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Naiad or Thalassa as seen by Voyager 2
(smearing has caused excessive elongation)
Discovered by Voyager Imaging Team
Discovered in September 1989
Orbital characteristics
Epoch 18 August 1989
Semi-major axis 48 227 ± 1 km
Eccentricity 0.0004 ± 0.0003
Orbital period 0.2943958 ± 0.0000002 d
Inclination 4.75 ± 0.03° (to Neptune equator)

4.75° (to local Laplace plane)

Is a satellite of Neptune
Physical characteristics
Dimensions 96×60×52 km
Mean radius 33 ± 3 km
Mass ~1.9 kg
(based on assumed density)
Mean density ~1.2 g/cm3 (estimate)
Rotation period assumed synchronous
Axial tilt ~zero presumably
Albedo (geometric) 0.07
Surface temp. ~51 K mean (estimate)
Atmosphere none
Naiad ( , , or as in Greek Ναϊάδ-ες), also known as Neptune III, is the innermost satellite of Neptune named after the Naiads of Greek legend.

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 after 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 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 Keck telescopemarker 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 Voyager 2. Still, Naiad has not been found. It is suspected that this is due to considerable errors in Naiad's ephemeris.


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