Top: Hubble Telescope image of the second ring plane crossing, August 10, 1995. Bottom: Images from the 256 x 256 IR camera on the European Southern Observatory 3.6-m telescope; left view shows, left to right, A ring, B ring, Janus, C ring; right view shows, left to right, C ring, B ring, A ring, Tethys and Mimas merged together. Brightening of the western extremity of the A ring is unexplained but could be caused by a newly discovered satellite or clumpy material in the F ring or Enke gap.

A team of astronomers using the Wide Field Planetary Camera II on the Hubble Space Telescope may have discovered several orbiting clumps of icy rubble that could be the remnants of recently shattered moonlets orbiting near the outer edge of Saturn's ring system. They may have discovered a new class of ephemeral, transitional objects in the solar system that provides new clues to the evolution of the rings.

The observations were made during the ring plane crossing on August 10, which provided a rare opportunity to seek out faint satellites in and near the ring plane. "Ring plane crossings" are those moments when the Earth or Sun crosses the plane of Saturn's rings, allowing them to be seen (or illuminated) edge on. The usually bright rings are seen only as a faint, thin line, and Saturn's smaller satellites become visible. These events are rare, occurring in groups of two or four at intervals of about 14.5 years.

The latest pictures gave astronomers an opportunity to confirm the presence of two new satellites first observed in Hubble images taken during the May 22 ring plane crossing. Rather than solving the moon question, however, the August observations presented a new mystery: "We realized these moons are too bright to have gone undetected when the Voyager spacecraft flew by Saturn in 1980 and 1981," said Phil Nicholson of the Cornell University observing team.

A further complication is that the August pictures seem to show at least three new objects, and in different orbits from the two May objects. "They also appear to be very elongated or arc-like, unlike a satellite should be. One possibility is that they are large clouds of debris from small satellites shattered by impacts with chunks of space debris (possibly comets), sometime during the 14 years since the Voyager 2 flyby."

The discovery of objects in this transitional phase is not totally unexpected, says Nicholson, because one theory for the origin of Saturn's ring system is that it is made up of countless fragments from several pulverized moons. This idea is reinforced by the fact the new objects orbit Saturn near the narrow F ring, which is a dynamic transition zone between the main rings and the larger satellites. Moonlets in this region can be easily disrupted by Saturn's tidal pull if they are fractured by an impact, forming a cloud of debris. Eventually such a cloud would spread around the moon's orbit to form a new ring.

The dynamics of this "bumper car" zone are also evident in Hubble's observations of the satellite Prometheus. Although a third object seen in the May images was first suspected to be another new satellite because its location did not match the predicted position for any of the known satellites charted by Voyager, it now appears that this body is in fact Prometheus, which has slipped in its orbit by 20 degrees from the predicted position. Nicholson suggests that this may be a consequence of a collision of Prometheus with the F ring, which is believed to have occurred in early 1993 as the moon passed close enough to one of the denser, lumpy regions of the F ring to have its orbit changed.

The researchers plan to observe the moons and rings again during the third ring plane crossing in November.