Proteus (moon)

Proteus (ˈproʊtiəs ), also known as Neptune VIII, is the second-largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology. Proteus orbits Neptune in a nearly equatorial orbit at a distance of about 4.75 times the radius of Neptune's equator. Despite being a predominantly icy body more than in diameter, Proteus's shape deviates significantly from an ellipsoid. It is shaped more like an irregular polyhedron with several slightly concave facets and relief as high as . Its surface is dark, neutral in color, and heavily cratered. Proteus's largest crater is Pharos, which is more than in diameter. There are also a number of scarps, grooves, and valleys related to large craters. Proteus is probably not an original body that formed with Neptune. It could have accreted later from the debris formed when the largest Neptunian satellite, Triton, was captured. Proteus was discovered from the images taken by the Voyager 2 space probe two months before its Neptune flyby in August 1989. Proteus was the third moon of Neptune to be discovered, 40 years after the discovery of Neptune's moon Nereid in 1949. Upon discovery, Proteus received the temporary provisional designation S/1989 N 1. Stephen P. Synnott and Bradford A. Smith announced its discovery on July 7, 1989, speaking only of "17 frames taken over 21 days", which gives a discovery date of sometime before June 16. On 16 September 1991, S/1989 N 1 was named after Proteus, the shape-changing sea god of Greek mythology. Proteus orbits Neptune at a distance of approximately , nearly equal to 4.75 times the equatorial radius of the planet. The orbit of Proteus is nearly circular, having a small orbital eccentricity, and is inclined by about 0.5 degrees to Neptune's equator. Proteus is tidally locked to Neptune, and rotates synchronously with its orbital motion, which means that Proteus always presents the same face to Neptune.