List of Neptune's moons

Neptune has a total of 16 known moons.  The largest moon is Triton, which was discovered by William Lassell just seventeen days after Neptune was found. One hundred years later, the second moon, Nereid was found.  The Hubble Telescope found the 14th moon in 2013.

Most of Neptune's moons are named after sea nymphs.

 Unusual orbits 
Neptune has moons which have unusual orbits. Triton, the largest moon of Neptune, has a retrograde orbit (which means Triton orbits or moves the opposite way from how Neptune spins).

Two moons, Psamathe (discovered in 2002) and Neso (discovered in 2003), are unusual because they have the largest orbits for any moons discovered in the solar system so far. They take about 25 years to make one orbit and are 125 times more distant from Neptune than our Moon is from Earth.

 How Neptune's moons formed 
Many astronomers and scientists think that Neptune's inner moons were not original bodies that formed with Neptune, but were remains of debris from Triton that slowly came together. Other scientists think that when Triton was executed in space by Neptune, Triton's mass and odd orbit caused Neptune's original moons to be destroyed by gravity, only to be reformed after Triton formed a stable orbit.

While many astronomers agree that Triton was not an original moon of Neptune, some think that Triton was captured in a "three-body-encounter". What must have happened was that Triton was part of a binary pair with another unknown celestial body. The pair was then captured but Triton survived and the other object being affected by Neptune's gravity.C.B. Agnor & D.P. Hamilton Neptune's capture of its moon Triton in a binary-planet gravitational encounter, Nature, 441 (2006), pp. 192. (pdf)

Halimede, discovered in 2002, was perhaps a piece of Nereid when the moon was hit with another object. Both moons seemed to have the same gray colors.T.Grav, M.Holman and W.Fraser, Photometry of Irregular Satellites of Uranus and Neptune, The Astrophysical Journal, 613 (2004), pp.L77–L80 (preprint)  Also Halimede was also calculated that the moon had a high chance of colliding with Nereid in the past.M.Holman, JJ Kavelaars, B.Gladman, T.Grav, W.Fraser, D.Milisavljevic,  P.Nicholson, J.Burns, V.Carruba, J-M.Petit, P.Rousselot, O.Mousis, B.Marsden, R.Jacobson
Discovery of five irregular moons of Neptune, Nature, 430 (2004), pp. 865-867. Final preprint(pdf)

 List of moons 
The Neptunian moons are listed here by its orbital period, from shortest to longest. Triton, which is not only massive enough for its surface to have collapsed into a sphere, but is comparable in size to our own moon, is highlighted in purple. Irregular (captured) moons are shown in grey; prograde in light grey and retrograde in dark grey. (Triton is also thought to be captured.)

 OrderOrder refers to the position among other moons with respect to their average distance from Neptune. Label NamePronunciation(key)Image Diameter(km)  Mass ( kg) Semi-major axis(km) Orbital period(d) Inclination(°)Jacobson, R.A. (2008) NEP078 - JPL satellite ephemerisEach moon's inclination is given relative to its local Laplace plane. Inclinations greater than 90° indicate retrograde orbits (in the direction opposite to the planet's rotation). Eccentricity Discoveryyear 1    Naiad  50px|center    19  48,227  0.294  4.691  0.0003  1989 2    Thalassa  50px|center    35  50,074  0.311  0.135  0.0002  1989 3    Despina      210  52,526  0.335  0.068  0.0002  1989 4    Galatea      212  61,953  0.429  0.034  0.0001  1989 5    Larissa      460  73,548  0.555  0.205  0.0014  1981 6    Hippocamp ˈhɪpoʊkæmp     n/a  104,200  0.9362  ~ 0.0  ~ 0.000  2013 7    Proteus    50px|center    4,400  117,646  1.122  0.075  0.0005  1989 8    Triton ♠    50px|center    354,759  5.877  156.865  0.0  1846 9    Nereid    50px|center   2,700  5,513,818  360.13  7.090  0.7507  1949 10    Halimede ♠       16  16,611,000  1,879.08  112.712  0.2646  2002 11   Sao       5.8  22,228,000  2,912.72  53.483  0.1365  2002 12    Laomedeia      5.0  23,567,000  3,171.33  37.874  0.3969  2002 13    Psamathe ♠       4.4  48,096,000  9,074.30  126.312  0.3809  2003 14    Neso ♠       15  49,285,000  9,740.73  136.439  0.5714  2002

 Irregular moons 

The picture at the right shows the orbits of Neptune's irregular moons discovered so far. The eccentricity of the orbits is represented by the yellow segments (extending from the pericentre to the apocentre) with the inclination represented on Y axis. The satellites above the X axis are prograde, the satellites beneath are retrograde. The X axis is labelled in Gm (million km) and the fraction of the Hill sphere's (gravitational influence) radius (~116 Gm for Neptune).

Given the similarity of their orbits, it was suggested that Neso and Psamathe could have a common origin in the break-up of a larger moon.Scott S. Sheppard, David C. Jewitt, Jan Kleyna, A Survey for "Normal" Irregular Satellites Around Neptune: Limits to Completeness (preprint)

Triton, the biggest moon following a retrograde but a quasi-circular orbit, also thought to be a captured satellite, is not shown. Nereid, on a prograde but very eccentric orbit is believed to be scattered during Triton's capture.goldreich, P.; Murray, N.; Longaretti, P. Y.; Banfield, D. Neptune's story, Science, 245, (1989), p. 500-504.

 Naming notes 
Some asteroids also have the same names as moons of Neptune: 74 Galatea, 1162 Larissa.

Note that Triton did not have an official name until the twentieth century. Although the name was suggested in 1880 by Camille Flammarion, it did not come into regular use until at least the 1930s. Usually, it was simply known as "the satellite of Neptune" (the second satellite, Nereid, was not discovered until 1949).

Related pages
 Neptune
 Satellite (natural), the word for moons of other planets

 Notes 

 References 

 Other websites 
 Neptune's Moons  by NASA's Solar System Exploration

Category:Astronomy lists