The Peregrinations of Prometheus

Details The Peregrinations of Prometheus The Peregrinations of Prometheus

Sep 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998dda....30.0204f&link_type=abstract

American Astronomical Society, DDA meeting #30, #02.04; Bulletin of the American Astronomical Society, Vol. 30, p.1141

Astronomy and Astrophysics

Astronomy

1

Scientific paper

One of the most surprising results from the 1995 Saturn RPX campaign was the discovery that Prometheus lagged some 19(deg) behind its predicted position based on an ephemeris derived from Voyager 1 and 2 observations (Nicholson et\ al.; Bosh and Rivkin 1996, Science 272, 509; 518). The most likely explanation seemed to be that a previously-undetected coorbital satellite with a 30-year libration period was in a horse-shoe orbit with Prometheus, resulting in the large orbital phase lag but permitting Prometheus to have the same mean motion in 1995 as during the Voyager encounters in 1980 and 1981. This explanation is no longer viable. Subsequent HST observations during the 1996 and 1997 oppositions show that Prometheus is now moving with a slower orbital mean motion than the Voyager epoch value of n=587.2890+/-0.0005(deg) /day, but only enough slower to account for about 10(deg) of the full 19(deg) accumulated lag. We fitted freely precessing equatorial elliptical models to the full set of observations, including the May, August and November 1995 HST RPX images, as well as the new results from HST programs 6806 and 7427. In Fit 1, we held the eccentricity fixed at the Voyager result. When the eccentricity and periapse longitude at epoch are fitted as well (Fit 2), the fit improves considerably and Prometheus's eccentricity is less than half that found from Voyager data. (The reference epoch is Aug 10.5 1995=JD 2449940.0.) Fit & n(deg/day) & e & omega ((deg)) & Delta lambda ((deg)) & N & rms (km) 1 & 587.287597 & (0.0024) & (236.35 ) & -18.906 & 48 & 283.2 & +/-0.000054 & & & +/-0.036& & 2 & 587.287751 & \phantom{+/-}0.00111 & 277.22 & -18.928 & 48 & 214.91 & +/-0.000045 &+/- 0.00024 & +/- 12.61 & +/-0.027& & Using Fit 2, the orbital lags with respect to the Voyager ephemeris are Delta lambda =(-18.992, -18.813, -18.962, -19.484, -20.053, -19.968, -20.102, -20.158) +/-0.04(deg) at JD-2400000=(49859., 49940.1, 50043., 50356.6, 50713.97, 50723.34, 50732.12, 50814.7).

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