NASA's Cassini spacecraft has been orbiting Saturn for only 5½ years, but in that time this remarkable craft has had occasion to pass near many of the planet's 61 moons (six of which were discovered in its images). It's an amazing bunch of bodies — variously big and small, smooth and cratered, gas-gushing and quiescent.
But the close-up of Prometheus seen here represents new territory, in a sense. This elongated body, just 74 miles (119 km) long, has an alien quality to it. Note how all the edges are rounded and the craters are filled in. We're just not used to seeing moons (or asteroids) with such muted features. Heck, I've seen potatoes with more detailed surfaces — but no mere spud has ever fascinated me nearly as much as this picture does!
Cassini captured this view on December 26th, from just 36,000 miles (59,000 km) away. We're not likely to see Prometheus with this kind of detail (about 1,150 feet, or 350 meters, per pixel) for a very long time, if ever again.
Discovered in 1980 by Voyager 1, Prometheus and like-sized Pandora are the F ring's gatekeepers, "shepherd satellites" that circle just inside and outside the faint ribbon of dust. Their gravitational influence confines the F ring, keeps its particles from dispersing, and gives a tortured appearance.
But all this gravitational jostling forces each moon into a very slightly eccentric orbit, one part in 450 (0.0022) for Prometheus and one in 240 (0.0042) for Pandora. The slight orbital eccentricity causes Prometheus to make headlong dives in the F ring once per orbit (every 15 hours) into Saturn's F ring. You'd lose the rough edges too if you endured all that peppering. Pandora, which got its Cassini close-up in 2005, looks somewhat the same — though not nearly as blanketed in ring dust.
I'll bet that Carolyn Porco, the lead scientist for Cassini's imaging team, finds this picture particularly satisfying. As a young scientist on the Voyager mission, she specialized in the gravitational interplay of rings and moons. While there's been no shortage of amazing sights to grab her attention throughout Cassini's long-running romp, I suspect that right now she's thinking long and hard about how this funny little moon, along with its buddies Pandora and Atlas (the A ring's shepherd), came to be.
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