One of the largest of → Neptune's known moons discovered in 1989 by the Voyager 2 space probe. Proteus revolves around Neptune at a distance of about 92,800 km, completing one orbit in 26 hours, 54 minutes. Proteus is about 400 m in diameter, larger than → Nereid. Orbiting the planet in the same direction as Neptune rotates, Proteus remains close to Neptune's equatorial plane. Proteus is irregularly shaped and heavily cratered, but it shows no sign of geological modification. In fact Proteus is about as large as a satellite can be without being pulled into a spherical shape by its own gravity. Proteus is one of the darkest objects in the solar system. Like Saturn's moon Phoebe, Proteus reflects only six percent of the sunlight that it receives.
Originally designated S/1989 N 1, Proteus is named after the shape-changing sea god of Greek mythology.