting).

In addition to new orbits for the objects S/1995 S5, S6 and S7 (Nicholson {\it et al.} 1995 {\it IAUC} 6243), we have obtained measurements and orbital parameters for S9, first reported in data taken at the CFHT (Roddier {\it et\ al.} 1995, {\it IAUC} 6515). S7 and S9 appear to be coorbital with Prometheus; while S5 and S6 fall ~500--600 km interior to the F ring.

\begin{tabular}{lcccr} \hline

Object & Mean motion (\deg/day) & e & RMS (km) & \Delta\lambda \\ \hline

Prometheus& 587.2902±0.0018 & 0.0024 & 281 & -18.85\\

Pandora & 572.7780±0.0024 & 0.0044 & 228 & -0.23 \\

Epimetheus & 518.4822±0.0020 & 0.0126 & 260 & 1.07 \\

Janus & 518.2383±0.0006 & 0.0066 & 132 & 0.08 \\

Mimas & 381.9894±0.0009 & 0.0201 & 272 & -0.25 \\

\hline \end{tabular}

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