Nereid (moon)
From Wikipedia, the free encyclopedia
Image of Nereid by Voyager 2
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Discovery
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| Discovered by | Gerard P. Kuiper[1] |
| Discovery date | May 1, 1949 |
| Periapsis | 1,372,000 km (0.00917 AU) |
| Apoapsis | 9,655,000 km (0.06454 AU) |
| Semi-major axis | 5,513,400 km (0.03685 AU) |
| Eccentricity | 0.7512[2] (mean) |
| Orbital period | 360.1362 d |
| Average orbital speed | 0.934 km/s |
| Inclination | 5.07° (to ecliptic) 32.55° (to Neptune's equator) 7.232° (to the local Laplace plane) |
| Satellite of | Neptune |
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Physical characteristics
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| Mean radius | 170±25 km[3] |
| Mass | 3.1×1019 kg (assumed)[3] |
| Mean density | 1.5 g/cm3 (assumed)[3] |
| Rotation period | 0.48 d (11 h 31 min)[4] |
| Albedo | 0.155 [3] |
| Temperature | ~50 K mean (estimate) |
Nereid (pronounced /ˈnɪəriɪd/ [UK, US], or /ˈnɛriɪd/ [UK]; Greek Νηρηΐδα), or Neptune II, is a moon of Neptune.
Nereid was discovered on 1 May 1949 by Gerard P. Kuiper, who proposed the name in the report of his discovery. It is named after the Nereids, sea-nymphs of Greek mythology.
Nereid was the outermost known moon of Neptune from its discovery until 2002. Nereid's diameter is 340 km, making it the third-largest moon in the Neptune system. Its orbit averages 5,513,400 km in radius, but is highly eccentric and varies from 1,372,000 to 9,655,000 kilometres. This is the highest mean eccentricity of any known satellite in the solar system.[2] But it may lose this title briefly to an outer irregular moon of a gas giant, such as Saturn's Bestla and Uranus' Margaret, because of the Kozai mechanism.[5] The unusual orbit suggests that it may be a captured asteroid or Kuiper belt object, or that it was perturbed during the capture of Neptune's largest moon Triton.
Halimede may be a fragment of Nereid that was broken off during a collision.[6]
Nereid was too far away from Voyager 2 to be properly imaged when the spacecraft visited the Neptune system in 1989. Photos sent back show only its highly irregular shape, and no surface features could be seen at the resolution available.
[edit] References
- ^ Gerard P. Kuiper (1949). "The second satellite of Neptune". Publications of the Astronomical Society of the Pacific 61: 175–176.
- ^ a b R. A. Jacobson, J. E. Riedel, and A. H. Taylor (1991) (2007-Jun-28). Planetary Satellite Mean Orbital Parameters. JPL/NASA. Retrieved on 2008-01-19.
- ^ a b c d Planetary Satellite Physical Parameters. JPL (Solar System Dynamics) (2006-Jul-13). Retrieved on 2008-01-24.
- ^ T. Grav, M. Holman, J. J. Kavelaars (2003). "The Short Rotation Period of Nereid". The Astrophysical Journal 591: 71–74. doi:.
- ^ IAU-MPC Natural Satellites Ephemeris Service. IAU: Minor Planet Center. Retrieved on 2008-01-26.
As of 2008, the Natural Satellites Ephemeris Service lists the current instantaneous eccentricities and epochs as:
Margaret: 0.7979 (2006 Mar. 6.0 TT = JDT 2453800.5)
Nereid: 0.7464 (2007 Oct. 27.0 TT = JDT 2454400.5)
Bestla: 0.7309 (2007 Apr. 10.0 TT = JDT 2454200.5)
(To use this utility: pick a gas giant, deselect Ephemerides and select Orbital Elements) - ^ T. Grav, M. J. Holman and W. Fraser, Photometry of Irregular Satellites of Uranus and Neptune, The Astrophysical Journal, 613 (2004), pp. L77–L80. Preprint
[edit] External links
- Transit of the Sun on 2006-Mar-28 as seen from Neptune
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