Naiad or Thalassa as seen by Voyager 2 (elongation is due to smearing)
|Discovered by||Voyager Imaging Team|
|Discovery date||September 1989|
|Epoch 18 August 1989|
|48 227 ± 1 km|
|Eccentricity||0.0004 ± 0.0003|
|0.2943958 ± 0.0000002 d|
|33 ± 3 km|
(based on assumed density)
|~1.3 g/cm³ (estimate)|
|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. The discovery was announced on September 29, 1989, in the IAU Circular No. 4867, but the text only talks of "25 frames taken over 11 days", giving a discovery date of sometime before September 18. The name was given on 16 September 1991.
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.
Naiad orbits about 23,500 km above Neptune's cloud tops. Since this is below the synchronous orbit radius, its orbit is slowly decaying due to tidal deceleration and it may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching. Naiad orbits Neptune well within its fluid Roche limit, and its density is expected to be low enough that it may be very close to its actual Roche limit already.
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. 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. The suspicion that the loss of positioning was due to considerable errors in Naiad's ephemeris proved correct as Naiad was ultimately located 80 degrees from its expected position.
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- "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2008-10-24. Retrieved 2008-12-13.
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- 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.
- "Lost Neptune Moon Re-Discovered". Retrieved 9 October 2013.
- 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.
- Naiad Profile by NASA's Solar System Exploration
- Neptune's Known Satellites (by Scott S. Sheppard)
- Archival Hubble Images Reveal Neptune's “Lost” Inner Moon (SETI : October 8, 2013)