Galatea (moon)


Galatea moon.jpg
Galatea as seen by Voyager 2 (elongation is due to smearing)
Discovered byStephen P. Synnott[1] and Voyager Imaging Team
Discovery dateJuly 1989
Neptune VI
Named after
Γαλάτεια Galateia
Orbital characteristics[4][5]
Epoch 18 August 1989
61 952.57  km
Eccentricity0.00022 ± 0.00008
0.42874431 ± 0.00000001 d
  • 0.052 ± 0.011° (to Neptune equator)
  • 0.06° (to local Laplace plane)
Satellite ofNeptune
Groupring shepherd
Physical characteristics
Dimensions204×184×144 km (±~10 km)[6][7]
Mean radius
87.4 ± 4.9 km[5]
Mass2.12 ± 0.08 ×1018 kg[8]
Mean density
~0.75 g/cm³ (estimate)[9]
~0.018 m/s2[a]
~0.056 km/s[b]
Temperature~51 K mean (estimate)

Galatea /ɡæləˈtə/, also known as Neptune VI, is the fourth-closest inner satellite of Neptune. It is named after Galatea, one of the fifty Nereids of Greek legend, with whom Cyclops Polyphemus was vainly in love.


Galatea inside of a faint ring arc near Neptune

Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe. It was given the temporary designation S/1989 N 4.[10] The discovery was announced (IAUC 4824) on 2 August 1989, and mentions "10 frames taken over 5 days", implying a discovery date of sometime before July 28. The name was given on 16 September 1991.[11]

Physical properties

Galatea 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.[12]


Galatea's orbit lies below Neptune's synchronous orbit radius, so it is slowly spiralling inward due to tidal deceleration and may eventually impact the planet or break up into a new planetary ring system upon passing its Roche limit due to tidal stretching.

Galatea appears to be a shepherd moon for the Adams ring that is 1,000 kilometres (620 mi) outside its orbit. Resonances with Galatea in the ratio 42:43 are also considered the most likely mechanism for confining the unique ring arcs that exist in this ring.[13] Galatea's mass has been estimated based on the radial perturbations it induces on the ring.[8]


  1. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: Gm/r2.
  2. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: 2Gm/r.

Since Galatea is irregularly shaped, the actual surface gravity and escape velocity will vary significantly between different positions on the surface.


  1. ^ Planet Neptune Data
  2. ^ "galatea". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  3. ^ AMIA (1999) Transforming health care through informatics
  4. ^ 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.
  5. ^ a b 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. Bibcode:2019Natur.566..350S. doi:10.1038/s41586-019-0909-9. PMC 6424524. PMID 30787452.
  6. ^ 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.
  7. ^ Williams, Dr. David R. (2008-01-22). "Neptunian Satellite Fact Sheet". NASA (National Space Science Data Center). Retrieved 2008-12-13.
  8. ^ a b Porco, C.C. (1991). "An Explanation for Neptune's Ring Arcs". Science. 253 (5023): 995–1001. Bibcode:1991Sci...253..995P. doi:10.1126/science.253.5023.995. PMID 17775342. S2CID 742763.
  9. ^ a b c "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2008-10-24. Retrieved 2008-12-13.
  10. ^ Marsden, Brian G. (August 2, 1989). "Satellites of Neptune". IAU Circular. 4824. Retrieved 2011-10-26.
  11. ^ Marsden, Brian G. (September 16, 1991). "Satellites of Saturn and Neptune". IAU Circular. 5347. Retrieved 2011-10-26.
  12. ^ 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.
  13. ^ Namouni, F.; C. Porco (2002). "The confinement of Neptune's ring arcs by the moon Galatea". Nature. 417 (6884): 45–7. Bibcode:2002Natur.417...45N. doi:10.1038/417045a. PMID 11986660. S2CID 4430322.

External links

  • Galatea Profile by NASA's Solar System Exploration
  • Neptune's Known Satellites (by Scott S. Sheppard)