Saturn ring toss: Hubble finds more moons.

Circling the gaseous globe like myriad brilliant belts, they've fascinated observers since the time of Galileo. Nothing quite compares with the rings of Saturn.

Except, perhaps, their absence.

Using the Hubble Space Telescope to observe Saturn during a rare moment when the planet's icy rings nearly vanish from sight, astronomers announced that they have discovered two previously unknown moons orbiting the planet.

Amanda S. Bosh of Lowell Observatory in Flagstaff, Ariz., and Andrew S. Rivkin of the University of Arizona in Tucson reported the findings in a July 26 circular of the International Astronomical Union.

Hubble made the observations on May 22, when Earth crossed the plane of Saturn's equator--the same plane in which the rings circle the planet. In this configuration, which only occurs every 15 years, the rings appear edge-on and their brilliance narrows to a razor-thin band of light that all but

disappears. Faint moons ordinarily hidden by the ring's glare then become visible. Astronomers have found 13 of Saturn's 18 known moons during such events.

Hubble data now appear to have added two more to that list. In analyzing 27 Hubble pictures taken over 10 hours, Bosh and Rivkin found four moons at distances between 137,000 and 147,000 kilometers from the center of Saturn. Two of the bodies, temporarily dubbed S1 and S2, probably correspond to known moons. But the other two, S3 and S4, don't fit any known orbit, Bosh and Rivkin assert. These bodies lie outside Saturn's narrow, outer F ring, with S3 adjacent to the ring and S4 about 6,000 km farther out.

After reviewing the projected orbits of Saturn's known moons, which NASA updates regularly for the planned Cassini mission to the planet, Robert A. Jacobson of NASA's Jet Propulsion Laboratory in Pasadena, Calif., concurs with that conclusion. But both he and Bosh caution that the calculated orbits rely on a set of quick observations taken in 1980 and 1981, when the two Voyager spacecraft flew past Saturn. "The projected orbits are from 15 years ago, and errors in position build up," says Bosh.

Such uncertainties hinder firm identification of S1 and S2. Both lie between Saturn's A and F rings. Although these moons have somewhat different orbital longitudes than predicted, Bosh and Rivkin suggest that S1 is Atlas and S2 is Prometheus, moons discovered by Voyager 1. Jacobson says it's possible that S1 is the moon Pan, while S2 could be either Prometheus or Atlas.

Just in time to help settle these questions, Hubble will watch as Earth again crosses Saturn's ring plane on Aug. 10. At the next two series of plane crossings, in 2009 and 2025, the sun's glare will hide Saturn. Astronomers won't get another chance for such observations until 2038.