Psamathe (moon)
Psamathe, also known as Neptune X, is a retrograde irregular satellite of Neptune. It is named after Psamathe, one of the Nereids. Psamathe was discovered by Scott S. Sheppard and David C. Jewitt in 2003 using the 8.2 meter Subaru telescope. Before the announcement of its name on February 3, 2007, it was known by the provisional designation S/2003 N 1.
Psamathe is about 38 kilometers in diameter. It orbits Neptune at a distance of between 25.7 and 67.7 million km and requires almost 25 Earth years to make one orbit. The orbit of this satellite is close to the theoretical stable separation from Neptune for a body in a retrograde orbit. Given the similarity of Psamathe's orbital parameters with Neso, it was suggested that both irregular satellites could have a common origin in the break-up of a larger moon. Both are further from their primary than any other known moon in the Solar System.
