Proteus
 Proteus, the second-largest moon of Neptune, is a bare chunk of dark rock orbiting close to the giant planet. Proteus was discovered in August 1989 by the Voyager 2 spacecraft, which beamed back this picture. [NASA/JPL]
Before Voyager 2, astronomers knew of only two moons orbiting Neptune. One, named Triton, was large. It was first sighted back in 1846, shortly after Neptune itself was discovered. The other moon, Nereid, was smaller, and was discovered in 1949.
But when Voyager 2 swept past Neptune in August of 1989, it found several more moons -- and one of them was larger than Nereid. Astronomers named it Proteus, for a Greek sea god. In mythology, Neptune himself was the Roman god of the sea.
Proteus is Neptune's second-largest moon, with a diameter of about 260 miles. Voyager pictures show that it's heavily cratered -- the victim of numerous impacts by smaller bodies. The most impressive crater is more than half the size of Proteus itself.
Although Proteus is larger than Nereid, it's much closer to Neptune, whose glare makes Proteus difficult to see from Earth. It's only a third as far from Neptune as our moon is from Earth. At that distance, it zips around the giant planet once every 27 hours.
As a result, Neptune's second-largest moon eluded detection until 20 years ago -- when an emissary from Earth went out to meet it.
Script by Ken Croswell, Copyright 2009
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