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Naiad (/ˈn.əd/ NAY-əd or /ˈn.əd/ NY-əd; Greek: Ναϊάδ-ες), also known as Neptune III, is the innermost satellite of Neptune, named after the Naiads of Greek legend.[6]

Naiad Voyager.png
Naiad or Thalassa as seen by Voyager 2 (elongation is due to smearing)
Discovered byVoyager Imaging Team
Discovery dateSeptember 1989
Orbital characteristics[1][2]
Epoch 18 August 1989
48 224.41  km
Eccentricity0.0047 ± 0.0018
0.2943958 ± 0.0000002 d
  • 4.75 ± 0.03° (to Neptune equator)
  • 4.75° (to local Laplace plane)
Satellite ofNeptune
Physical characteristics
Dimensions96×60×52 km[3][4]
Mean radius
30.2 ± 3.2 km[2]
Mass~1.9×1017 kg
(based on assumed density)
Mean density
~1.3 g/cm³ (estimate)[5]
~0.012 m/s2[a]
~0.028 km/s[b]
Temperature~51 K mean (estimate)


Naiad was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe. The last moon to be discovered during the flyby, it was designated S/1989 N 6.[7] The discovery was announced on 29 September 1989, in the IAU Circular No. 4867, and mentions "25 frames taken over 11 days", implying a discovery date of sometime before September 18. The name was given on 16 September 1991.[8]

Physical characteristicsEdit

A simulated view of Naiad orbiting Neptune.

Naiad is irregularly shaped and probably has not been modified by any internal geological processes since its formation. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.[9]


Since the Voyager 2 flyby, the Neptune system has been extensively studied from ground-based observatories and the Hubble Space Telescope as well. In 2002–03 the Keck telescope observed the system using adaptive optics and detected easily the largest four inner satellites. Thalassa was found with some image processing, but Naiad was not located.[10] Hubble has the ability to detect all the known satellites and possible new satellites even dimmer than those found by Voyager 2. On October 8, 2013 the SETI Institute announced that Naiad had been located in archived Hubble imagery from 2004.[11] The suspicion that the loss of positioning was due to considerable errors in Naiad's ephemeris[12] proved correct as Naiad was ultimately located 80 degrees from its expected position.


  1. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: Gm/r2.
  2. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: 2Gm/r.


  1. ^ Jacobson, R. A.; Owen, W. M., Jr. (2004). "The orbits of the inner Neptunian satellites from Voyager, Earthbased, and Hubble Space Telescope observations". Astronomical Journal. 128 (3): 1412–1417. Bibcode:2004AJ....128.1412J. doi:10.1086/423037.
  2. ^ a b Showalter, M. R.; de Pater, I.; Lissauer, J. J.; French, R. S. (2019). "The seventh inner moon of Neptune" (PDF). Nature. 566 (7744): 350–353. doi:10.1038/s41586-019-0909-9.
  3. ^ a b Karkoschka, Erich (2003). "Sizes, shapes, and albedos of the inner satellites of Neptune". Icarus. 162 (2): 400–407. Bibcode:2003Icar..162..400K. doi:10.1016/S0019-1035(03)00002-2.
  4. ^ Williams, Dr. David R. (2008-01-22). "Neptunian Satellite Fact Sheet". NASA (National Space Science Data Center). Retrieved 2008-12-13.
  5. ^ a b c "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2008-10-24. Retrieved 2008-12-13.
  6. ^ "Planet and Satellite Names and Discoverers". Gazetteer of Planetary Nomenclature. USGS Astrogeology. July 21, 2006. Retrieved 2006-08-05.
  7. ^ Green, Daniel W. E. (September 29, 1989). "Neptune". IAU Circular. 4867. Retrieved 2011-10-26.
  8. ^ Marsden, Brian G. (September 16, 1991). "Satellites of Saturn and Neptune". IAU Circular. 5347. Retrieved 2011-10-26.
  9. ^ Banfield, Don; Murray, Norm (October 1992). "A dynamical history of the inner Neptunian satellites". Icarus. 99 (2): 390–401. Bibcode:1992Icar...99..390B. doi:10.1016/0019-1035(92)90155-Z.
  10. ^ Marchis, F.; Urata, R.; de Pater, I.; Gibbard, S.; Hammel, H. B.; Berthier, J. (May 2004). "Neptunian Satellites observed with Keck AO system". American Astronomical Society, DDA meeting #35, #07.08; Bulletin of the American Astronomical Society. 36. p. 860. Retrieved 2006-08-05.
  11. ^ "Lost Neptune Moon Re-Discovered". Retrieved 9 October 2013.
  12. ^ Showalter, M. R.; Lissauer, J. J.; de Pater, I. (August 2005). "The Rings of Neptune and Uranus in the Hubble Space Telescope". American Astronomical Society, DPS meeting #37, #66.09; Bulletin of the American Astronomical Society. 37. p. 772. Retrieved 2006-08-05.

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