Moons of Neptune
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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 so 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.
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[edit] Theories of origin
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 (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 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).
[edit] 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) |
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 | <a href="/wiki/L | ||||||||