This spectacular view of the sun setting over Saturn's rings is part of a group of images recently released by the Space Telescope Science Institute that takes advantage of an unusual alignment among the sun, Earth and Saturn (the images also celebrate the Hubble Space Telescope's sixth anniversary).

During this rare astronomical event, known as a Saturn ring-plane crossing, the planet's beautiful, mysterious rings appear edge on to terrestrial viewers. In the current crossing, the Earth passed through the plane of Saturn's rings three times, on May 22 and August 10 of 1995 and on February 11, 1996; the Sun passed through the ring plane once, on November 19, 1995. All but the February 11 event were visible to the Space Telescope.

Saturn's complex system of rings is so thin, the edge-on view makes them almost invisible from Earth, just as a sheet of paper would be invisible if viewed perfectly edge-on from a distance. In the absence of the usual bright glare of sunlight reflected from the rings, astronomers could study the vertical structure within the rings; search for faint, undiscovered Saturnian moons; and measure the tenuous "atmosphere" that surrounds the rings like a halo. The Hubble Space Telescope participated in all of these observations.

The recent ring crossing was literally a once-in-a-lifetime opportunity for many astronomers. The orbital configuration that causes the rings to appear edge-on occurs roughly every 15 years. But the next two times it happens, Saturn will be situated nearly on the other side of the sun and hence out of the Earth's view. The next chance to get a good edge-on view of Saturn's ring system will not occur again until a triple passage in 2038- 39.

The Space Telescope did not disappoint astronomers. It recorded starkly beautiful images of the rings edge on. It also collected a sequence of spectacular pictures of Saturn's moons.

Much of the data collected during the ring crossing are still being interpreted. But already, the observations are leading into intriguing new insights into the nature of Saturn's rings. Researchers have known for decades that the rings are composed of innumerable particles (ranging in size from fine dust to chunks the size of a house), each composed at least in part of frozen water. One theory about how the ring system formed is that small icy moons in orbit around Saturn collided with comets or meteoroids. The moons would have shattered, their debris gradually dispersing around Saturn to become part of the intricate system of rings.

If that is the case, scientists wonder, how often do such events happen, and how long do the rings survive? Astronomers long ago predicted that the rings must experience a kind of erosion process. Bombardment by solar radiation, charged particles, dust-size meteorites or other ring particles would dehydrate the icy ring particles, blowing off an extended but very thin cloud of water vapor. Solar radiation would then split the water vapor molecules into hydrogen atoms and hydroxyl molecules (an atom of hydrogen bound to an atom of oxygen).

A team of investigators used the Hubble to search for hydroxyl emissions and reported their findings in the journal Science. They confirmed the reality of this erosion process and determined that the ring system is losing up to 3,000 kilograms (6,600 pounds) of frozen water per second. Even at this rate, the researchers estimate that the rings probably will survive at least another billion years. Based solely on the measured erosion rate, the rings might have formed a billion or more years ago.

Other studies, also based in part on Hubble observations, provide more information about the dynamics of the ring system. Gravitational interactions between Saturn's satellites and the ring particles, should cause the rings to spiral slowly into the planet. Calculations indicate that this process should clear out the rings over a period of about 100 million years, which suggests that we are fortunate to be seeing Saturn during a comparatively brief ringed period. Hubble studies of the motions of the inner satellites (possible only during ring crossing) were supposed to help settle the age of the rings; instead, it seems that even the satellites follow complicated, difficult-to- predict paths.

At least Hubble has helped nail down the thickness of the rings: no more than 1.5 kilometers thick at the outer "F ring," as reported in another paper in Science, and probably much thinner closer in.

Hubble was hardly the only telescope observing the ring crossing; ground-based telescopes around the world were also pointed at the ringed planet. These included observatories at Kitt Peak (near Tucson, Arizona), Pic-du-Midi, France, the Canary Islands; and the European Southern Observatory in Chile. Complementary observations made at near-infrared wavelengths, just longer than visible light, were conducted at Palomar Observatory, California; NASA's Infrared Telescope Facility, Hawaii; Pic du Midi; and Calar Alto, Spain.

Terrestrial images may lack the crisp clarity of those from Hubble, but they remain extremely important to astronomers. Researchers working at Kitt Peak, for example, prepared animations of images made during the ring crossing; these will be analyzed to search for even fainter moons or ring clumps not evident in single images.