Thalassa (/θəˈlæsə/ thə-LASS; 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".

Neptune Trio.jpg
A Voyager 2 image of Thalassa (1989 N5), Naiad (1989 N6) and Despina (1989 N3)
Discovered byRichard J. Terrile[1] and Voyager Imaging Team
Discovery dateSeptember 1989
Orbital characteristics[2][3]
Epoch 18 August 1989
50 074.44  km
Eccentricity0.00176 ± 0.00054
0.31148444 ± 0.00000006 d
  • 0.21 ± 0.02° (to Neptune equator)
  • 0.21° (to local Laplace plane)
Satellite ofNeptune
Physical characteristics
Dimensions108×100×52 km[4][5]
Mean radius
40.7 ± 2.8 km[6]
Mass~3.5×1017 kg
(based on assumed density)
Mean density
~1.2 g/cm³ (estimate)[6]
Temperature~51 K mean (estimate)


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.[7] The discovery was announced (IAUC 4867) on September 29, 1989, and mentions "25 frames taken over 11 days", implying a discovery date of sometime before September 18. The name was given on 16 September 1991.[8]

Physical propertiesEdit

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.[9] 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.

A simulated view of Thalassa orbiting Neptune.


  1. ^ Planet Neptune Data
  2. ^ 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.
  3. ^ Showalter, M. R.; de Pater, I.; Lissauer, J. J.; French, R. S. (2019). "The seventh inner moon of Neptune" (PDF). Nature. 566 (7744): 350–353. doi:10.1038/s41586-019-0909-9.
  4. ^ a b 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.
  5. ^ Williams, Dr. David R. (2008-01-22). "Neptunian Satellite Fact Sheet". NASA (National Space Science Data Center). Retrieved 2008-12-13.
  6. ^ a b c d "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2010-10-18. Retrieved 2011-10-11.
  7. ^ Green, Daniel W. E. (September 29, 1989). "Neptune". IAU Circular. 4867. Retrieved 2011-10-26.
  8. ^ Marsden, Brian G. (September 16, 1991). "Satellites of Saturn and Neptune". IAU Circular. 5347. Retrieved 2011-10-26.
  9. ^ 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.

External linksEdit