List of Neptune's moons

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Neptune has a total of 13 moons. The largest moon is Triton which was discovered by William Lassell just seventeen days after Neptune was found. Then it took one hundred years later to find the second moon, Nereid.

Contents 1 Unusual orbits 2 How Neptune's moons formed 3 List of moons 3.1 Irregular moons 4 Naming notes 5 See also 6 References 7 Other websites

[change] Unusual orbits

Neptune has a lot of moons who have unusual orbits. Triton which is the largest moon of Neptune, orbits in 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 also 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 farther than how our Moon orbits away from Earth.

[change] How Neptune's moons formed

Many astronomers and scientists think that Neptune's inner moons weren't original bodies that formed with Neptune, but were instead remains of debris from Triton that slowly builded up together. Other scientists think that when Triton was captured 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. ^[1]

While many astronomers also agree that Triton wasn't 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 pair¹ with another unknown celestial body. The pair was then captured by Triton survived and the other object being affected by Neptune's gravity.^[2]

Another possible event had Halimede, which was discovered in 2002, was thought to been originally a piece of Nereid when the moon was possibly hit with another object, since both moons seemed to have the same gray colors.^[3] Also Halimede was also calculated that the moon had a high chance of colliding with Nereid in the past.^[4]

¹Binary pairs are two astronomical objects orbiting closely to each other.

[change] 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.)

Order	Name (spheroidal moon in bold) (Pronunciation key)			Diameter (km)	Mass (1016 kg)	Semi-major axis (km)	Orbital period‡ (d)	Inclination (°) to Neptune's Equator	Discovery Date
1	Neptune III	Naiad	ˈnaɪ.əd	67 (96×60×52)	~19	48,227	0.294	4.7°	1989
2	Neptune IV	Thalassa	θəˈlæs.ə	83 (108×100×52)	~35	50,075	0.311	0.2°	1989
3	Neptune V	Despina	dɛsˈpi.nə	152 (180×150×130)	~210	52,526	0.335	0.1°	1989
4	Neptune VI	Galatea	ˌgæl.əˈti.ə	175 (204×184×144)	212	61,953	0.429	0.1°	1989
5	Neptune VII	Larissa	ləˈrɪs.ə	195 (216×204×164)	~420	73,548	0.555	0.2°	1989
6	Neptune VIII	Proteus	ˈproʊ.ti.əs	418 (436 × 416 × 402)	~5,000	117,647	1.122	0.6°	1989
7	Neptune I	Triton	ˈtraɪ.tən	2707	2,140,000	354,800	−5.877	156.8°	1846
8	Neptune II	Nereid	ˈnir.i.ɨd	340	~3,100	5,513,400	360.14	27.6°	1949
9	Neptune IX	Halimede	ˌhæl.əˈmi.di	60	~9	15,728,000	−1879.71		2002
10	Neptune XI	Sao	ˈseɪ.oʊ	38	~9	22,422,000	2914.07		2002
11	Neptune XII	Laomedeia	ˌleɪ.ə.məˈdi.ə	38	~9	23,571,000	3167.85		2002
12	Neptune X	Psamathe	ˈsæm.ə.θi	28	~1.5	46,695,000	−9115.91		2002
13	Neptune XIII	Neso	ˈni.soʊ	60	~9	48,387,000 (0.32 AU)	−9373.99		2003

^‡ Negative orbital periods show a retrograde orbit around Neptune (opposite to the planet's rotation)

[change] 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.^[5]

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.^[6]

[change] 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).

[change] See also

• Neptune
• Moon

[change] References

[change] Other websites

• Neptune's Moons by NASA's Solar System Exploration

v • d • e Neptune
Moons	Naiad · Thalassa · Despina · Galatea · Larissa · Proteus · Triton · Nereid · Halimede · Sao · Laomedeia · Psamathe · Neso
Characteristics	Rings of Neptune · Great Dark Spot
Discovery	John Couch Adams · Johann Gottfried Galle · William Lassell · Urbain Le Verrier
Exploration	Voyager program · Voyager 2
Neptune Trojans	2001 QR322 · 2004 UP10 · 2005 TN53 · 2005 TO74 · 2006 RJ103 · 2007 RW10

v • d • e The Solar System
The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris
Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
Small bodies:   Meteoroids · Asteroids/Asteroid moons (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also: List of solar system objects