The Cassini spacecraft is about to embark on a new mission phase that will give it a ringside seat at Saturn -- literally. After concentrating on flybys of the stately planet's moons since arriving last year, Cassini will begin five months of extensive study of Saturn's magnificent rings. Knowing how the rings form and how long they have been there is a central question for the Cassini mission.

Cassini will view the rings on their lit and unlit faces, both toward the Sun and away from the Sun. This range of geometries will allow all of Cassini's various instruments to observe the rings as never before.

Saturn's Crown Jewels

From a distance, the majestic rings of Saturn look like symmetrical hoops surrounding the planet. Up close, however, the rings turn out to be a splendid but somewhat unruly population of ice and rock particles jostling against each other or being pushed and pulled into uneven orbits by bigger particles and by Saturn's many moons.

Their origin is a mystery. Scientists think the rings did not form out of the initial cloud of gas and dust that surrounded Saturn as it formed, but are actually much younger than the planet. However, they do not know if the rings formed after an incoming comet was torn apart by Saturn's gravity, or if some previous moon of Saturn was smashed to bits by an incoming comet.

Although the rings stretch over 282,000 kilometers (175,000 miles) -- about three-fourths of the distance from Earth to the moon -- they may be as little as 30 meters (roughly 100 feet) in thickness. The mass of all the ring particles measured together would comprise a moon about the size of Mimas, one of Saturn's medium-small moons. The rings may in fact be at least partly composed of the remnants of such a moon or moons, torn apart by gravitational forces.

Named in order of discovery, the labels scientists have assigned to the major rings do not indicate their relative positions. From the planet outward, they are known as the D, C, B, A, F, G and E rings.

Running Rings Around Saturn

Cassini's tour of Saturn has been planned to include three ring observation periods. Much of Cassini's flight path so far has been along the plane of the rings, where basically the spacecraft sees the rings edge-on.

The first sequence of ring observations, about to begin, runs through early September and will take Cassini seven times around Saturn and its rings. These orbits will be inclined from the ring plane by 24 degrees. The second set of ring orbits occurs between summer 2006 and summer 2007, when the inclination gets up to 53 degrees. Late in the Cassini tour, starting in the fall of 2007, the third set of inclined orbits begins, and by the end of the mission in summer 2008, the inclination of the orbit reaches nearly 80 degrees. This will mean viewing the rings from almost straight above. Naturally, many new images will be taken, including the first complete global studies of several interesting regions in the rings, including the kinky F ring. Some other first-time events are: high-resolution, full-color images of the rings; radio wavelength mapping of the rings, using Cassini's main antenna; in-depth studies of thermal emission from the rings over a range of geometries; complete, high-resolution radial scans of the rings in near-infrared, which will provide information on composition of the rings. There will also be a number of new "occultations" of stars by the rings, when stars pass behind the rings from Cassini's point of view; these will be studied by two different instruments. Also in store are the first occultations by the rings of Cassini's radio signal, meaning that the signal will pass through the rings en route from the spacecraft to Earth. These will be studied at three radio wavelengths.

During these occultations, scientists will watch how a beam of light from a star or the radio waves from Cassini's transmitter are affected by the ring material as they pass through the ring. Each occultation experiment provides an opportunity for an extremely high-resolution study of a single path across the rings with resolutions of about 100 meters (330 feet) - some even have resolution as fine as 10 to 20 meters (33 to 66 feet). NASA's two Voyager spacecraft conducted one radio occultation and one stellar occultation by the rings. During its lifetime, Cassini will obtain 14 radio occultations and 80 stellar occultations, giving far more detailed studies of the ring structures.

The radio experiments will use Cassini's radio antennas and the ground-based antennas of NASA's Deep Space Network. From these measurements, scientists can derive information about the structures, composition, densities and sizes of ring particles. New moons may also be discovered from their effects on the ring material.