In the near-infrared part of the spectrum the surface becomes less reflective around 2 μm pointing to a possible presence of complex organic compounds such as hydrocarbons or cyanides. The color of its surface is neutral as the reflectivity does not change appreciably with the wavelength from violet to green. The surface of Proteus is dark-its geometrical albedo is about 10%. It was not discovered by Earth-based telescopes because it is so close to Neptune that it is lost in the glare of reflected sunlight. It is about 420 kilometres in diameter, larger than Nereid, the second to be discovered. Proteus is the second largest moon of Neptune. It rotates synchronously with the orbital motion, which means that one face always points to the planet. Proteus is the largest of the regular prograde satellites of Neptune. Its orbit has a small eccentricity and is inclined by about 0.5° to the planet's equator. Proteus orbits Neptune at the distance approximately equal to 4.75 equatorial radii of the planet. On 16 September 1991 S/1989 N 1 was named after Proteus, the shape-changing sea god of Greek mythology. 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. It received the temporary designation S/1989 N 1. Proteus was discovered from the images taken by Voyager 2 space probe two months before its Neptune flyby in August 1989. Proteus is probably not an original body that formed with Neptune it may have accreted later from the debris created when the largest Neptunian satellite Triton was captured. There are also a number of scarps, grooves, and valleys related to large craters. Proteus's largest crater is Pharos, which is more than 230 km in diameter. Its surface is dark, neutral in color and heavily cratered. It is shaped more like an irregular polyhedron with several slightly concave facets and relief as high as 20 km. Proteus orbits Neptune in a nearly equatorial orbit at the distance of about 4.75 equatorial radii of the planet.ĭespite being a predominantly icy body more than 400 km in diameter, Proteus's shape deviates significantly from an ellipsoid. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology. Proteus ( / ˈ p r oʊ t iː ə s/ Greek: Πρωτεύς), also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite.