Thalassa (moon)

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Naiad or Thalassa as seen by Voyager 2
(smearing has caused excessive elongation)
Discovered by Richard J. Terrile and Voyager Imaging Team
Discovered in September 1989
Orbital characteristics
Epoch 18 August 1989
Semi-major axis 50 075 ± 1 km
Eccentricity 0.0002 ± 0.0002
Orbital period 0.31148444 ± 0.00000006 d
Inclination 0.21 ± 0.02° (to Neptune equator)

0.21° (to local Laplace plane)

Is a moon of Neptune
Physical characteristics
Dimensions 108×100×52 km
Mass ~3.5×1017 kg
(based on assumed density)
Mean density ~1.2 g/cm3 (estimate)
Rotation period assumed synchronous
Axial tilt ~zero presumably
Albedo (geometric) 0.09
Surface temp. ~51 K mean (estimate)
Atmosphere none
A simulated view of Thalassa orbiting Neptune.

Thalassa or Neptune IV, is the second closest moon to Neptune. It was named after a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".

Thalassa was discovered sometime before mid-September, 1989 from the images taken by the Voyager 2 probe. It was given the designation S/1989 N 5. The discovery was said (IAUC 4867) on September 29, 1989, 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.

Thalassa is not a sphere and shows no sign of any geological changes. Unusually for a non-spherical moon, it appears to be oddly shaped not able to be explained.

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