NEREID

Neptune’s boomerang

Mass: 3.1 x 1019kg (6.8 x 1019lbs) Diameter: 340km (211 miles) Parent planet: Neptune Discovered: 1949, Gerard Kuiper

Nereid was the second moon found to orbit Neptune, and its claim to fame arises from its extreme orbit. Nereid’s distance from Neptune ranges between 1.4 million and 9.7 million kilometres (870,000 and 6 million miles). This orbit is usually typical of captured satellites – asteroids and comets swept up into highly eccentric orbits by the gravity of the giant outer planets – but Nereid’s unusually large size suggests a rather more interesting story.

Evidence from Voyager 2’s 1989 flyby suggests that Triton was captured into orbit from the nearby Kuiper Belt. Triton would have disrupted the orbits of Neptune’s original moons, ejecting many of them. But many astronomers believe Nereid could be a survivor, clinging on at the edge of Neptune’s gravitational reach.