The second-largest satellite of Neptune, 436 × 416 × 402 km in size; also known as Neptune VIII. Proteus was discovered in 1989 on images from the Voyager 2 spacecraft, and orbits the planet in 1.122 days at a distance of 117 646 km. It has a rough surface covered with impact craters, including one over 200 km in diameter, named Pharos.