Larissa (moon)

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Larissa 1.jpg
Larissa from Voyager 2
Discovered by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky, and David J. Tholen
Discovery date May 24, 1981
Orbital characteristics[1]
Epoch 18 August 1989
73 548 ± 1 km
Eccentricity 0.001393 ± 0.00008
0.55465332 ± 0.00000001 d
  • 0.251 ± 0.009° (to Neptune equator)
  • 0.205° (to local Laplace plane)
Satellite of Neptune
Physical characteristics
Dimensions 216×204×168 km (± ~10 km)[2][3]
Mean radius
97 ± 3 km[4]
Volume ~3.5×106km³
Mass ~4.2×1018 kg (estimate)[a]
Mean density
~1.2 g/cm³ (estimate)[4]
~0.03 m/s2[b]
~0.076 km/s[c]
Albedo 0.09[2][4]
Temperature ~51 K mean (estimate)

Larissa (/ləˈrɪsə/ lə-RISS; Greek: Λάρισσα), also known as Neptune VII, is the fifth-closest inner satellite of Neptune. It is named after Larissa, a lover of Poseidon (Neptune) in Greek mythology and eponymous nymph of the city in Thessaly, Greece.


It was first discovered by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Tholen, based on fortuitous ground-based stellar occultation observations[6] on May 24, 1981, given the temporary designation S/1981 N 1 and announced on May 29, 1981.[7] The moon was recovered and confirmed to be the only object in its orbit during the Voyager 2 flyby in 1989[8] after which it received the additional designation S/1989 N 2 on August 2, 1989.[9] The announcement by Stephen P. Synnott spoke of “10 frames taken over 5 days”, which gives a recovery date sometime before July 28. The name was given on September 16, 1991.[10]


Front and back of Larissa
Map of Larissa
A simulated view of Larissa orbiting Neptune. The surface details are fictional.

The fourth-largest satellite of Neptune, Larissa is irregular (non-spherical) in shape and appears to be heavily cratered, with no sign of any geological modification. Little else is known about it. It is likely that Larissa, like the other satellites inward of Triton, 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.[11]

Larissa's orbit is circular but not perfect and lies below Neptune's synchronous orbit radius, so 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, similar to how Triton will crash into Neptune or break into a planetary ring.


Larissa has only been visited by Voyager 2. The probe was able to get photographs of Larissa.


  1. ^ The mass estimate is based on the assumed density of 1.2 g/cm³, and a volume of 3.5 ×106 km³ obtained from a detailed shape model in Stooke (1994).[5]
  2. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: Gm/r2.
  3. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: 2Gm/r.


  1. ^ 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.
  2. ^ 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.
  3. ^ Williams, Dr. David R. (2008-01-22). "Neptunian Satellite Fact Sheet". NASA (National Space Science Data Center). Retrieved 2008-12-13.
  4. ^ a b c d "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2010-10-18. Retrieved 2011-10-11.
  5. ^ Stooke, Philip J. (1994). "The surfaces of Larissa and Proteus". Earth, Moon, and Planets. 65 (1): 31–54. Bibcode:1994EM&P...65...31S. doi:10.1007/BF00572198.
  6. ^ Reitsema, H. J.; Hubbard, W. B.; Lebofsky, L. A.; Tholen, D. J. (1982). "Occultation by a Possible Third Satellite of Neptune". Science. 215 (4530): 289–291. Bibcode:1982Sci...215..289R. doi:10.1126/science.215.4530.289. PMID 17784355.
  7. ^ Marsden, Brian G. (May 29, 1981). "S/1981 N 1". IAU Circular. 3608. Retrieved 2011-10-26.
  8. ^ Smith, B. A.; Soderblom, L. A.; Banfield, D.; Barnet, C.; Basilevsky, A. T.; Beebe, R. F.; Bollinger, K.; Boyce, J. M.; Brahic, A. (1989). "Voyager 2 at Neptune: Imaging Science Results". Science. 246 (4936): 1422–1449. Bibcode:1989Sci...246.1422S. doi:10.1126/science.246.4936.1422. PMID 17755997. [on page 1435]
  9. ^ Marsden, Brian G. (August 2, 1989). "Satellites of Neptune". IAU Circular. 4824. Retrieved 2011-10-26.
  10. ^ Marsden, Brian G. (September 16, 1991). "Satellites of Saturn and Neptune". IAU Circular. 5347. Retrieved 2011-10-26.
  11. ^ 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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