LORD OF THE RINGS
If the Voyager spacecraft showed too many rings to number, the view from Earth is much simpler—we see only three. The outermost is called the A ring and it is 9,000 miles (14,500 km) wide. It is separated from the B ring by the dark gap of the Cassini Divi-sion, 2,600 miles (4,200 km) across. The B ring is both the brightest and, at 16,000 miles (26,000 km) across, the widest. On its inner edge is the gauzy C (or crepe) ring, 10,500 miles (17,500 km) wide.
While the B ring looks solid enough to walk on, in reality it, like the other rings, is a loose collection of particles orbiting in complex ways. This was dramatically illustrated in 1989, when Saturn passed in front of the star 28 Sagittarii. All over the world, professional and amateur astronomers watched the star flicker as it appeared to traverse the rings. The occultation revealed even more structure in the rings than Voyager had shown.
The Cassini Division is hard to see when the rings are nearly edge-on, as in the 1990s, but it becomes easier to see as the tilt increases. Voyager showed that the division is not empty, but simply contains less materi-al than the rings. The division occurs because the gravity of the moon Mimas perturbs ring particles that orbit there, selectively removing them.
The C ring is also hard to view when the rings are rela-tively closed. When the rings are open, it is easy to spot the C ring in front of Saturn's disk.
Saturn's rings are probably transitory. They originated when one or more moons strayed too close to Saturn to withstand its tidal force and broke apart. Many scientists now think the rings will last only a few tens of millions of years before mutual collisions rob the particles of energy and they spiral into Saturn.