As the Cassini spacecraft steams toward Saturn, the pictures are getting more detailed. Yesterday, mission managers released one that shows Prometheus and Pandora, two so-called "shepherding" moons with unpredictable orbits. Both moons have been imaged before, but not by Cassini.

Cassini's rings have clear divisions, and the moons play a role. At 63 miles wide (102 kilometers) Prometheus is visible left of center in the image, inside the F-ring. Pandora is 42 miles in diameter (84 kilometers) and appears above center, outside the ring.

A dark shadow cast by the planet stretches more than halfway across the A-ring, the outermost main ring, near the bottom of the picture. The mottled pattern appearing in the dark regions of the image is 'noise' in the signal recorded by the camera system.

Astronomers expect Cassini to help them understand when and how the mysterious rings formed. The craft is also likely to discover more, smaller moons embedded in the rings.

The F-ring is a narrow, ribbon-like structure, with a width seen in this geometry equivalent to a few kilometers. The two small, irregularly shaped moons exert a gravitational influence on particles that make up the F ring, confining it and possibly leading to the formation of clumps, strands and other structures observed there, according to astronomers at the Space Science Institute, which operates Cassini's camera for NASA.

Pandora prevents the F-ring from spreading outward and Prometheus prevents it from spreading inward. However, their interaction with the ring is complex and not fully understood.

The moons, which were discovered by the Voyager 1 spacecraft, are in chaotic orbits that can change unpredictably when the moons get very close to each other. This strange behavior was first noticed in ground-based and Hubble Space Telescope observations in 1995.

Credit: NASA/JPL/Space Science Institute