Moons of Neptune

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 took about one hundred years to discover the second natural satellite, Nereid.

Triton is massive enough to have achieved hydrostatic equilibrium, and would be considered a dwarf planet if it were in direct orbit about the Sun. Triton has a very unusual orbit that is circular but retrograde and inclined. Inward of Triton are six regular satellites, which all have prograde orbits that are not greatly inclined with respect to Neptune's equatorial plane. Some of these orbit among Neptune's rings.

Neptune also has six outer irregular satellites, including Nereid, whose orbits are much farther from Neptune, have high inclinations, and are mixed between prograde and retrograde. 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. Neptune has the largest Hill sphere in the solar system, owing primarily to its large distance from the Sun; this allows it to retain control of such distant moons.

Neptune's moons are named for aquatic personages in Greek and Roman mythology, many of them after Nereids, in keeping with Neptune's position as god of the sea.

Discovery

Simulated view of Neptune in the hypothetical sky of Triton

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 further 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 of Neptune to eight.

In 2002 and 2003 telescopic surveys found five additional outer moons from Earth observatories bring the total to thirteen. The five new outer moons are Halimede, Sao, Psamathe, Laomedeia, and Neso.

Names

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. The name "Triton" was suggested by Camille Flammarion in his 1880 book Astronomie Populaire,^[1] but it did not come into common use until at least the 1930s.^[2] Until this time it was usually simply known as "the satellite of Neptune" (the second satellite, Nereid, was not discovered until 1949).

Characteristics

Triton

This section requires expansion.

The relative masses of the Neptunian moons.

Irregular moons

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.^[3]

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.^[4]

Formation

The mass distribution of the Neptunian moons is the most lopsided of any planet. One moon, Triton, makes up nearly all of the mass of the system, with all other moons together comprising only one third of one percent (see diagram). This may be because the capture of Triton destroyed much of the original Neptunian system.^{[citation needed]}

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 orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble. Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites ^[5].

The mechanism of the Triton’s capture has been the subject of several theories over the years. The most recent as of 2006 postulates that Triton was captured in a three-body encounter. In this scenario, Triton is the surviving member of a binary object^{[note 1]} disrupted by its encounter with Neptune.^[6].

Numerical simulations show that a moon discovered in 2002, Halimede, has had a high probability of colliding with Nereid during the lifespan of the system.^[7] As both moons appear to have similar (grey) colours, the satellite could be a fragment of Nereid.^[8]

Table

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 [note 2]	Label [note 3]	Name	Pronunciation (key)	Image	Diameter (km)[note 4]	Mass ( × 1016 kg)[note 5]	Semi-major axis (km)[9]	Orbital period (d)[9]	Inclination (°)[9][note 6]	Eccentricity[9]	Discovery year[10]	Discoverer [10]
1	Neptune III	Naiad	ˈneɪəd		66 (96×60×52)	~19	48 227	0.294	4.691°	0.0003	1989
2	Neptune IV	Thalassa	θəˈlæsə		82 (108×100×52)	~35	50 074	0.311	0.135°	0.0002	1989
3	Neptune V	Despina	dɨsˈpiːnə		150 (180×148×128)	~210	52 526	0.335	0.068°	0.0002	1989
4	Neptune VI	Galatea	ˌɡæləˈtiːə		176 (204×184×144)	212	61 953	0.429	0.034°	0.0001	1989
5	Neptune VII	Larissa	ləˈrɪsə		194 (216×204×168)	~420	73 548	0.555	0.205°	0.0014	1981
6	Neptune VIII	Proteus	ˈproʊtiəs		420 (436 × 416 × 402)	~5 000	117 646	1.122	0.075°	0.0005	1989
7	Neptune I	‡♠Triton	ˈtraɪtən		2707	2 140 000	354 759	−5.877	156.865°	0.0	1846
8	Neptune II	Nereid	ˈniːriː.ɪd		340	~3 100	5 513 818	360.13	7.090°	0.7507	1949
9	Neptune IX	♠Halimede	ˌhælɨˈmiːdiː		62	~16	16 611 000	1 879.08	112.712°	0.2646	2002
10	Neptune XI	Sao	ˈseɪ.oʊ		44	~5.8	22 228 000	2 912.72	53.483°	0.1365	2002
11	Neptune XII	Laomedeia	ˌleɪ.ɵmɨˈdiːə		42	~5.0	23 567 000	3 171.33	37.874°	0.3969	2002
12	Neptune X	♠Psamathe	ˈsæməθiː		38	~3.7	48 096 000	9 074.30	126.312°	0.3809	2003
13	Neptune XIII	♠Neso	ˈniːsoʊ		60	~15	49 285 000 (0.32 AU)	9 740.73	136.439°	0.5714	2002

Notes

1. ^ Binary objects, objects with moons such as the Pluto–Charon system, are quite common among the larger Trans-Neptunian objects.
2. ^ Order refers to the position among other moons with respect to their average distance from Neptune.
3. ^ Label refers to the Roman numeral attributed to each moon in order of their discovery.
4. ^ Diameters with multiple entries such as "60×40×34" reflect that the body is not spherical and that each of its dimensions has been measured well enough.
5. ^ Mass of the small irregular moons (Halimede through Neso) was calculated assuming a density of 1.3 g/cm³. Unless otherwise noted, the uncertainty in the reported masses is not available.
6. ^ Each 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).

References

External links

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

v • d • e 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

v • d • e Natural satellites of the Solar System Planetary satellites Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian Other satellite systems Plutonian · Eridian · Haumean · Asteroid satellites Largest satellites Ganymede · Titan · Callisto · Io · Moon · Europa · Triton Titania · Rhea · Oberon · Iapetus · Charon · Umbriel · Ariel · Dione · Tethys  · Enceladus · Miranda · Proteus · Mimas Inner satellites · Trojans · Irregulars · List · Timeline of discovery · Naming

v • d • e The Solar System The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Haumea · Makemake · Eris Moons (Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Haumean · Eridian) · Rings (Jovian · Saturnian · Uranian · Neptunian) Meteoroids · Minor planets (Asteroids (Main belt) · Centaurs · TNOs (Kuiper belt · Scattered disc) ) · Comets (Oort cloud) Related articles: astronomical object, star, planet, dwarf planet, small body and planetary system See also the Solar System's list of objects, sorted by size, the list of minor planets, or the Solar System Portal