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Proteus: Overview
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Fuzzy image of the moon Proteus.
Proteus' irregular shape suggests it has been cold and rigid throughout its history and subject to significant impact cratering.

Proteus is one of the largest of Neptune's known moons, although it is not as big as Triton. The moon has an odd box-like shape and if it had just a little more mass it would be able to transform into a sphere. Proteus orbits Neptune about every 27 hours.

Proteus is irregularly shaped and heavily cratered, but it shows no sign of geological modification. Circling the planet in the same direction as Neptune rotates, Proteus remains close to Neptune's equatorial plane. Proteus is one of the darkest objects in our solar system. Like Saturn's moon Phoebe, Proteus reflects only 6 percent of the sunlight that hits it.

Proteus was discovered in 1989 by the Voyager 2 spacecraft. This is unusual since a smaller moon, Nereid, was discovered 33 years earlier using an Earth-based telescope. Proteus was most likely overlooked because it is so dark and the distance between Earth and Neptune is so great.

How Proteus Got its Name:
Moons of Neptune are named for characters from Greek or Roman mythology associated with Neptune or Poseidon, or the oceans. Irregular satellites are named for the Nereids, daughters of Nereus and Doris and the attendants of Neptune.

Proteus is named after the shape-changing sea god of Greek mythology. Proteus was originally designated S/1989 N 1.

Just the Facts
Orbit Size (semi-major axis):  117,646 km
Mean Radius:  210.0 km
Volume:  38,792,386 km3
Mass:  50,355,259,352,065,700,000 kg
22 Jan 2003: 
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Last Updated: 12 Aug 2013