 Naiad or Thalassa as seen by Voyager 2 (smearing has caused excessive elongation) | |
| Discovery | |
|---|---|
| Discovered by | Richard J. Terrile[1] and Voyager Imaging Team |
| Discovered in | September 1989 |
| Orbital characteristics[2] | |
| | |
| 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 satellite of | Neptune |
| Physical characteristics | |
| Dimensions | 108×100×52 km[3][4] |
| Mean radius | 41 ± 3 km[5] |
| 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[3][4] |
| Surface temp. | ~51 K mean (estimate) |
| Atmosphere | none |
Thalassa (pronounced /θəˈlæsə/ thə-LASS-ə, or as in Greek Θάλασσα), also known as Neptune IV, is the second innermost satellite of Neptune. Thalassa 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 temporary designation S/1989 N 5.[6] The discovery was announced (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[7].
Thalassa is irregularly shaped and shows no sign of any geological modification. 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.[8] Unusually for irregular bodies, it appears to be roughly disk-shaped.
Since the Thalassian orbit is below Neptune's synchronous orbit radius, it is slowly spiralling inward due to tidal decceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching. Relatively soon after, the spreading debris may impinge upon Despina's
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