In 1989, the Voyager space probe discovered six moons of Neptune that had hitherto been unknown. The largest of these moons is even larger than Nereid, a moon that had been discovered from the Earth. Nereid has a diameter of between 300 and 400 kilometers and revolves about Neptune in an elliptical orbit. Its period of revolution is approximately one year.
Nereid's orbit is also highly eccentric so that the distance to the planet at the farthest point is seven times the distance between the satellite and the planet when it is closest to Neptune. Astronomers assume that Nereid was captured by the planet's gravitational field at a relatively late stage.
The largest Neptune moon is Triton. It has a diameter of 2700 kilometers and a mean distance to the planet of 350,000 kilometers, which is comparatively close.
Triton is also the only larger moon in the solar system with a retrograde orbit, e.g. revolving around the planet in the opposite direction to the other moons. It also stands out as the object where the lowest temperature was ever measured within our solar system. On this frosty satellite, it is minus 235┬░ Celsius cold. It features a thin atmosphere of nitrogen fed by its polar caps and from volcanoes spewing nitrogen ice crystals.
The images taken of Triton by the Voyager probe and sent to Earth are breathtaking. Triton's surface shows numerous fissures that are believed to have been formed when the moon's surface repeatedly melted and re-froze. Compared with Triton, the inner moons of Neptune are rather unimpressive featureless rocks of irregular shape. These satellites have diameters of between 50 and 420 kilometers.
