Moon and Saturn
 Saturn's rings curve gracefully around the giant planet in this view from the Cassini spacecraft, which is orbiting Saturn. The rings span almost a quarter-million miles, yet are only a few yards thick. The thousands of individual rings are held in check by the gravity of small moons, known as shepherds, that orbit between the rings. [NASA/JPL/SSI]
Appropriately enough, these moons are known as shepherd satellites. The biggest are around a hundred miles in diameter, but most are much smaller -- chunks of rock and ice about the size of a mountain.
Thousands of individual rings encircle Saturn. The ring system spans the distance from Earth to the Moon, yet most of the rings are no thicker than the height of a three-story building.
The rings consist of bits of ice, or ice mixed with rock. Most are no bigger than ice cubes, so they're easily pushed around. Collisions with other particles could knock them out of the rings entirely. But the shepherd moons prevent that. As they orbit within the ring system, their gravity helps nudge wayward particles back into place.
The interplay between moons and rings is fascinating to watch. Images from the Cassini spacecraft show that a passing moon can create ripples in the rings, like the wake of a boat. It can also create kinks and braids, like twisted strands of spaghetti. So the flock may get a little unruly, but thanks to the shepherd moons, it stays in place.
Look for Saturn near our own moon tonight. They climb into view by around midnight, and stand high in the south at first light. Saturn looks like a bright golden star to the left of the Moon.
Script by Damond Benningfield, Copyright 2008
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