Geometry of the Saturn System from the 3 July 1989 Occultation of 28 Sgr and Voyager Observations

Abstract Observations of the 3 July 1989 Saturn occultation of 28 Sgr are combined with Voyager 1 and 2 occultation measurements to determine Saturn's pole direction and the radius scale for the ring system. Measurements of event times for circular ring features are presented for 3.9-μm imaging measurements from the Palomar 5-m telescope, 2.1-μm imaging measurements using the McDonald Observatory 2.7-m telescope, 3.3-μm imaging results (Harrington et al . 1993, this issue) from the NASA Infrared Telescope Facility and 3.4-μm aperture photometry from the European Southern Observatory 2.2- and 1-m telescopes. An atlas of ring features is provided, extending the catalog of P. D. Nicholson, M. L. Cooke, and E. Pelton (1990b, Astron. J. 100, 1339-1362) to include B Ring features. Voyager 1 radio science and Voyager 2 Photopolarimeter (PPS) occultation observations were reanalyzed by converting the optical depth profiles to intensity profiles expected for the 28 Sgr occultation geometry. Ring feature times were measured from these rescaled lightcurves, including new measurements of B Ring features from the PPS data. Measurements of 30 presumed circular features, together with additional 28 Sgr observations from Catalina Station, Kitt Peak, San Pedro Martir, United Kingdom Infrared Telescope, and Cerro Tololo Interamerican Observatory, described by Hubbard et al . (1993, this issue), were used to determine ring radii and Saturn's pole direction. Two separate techniques for computing ring orbit models are described; a sky-plane method, in which individual ring features are projected into the plane of the sky perpendicular to the star direction, and a solar system barycentric vector approach, which retains explicitly the 3-dimensional nature of the problem. The results of the two methods agree to high precision. The pole direction and radius scale cannot be accurately determined from 28 Sgr observations alone. The theoretical period of the forced precession of Saturn's pole due to solar torques on both Saturn's oblate figure and the equatorial satellites is estimated to be 1.76 × 10 6 year. A joint solution including both 28 Sgr and Voyager measurements, and accounting for this polar precession, gives a pole direction of α P (B1950.0) = 38.4168° ± 0.0035 and δ P (B1950.0) = 83.32329° ± 0.00017 (α P (J2000.0) = 40.5955° ± 0.0036, δ P (J2000.0) = 83.53812° ± 0.00018) at a reference epoch of 1980 November 12 at 23:46:32 UTC, consistent with the pole derived by P. D. Nicholson, M. L. Cooke, and E. Pelton (1990b, Astron J. 100, 1339-1362) from Voyager data alone. The pole uncertainty in this joint solution is dominated by uncertainties in the Voyager 1 and 2 trajectories. The ring radius scale is systematically about 1 km smaller than that found by P. D. Nicholson, M. L. Cooke, and E. Pelton (1990b, Astron. J. 100, 1339-1362). If the a priori Voyager trajectory uncertainties are assumed to be negligible, then the observations can be used to determine the precession rate of Saturn's pole about the invariable pole of the Solar System. The corresponding rate of motion of Saturn's pole on the sky is found to be 0.86 ± 0.31 times the predicted rate of 0.339″ year -1 .