Nereid is the third largest Neptunian moon, with a highly eccentric orbit. Nereid is the 21st largest moon in the Solar System.
Discovery
Nereid was discovered on May 1, 1949, by Gerard P. Kuiper, on photographic plates taken with the 82-inch telescope at the McDonald Observatory.
Naming
The moon is named after the Nereids, sea-nymphs of Greek mythology and attendants of the god Neptune.
In Greek mythology, the Nereids are sea nymphs, the fifty daughters of Nereus and Doris, sisters to Nerites. They often accompany Poseidon and can be friendly and helpful to sailors fighting perilous storms.
Stats
Diameter (mean): 340 km
Semi-major axis: 5,513,818 km
Orbital Period: 360.14 days
Rotation Period: 11 hrs 31 minutes
Orbit
Nereid orbits Neptune in the prograde direction at an average distance of 5,513,818 km, but its high eccentricity of 0.7507 takes it as close as 1,372,000 km and as far as 9,655,000 km from the planet.
The unusual orbit suggests that it may be either a captured asteroid or Kuiper belt object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon Triton.
Physical characteristics
Nereid is rather large for an irregular satellite. The shape of Nereid is not known.
Spectrally Nereid appears neutral in colour and water ice has been detected on its surface. Its spectrum appears to be intermediate between Uranus's moons Titania and Umbriel, which suggests that Nereid's surface is composed of a mixture of water ice and some spectrally neutral material.
Nereid's spectrum is markedly different from the outer-Solar-System minor planets, centaurs Pholus, Chiron and Chariklo, suggesting that Nereid possibly formed around Neptune rather than being a captured body.
Exploration
The only spacecraft to visit Nereid is Voyager 2, which passed Nereid at a distance of 4,700,000 km between April 20 and August 19, 1989.
Voyager 2 obtained 83 images of the moon with observation accuracies of 70 km to 800 km.
Not much details is known about Nereid.
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