This article has reported on the determination of the orbit of Phoebe using a numerical integration fit to Earthbased astrometric observations and imaging observations obtained from the Voyager 2 spacecraft. It has included a description of the dynamical models used in the integration, a general discussion of the observations available, an overview of the observation processing procedure, an assessment of the accuracy of the final orbit, and a comparison of the orbit with previously published integrations.
Because of its distance from Saturn, astrometric measures of Phoebe are mostly and will probably continue to be absolute positions. Any future determination of an improved orbit incorporating such positions will require both high quality observations and careful reduction with new star catalogues closely tied to the IERS/J2000 reference frame. Re-reduction of the existing observations against such catalogues would also provide significant benefit.
The determination of the orbits of most other planetary satellites relies heavily on intersatellite and planet relative positions, observations which are independent of star catalogue related errors. Among the existing Phoebe observations, there are a number of relative positions, but most are no better than the best absolute positions. New precise wide field simultaneous observations of Phoebe together with Saturn or other Saturnian satellites could provide invaluable relative positions leading to a greatly improved orbit.
An ephemeris based on the orbit described in this article is available electronically from the JPL Horizons on-line solar system data and ephemeris computation service (Giorgini et al. 1996).
Acknowledgements
The research described in this publication was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Adminstration. The author wishes to thank all of the astronomers who have provided the astrometric observations of Phoebe. The author also wishes to thank J.H. Lieske for his assistance in interpreting many of the old publications.