Luke Dones

Observations of Saturn's Ring-Plane Crossings in August and November 1995

Observations of Saturns ring system with the Hubble Space Telescope during the 10 August 1995 Earth crossing and the 17 to 21 November 1995 solar crossing indicate that the F ring dominates their apparent edge-on thickness of 1.2 to 1.5 kilometers. The F ring is slightly inclined with respect to the A ring, which may explain the approximately 50-minute difference in apparent crossing times for the east and west ring ansae in August. Prometheus lags its predicted position by about 19 degrees in longitude. The faint G ring is neutral or reddish in color and is confined to a radial range of 2.72 to 2.85 Saturn radii. The broad, distinctly blue E ring flares outward to a maximum thickness of about 15,000 kilometers at 7.5 Saturn radii and appears to have a spatially uniform particle size distribution.

The Rings of the Outer Planets

All four of the giant planets are encircled by rings. Saturn’s rings were discovered by Galileo in 1610. Huygens was the first to realize, during the 1655–1656 ring-plane crossing, that the changing appearance of the rings results from their varying tilt as Saturn orbits the Sun (van Helden, 1984). Until 1977, Saturn’s rings were thought to be unique. In that year, the narrow, opaque uranian rings were discovered during the occultation of a bright star by Uranus (Elliot and Kerr, 1984). Two years later, Voyager 1 returned the first image of the broad, faint jovian ring, whose existence had been hinted at by a drop in particle fluxes measured by Pioneer 10 in 1976 (Burns et al. 1984). Finally, a stellar occultation in 1984 revealed the existence of incomplete ring ‘arcs’ around Neptune (Hubbard et al. 1986). Voyager 2 subsequently imaged the neptunian rings, and showed that the arcs were the densest component of an extensive system of faint rings (Smith et al. 1989).