Neptune and Triton [image: Voyager 2]

Neptune is the last major planet out from the Sun. Beyond Neptune is the Kuiper Belt, an enormous region of millions of icy objects. Some of them, like Pluto, are big enough to be dwarf planets.

Fourteen known moons circle Neptune, and they're a mixture. One of them – Triton – has over 99% of the total mass of Neptune's moons. Thirteen little moons share what's left. Only two of the moons were discovered before 1981. They all have names from Greek mythology relating to the Greek sea god Poseidon. (The Romans called the sea god Neptune.)

Triton
Triton was the son of Poseidon. When a moon forms along with its planet, it usually orbits in the same direction as the planet's spin. But Triton orbits in the opposite direction – we call this retrograde. Triton's orbit is also highly tilted instead of being close above the equator. This is evidence of a captured body. Since Triton is very similar to Pluto, it's likely to have come from the Kuiper Belt.

Inner moons – crash, bang, reform
When Triton was pulled into the Neptunian system, it caused gravitational chaos. The existing moons smashed into each other and broke up into a disk of rubble above Neptune's equator. After Triton's orbit became stable, new moons formed from the rubble. We don't know what the original moons were like. But now there are seven small moons closer to Neptune than Triton is.

Voyager 2 discovered five of the inner moons in 1989. They were Naiad, Thalassa, Despina, Galatea, and the biggest of the small moons, Proteus. The spacecraft also imaged Larissa, which was detected from Earth in 1981. These five moons have circular orbits and move in the direction of Neptune's spin. That's called prograde.

Proteus is the biggest of the small moons. It's around 400 km (250 mi) across. It's almost massive enough to be spherical, but didn't quite make it. Voyager got an image of Proteus that shows a crater that's over 150 km (95 mi) across.

In 2013, a seventh inner moon was discovered in some old Hubble Space Telescope data. It's between Larissa and Proteus. It's named Hippocamp, after a mythical sea monster. Hippocamp is much smaller than the other inner moons. In fact, it's probably a piece broken off from Proteus. As it's only about 18 km (11 mi) across, it's no wonder nobody noticed it before.

Nereid – homemade or captured?
Neptune was discovered in 1846, and Triton a few weeks later. With the distance and small size of the other moons, it's not surprising that it took until 1949 to discover a second moon. Gerard Kuiper discovered it and chose the name Nereid. The Nereids were sea nymphs.

Nereid has a prograde orbit. The orbit is highly eccentric, which means its orbit is stretched out instead of being round. In fact, it has the most eccentric orbit of any known Solar System moon. Its distance from Neptune varies from about 1,400,000 km (850,000 mi) to nearly 10 million km (6 million mi). This strange orbit suggests that Nereid is a captured object. But other evidence says it could have been an inner moon whose orbit was disrupted when Triton was captured.

Since Proteus is bigger than Nereid, we might expect that it would be discovered first. But Nereid is easier to see. Proteus is close to Neptune and covered in a dark, reddish material. Its discovery had to wait for a visiting space probe.

Outer moons – gotcha!
Neptune captured its five outermost moons. Halimede, Sao, Laomedeia, Psamathe and Neso are all named for Nereids. They're small, between 40-62 km (25-38 mi) across, with irregular shapes and very eccentric orbits.

Two of these little moons have retrograde orbits, and three have prograde orbits. Psamathe and Neso are farther away from their planet than any other known moons in the Solar System. It takes them over 25 years to orbit Neptune. Their orbits suggest they could be two pieces thrown out from a collision in the Kuiper Belt.