The moons of Neptune
Sen—Neptune has 14 known moons ranging in size from 16 to 2,707 km in diameter. The earliest was discovered on Oct. 10, 1846 and the latest on Jul. 1, 2013. Of those, the seven closest to the planet are classed as regular, thought to have formed at the same time as Neptune. The other seven are classed as irregular, meaning they were likely captured later on. The latest discovery was made using the Hubble Space Telescope and has the provisional designation S/2004 N1.
It was only 17 days after the discovery of Neptune that the first of its moons was also found. The planet was identified by German astronomers Johann Galle and Heinrich Louis d'Arrest on Sep. 23, 1846—following mathematical predictions by Urbain La Verrier and John Couch Adams in France and England, respectively. Then the English astronomer William Lassell spotted Triton, which orbits Neptune every 5 days, 21 hours and 2 minutes.
With a mean diameter of 2,707 km, Triton is Neptune's largest moon. Its orbit is unusual in that it is opposite to the direction of the planet's rotation. Although some of Neptune's other moons do this too—as well as other moons in the Solar System—Triton is by far the largest. This so-called 'retrograde' motion strongly suggests that Triton did not form from the same disc of material that made Neptune. Instead it is highly likely that Triton came from the nearby Kuiper Belt, a vast band of icy/rocky bodies that encircles the Solar System. This makes Triton an irregular moon. It is slightly larger than another, more famous Kuiper Belt object: the dwarf planet Pluto.
A Hubble Space Telescope image showing the latest of Neptune's moons to be discovered: S/2004 N1. Also seen are four of the planet's other moons, as well as its ring system. Image credit: NASA. ESA, Mark Showalter (SETI).
Like Pluto, Triton is partially covered by frozen nitrogen. Frozen water and carbon dioxide are also present. Triton is one of the coldest places in the Solar System, with a temperature of -237°C, and is actually a few degrees colder than the more-distant Pluto.
Like our own Moon is to Earth, Triton is in synchronous rotation with Neptune, meaning that one side always faces the planet.
In 1989 NASA's Voyager 2 probe spotted geysers of dust and nitrogen from a polar ice cap that created plumes up to 8 km high, driven by the Sun.
After Lassell's discovery it was over a century before the next moon was discovered on May 1, 1949. The Dutch-born American astronomer Gerard Kuiper spotted Nereid using an 82-inch telescope at the McDonald Observatory. With an average diameter of between 290—390 km, it is Neptune's third-largest moon. Like Triton it is thought to have been captured from the Kuiper Belt. Nereid has a long orbit for a moon—360 days, almost as long as a year on Earth. Its orbit is very eccentric, meaning that it comes as close as 1,372,000 km to Neptune, and out as far as 9,655,000 km.
But outermost Neso is more distant still, orbiting between 48 and 72 million km of Neptune. This means it is more distant from Neptune than Mercury is from the Sun by 14 million km at times. Neso is, in fact, the most distant moon in the Solar System from its planet. It takes nearly 27 years to complete one orbit. By contrast the closest moon, Naiad—23,500 km from Neptune— completes an orbit in just seven hours and six minutes. Naiad's orbit is decaying and it will eventually crash into Neptune.