Neptune's natural satellites

Neptune (top) and Triton (bottom), 3 days after the Voyager 2 flyby.

Neptune has thirteen known moons. The largest by far is Triton, discovered by William Lassell just seventeen days after the discovery of Neptune itself; it is the only one massive enough to have achieved hydrostatic equilibrium. It took about one hundred years to discover the second natural satellite, Nereid.

Unusual orbits

Triton orbits Neptune on a circular but retrograde orbit.

Two natural satellites discovered in 2002 and 2003, Psamathe and Neso, have the largest orbits of any natural satellites discovered in the Solar system to date. They take 25 years to orbit Neptune at an average of 125 times the distance between Earth and the Moon.

Theories of origin

It is likely that Neptune's inner satellites are not the original bodies that formed with Neptune but accreted rubble from the havoc that was wreaked after Triton's capture. Triton's original captured orbit would have been highly eccentric, and caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and become reduced to a rubble disc. Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites ^[1].

The mechanism of the Triton’s capture have been the subject of a few theories over the years. The most recent postulates that Triton was captured in a three body encounter. In this scenario, Triton is the surviving member of a binary object¹ disrupted by the encounter with Neptune.^[2].

Numerical simulations show that another moon discovered in 2002, Halimede has had a high probability of collision with Nereid during the lifespan of the system.^[3] As both moons appear to have similar (grey) colours, the satellite could be a fragment of Nereid.^[4]

¹Binary objects, gravitational association of two objects, are quite common among Trans-Neptunian Objects (>10%; the most known is Pluto-Charon) and less so among the asteroids (e.g. Ida and Dactyl).

The moons

The Neptunian moons are listed here by orbital period, from shortest to longest. Triton, which is not only massive enough for its surface to have collapsed into a spheroid, 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) (Help:Pronunciation)			Image	Diameter (km)	Mass (1016 kg)	Semi-major axis (km)	Orbital period‡ (d)	Inclination (°) to Neptune's Equator	Discovery Date
1	Neptune III	Naiad	ˈneɪəd		67 (96×60×52)	~19	48,227	0.294	4.7°	1989
2	Neptune IV	Thalassa	θəˈlæsə		83 (108×100×52)	~35	50,075	0.311	0.2°	1989
3	Neptune V	Despina	dɨsˈpiːnə		152 (180×150×130)	~210	52,526	0.335	0.1°	1989
4	Neptune VI	Galatea	ˌgæləˈtiːə		175 (204×184×144)	212	61,953	0.429	0.1°	1989
5	Neptune VII	Larissa	ləˈrɪsə		195 (216×204×164)	~420	73,548	0.555	0.2°	1981
6	Neptune VIII	Proteus	ˈproʊtiəs		418 (436 × 416 × 402)	~4,400	117,647	1.122	0.6°	1989
7	Neptune I	Triton	ˈtraɪtən		2707	2,140,000	354,800	−5.877	156.8°	1846
8	Neptune II	Nereid	ˈnɪəriɪd		340	~2,200	5,513,400	360.14	27.6°	1949
9	Neptune IX	Halimede	ˌhælɨˈmiːdi		60	~9	15,728,000	−1879.71		2002
10	Neptune XI	Sao	ˈseɪoʊ		38	~9	22,422,000	2914.07		2002
11	Neptune XII	Laomedeia	ˌleɪəmɨˈdiːə		38	~9	23,571,000	3167.85		2002
12	Neptune X	Psamathe	ˈsæməθi		28	~1.5	46,695,000	−9115.91		2003
13	Neptune XIII	Neso	ˈniːsoʊ		60	~9	48,387,000 (0.32 AU)	−9373.99		2002

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

Irregular moons

Neptune has the most lopsided of all moon systems. One moon, Triton, makes up nearly all of the mass of the system, with all other moons together only one third of one percent. This may be because the capture of Triton destroyed much of the original Neptunian system.

Neptune's irregular satellites.

The diagram illustrates 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 conjectured to be a captured satellite, is not shown. Nereid, which has a prograde but very eccentric orbit, is believed to have been scattered during Triton's capture.^[6]

Discovery

Triton was discovered by William Lassell in 1846, seventeen days after Neptune was discovered. Nereid was discovered by Gerard P. Kuiper in 1949. In 1981 Larissa was first observed by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Thole.

No futher moons were found until Voyager 2 flew by Neptune in 1989. Voyager 2 recovered Larissa and discovered five new inner moons, bringing the total of known moons to eight.

From 2002 onwards, telescopic surveys found the remaining five outer moons.

Naming notes

Some asteroids share the same names as moons of Neptune: 74 Galatea, 1162 Larissa. See also Name conflicts of solar system objects.

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 common 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).

References

See also

• Natural satellites of Mars · Jupiter · Saturn · Uranus · Pluto
• Timeline of discovery of Solar System planets and their natural satellites
• Naming of natural satellites

External links

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

Neptune Major moons Triton · Proteus  · Nereid Characteristics Rings · Great Dark Spot · Trojans · Moons Discovery John Couch Adams · Johann Gottfried Galle · Heinrich d'Arrest · William Lassell · Urbain Le Verrier Exploration Voyager program · Voyager 2 Trojans 2001 QR322 · 2004 UP10 · 2005 TN53 · 2005 TO74 · 2006 RJ103 · 2007 RW10 Related topics Neptune-crosser asteroid · Neptune in fiction

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