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Psamathe (sam'-ə-thee,, Latin Psamathē from Greek Ψαμάθη) or Neptune X, is a retrograde irregular satellite of Neptune.

Psamathe was discovered by S.Sheppard, D. Jewiit and J. Kleyna in September 2003. They reported a satellite which proved to be the same as one of the moons also observed by M. Holman and given the provisional designation S/2003 N 1.

It is named after Psamathe, one of the Nereids.

Psamathe orbits Neptune at a distance of about 46.7 Gm and remarkably takes almost 25 Earth years to make one orbit. It's diameter is estimated at 28 kilometers, assuming albedo of 0.04.

The diagram illustrates its orbit in relation to other irregular satellites of Neptune. The eccentricity of selected orbits is represented by the yellow segments (extending from the pericentre to the apocentre).

Given the similarity of the orbit's parameters with S/2002 N 4, it was suggested that both irregular satellites could have a common origin in the break-up of a larger moon.