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Voyager 2 view of Nereid.

Nereid (neer'-ee-id (UK, US), nair'-ee-id (UK), Greek Νηρηΐδα) is a moon of Neptune.

Nereid was discovered on 1949-05-01 by Gerard P. Kuiper. It is named after the Nereids, sea-nymphs of Greek mythology. It is also designated Neptune II.

It was the outermost of Neptune's known moons for a long time (S/2002 N 1 through S/2003 N 1 have since taken that honour), and is the third largest, with a diameter of 340 km. Its orbit averages 5,513,400 km in radius, but is highly eccentric and varies from 1,353,600 to 9,623,700 kilometres. This is the most highly eccentric orbit of any known satellite in the solar system. The unusual Nereidian orbit suggests that it may be a captured asteroid or Kuiper belt object, or possibly that it was perturbed during the capture of Neptune's largest moon Triton. Very little else is known of Nereid.

Nereid was too far to be properly imaged by the Voyager 2 probe as it visited the Neptune system in 1989. Photos sent back show only its highly irregular shape as no surface features could be seen at the resolution available.

A simulated view of Nereid with Neptune in the distance


  • Orbital radius: 5,513,400 km
  • Periposeidion: 1,353,600 km
  • Apoposeidion: 9,623,700 km
  • Diameter: 340 km
  • Mass: 3.1×1019 kg
  • Orbital period: 360.14 d
  • Orbital inclination: 32.55° (to Neptune's equator), 7.232° (to the local Laplace plane), 5.07° (to the ecliptic)
  • Rotation period: 0.48 d (11.52 h)


Kuiper, Gerard P., "The second satellite of Neptune", PASP 61 (1949) 175–176

S/2002 N 1 | S/2002 N 2 | S/2002 N 3 | S/2003 N 1 | S/2002 N 4

Astronomy Encyclopedia

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