Naiad or Thalassa as seen by Voyager 2 (elongation is due to smearing)
|Discovered by||Richard J. Terrile and Voyager Imaging Team|
|Discovery date||September 1989|
|Epoch 18 August 1989|
|50 075 ± 1 km|
|Eccentricity||0.0002 ± 0.0002|
|0.31148444 ± 0.00000006 d|
|41 ± 3 km|
(based on assumed density)
|~1.2 g/cm³ (estimate)|
|Temperature||~51 K mean (estimate)|
Thalassa (// thə-LASS-ə; Greek: Θάλασσα), also known as Neptune IV, is the second innermost satellite of Neptune. Thalassa was named after sea goddess Thalassa, 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. 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.
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. 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 deceleration 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 orbit.
- Planet Neptune Data http://www.princeton.edu/~willman/planetary_systems/Sol/Neptune/
- 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.
- 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.
- Williams, Dr. David R. (2008-01-22). "Neptunian Satellite Fact Sheet". NASA (National Space Science Data Center). Retrieved 2008-12-13.
- "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2010-10-18. Retrieved 2011-10-11.
- Green, Daniel W. E. (September 29, 1989). "Neptune". IAU Circular 4867. Retrieved 2011-10-26.
- Marsden, Brian G. (September 16, 1991). "Satellites of Saturn and Neptune". IAU Circular 5347. Retrieved 2011-10-26.
- 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.
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