StarDate: August 22, 2009

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Twenty years ago today, the Voyager 2 spacecraft was speeding toward Neptune for the first and only encounter with the giant planet. Neptune is almost three billion miles away -- so far that sunlight takes more than four hours to reach it. That's also how long it took Voyager's radio signals to reach Earth.

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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