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Image of Nereid taken by Voyager 2
Date of discovery May 1, 1949[1]
Name of discoverer Gerard P. Kuiper[1][2]
Name origin Any of fifty women, daughters of Nereus and Doris, who attended Neptune
Orbital characteristics
Primary Neptune
Order from primary 8
Periposeidion 1,371,734 km[3]
Apoposeidion 9,655,066 km[3]
Semi-major axis 5,513,400 km[4]
Orbital eccentricity 0.7512[4]
Sidereal month 360.13619 da[4]
Avg. orbital speed 1.12 km/s[5]
Inclination 27.6°[5] to Neptune's equator
Rotational characteristics
Sidereal day 11.52 hr[6]
Rotational speed 0.02576 km/s[3]
Physical characteristics
Mass 3.09 * 1019 kg[3]
Density 1,500 kg/m³[7]
Mean radius 170 km[7]
Surface gravity 0.713 m/s²[3]
Escape speed 0.156 km/s[3]
Surface area 363,158 km²[3]
Composition Ice and silicates[8]
Color Neutral gray
Albedo 0.155[7]
Nereid, or Neptune II, is the second moon of Neptune to be discovered. It is also the third most massive of Neptune's thirteen moons, and eighth in orbit around Neptune.

Discovery and naming

Gerard P. Kuiper discovered Nereid in 1949. He gave it the generic name of the fifty female attendants of Neptune, the Roman god of the sea.

Orbital characteristics

Nereid is in a highly eccentric orbit around Neptune, and in fact its orbit is the most eccentric orbit of any solar system body.[9] Its sidereal month is about 360.14 Julian days. Nereid's orbit is inclined about 28 degrees from Neptune's equator, but is inclined only 7.23 degrees from the local Laplace plane.[4] Almost no astronomer believes that Nereid formed with the planet, and most astronomers believe that Nereid is a captured Kuiper belt or other trans-Neptunian object.[10]

Rotational characteristics

Nereid's sidereal day is 11.52 hours and is considered regular.[6]

Physical characteristics

Nereid has a mass of 3.09 * 1019 kg, the third greatest mass of the moons of Neptune. Its color is neutral-gray. Another moon of Neptune, Halimede (formerly S/2002 N 1), has the same neutral color, leading some astronomers to suggest that Halimede is a fragment of Nereid broken off in a collision with another, unknown object.[11]

Observation and Exploration

Telescopic observation has revealed little about Nereid beyond its orbital elements. The one spacecraft that visited the Neptunian system, Voyager 2, captured one low-resolution image of Nereid but did not fly close enough to it to discover more.


  1. 1.0 1.1 "Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers." US Geological Survey, Jennifer Blue, ed. March 31, 2008. Accessed May 9, 2008.
  2. Kuiper, Gerard P. "The Second Satellite of Neptune." Publications of the Astronomical Society of the Pacific, 61:175-176, 1949. Accessed June 8, 2008.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Calculated
  4. 4.0 4.1 4.2 4.3 Neptunian Satellite Fact Sheet, NASA, January 22, 2008. Accessed May 9, 2008.
  5. 5.0 5.1 Hamilton, Calvin J. "Entry for Nereid." Views of the Solar System, 2000. Accessed June 8, 2008.
  6. 6.0 6.1 Grav, Tommy, Holman, Matthew J., and Kavelaars, J. J. "The Short Rotation Period of Neptune." Astrophysical Journal Letters 591:L71-L74, July 1, 2003. arXiv:astro-ph/0306001 doi:10.1086/377067 Accessed June 8, 2008.
  7. 7.0 7.1 7.2 "Planetary Satellite Physical Parameters." JPL, NASA. Retrieved June 8, 2008.
  8. "Nereid." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 8 June 2008.
  9. Arnett, Bill. "Entry for Nereid." The Nine 8 Planets, May 6, 1995. Accessed June 8, 2008.
  10. Tytell, David. "Neptune's Stowaway." Sky and Telescope, September 11, 2003. Accessed June 8, 2008.
  11. Grav, T., Holman M. J., and Fraser W. "Photometry of Irregular Satellites of Uranus and Neptune." Astrophys. J. 613(2004):L77-L80. arXiv:astro-ph/0405605v1 Accessed June 8, 2008.