A comparatively small and little-known moon of the planet Neptune, named for the Nereids, the sea-nymph handmaidens of the god Neptune. (It is unusual for a moon to be named after a class of beings like this, rather than a single mythological entity, but not unique: Naiad, another of Neptune's moons, derives its name in the same way.)
Origins and Orbit
Nereid pursues a wildly eccentric orbit around Neptune. At times it approaches the planet to within 1.4 million kilometres, while at other points on its orbit it can reach a distance of some 9.7 million kilometres. To put these figures into some perspective, at its most distant point from Neptune, Nereid is nearly thirty times as distant as Neptune's major moon Triton.
This extreme orbit implies that Nereid has had a turbulent past. It may be a captured object from outside the Neptune system, but the best evidence suggests that it started life as a typical inner moon that suffered some effect significant enough to throw it out into its current extreme orbit. Another of Neptune's minor moons, Halimede, seems to share Nereid's physical parameters, implying that Halimede fractured away from Nereid at some point in the past, probably due to some cataclysmic collision.
Nereid is the third largest of Neptune's moons, after Triton (by far the largest of Neptune's moons) and Proteus. It has an irregular shape, but its average diameter is some 340 km (so in cross section it would be roughly the size of the island of Ireland). Very little is known about Nereid's precise form and composition, but it does appear to have at least some water ice on its surface.