Moons

Prior to Voyager 2's encounter, Neptune's only known moons were Triton, discovered visually through a telescope in October 1846 by English astronomer William Lassell shortly after the discovery of Neptune, and Nereid, discovered in telescopic photographs more than a century later in 1949 by American astronomer Gerard Kuiper. In 1989, Voyager's observations added six previously unknown moons to Neptune's system. All are less than half of Triton's distance from Neptune and are regular moons—i.e., they travel in prograde, nearly circular orbits that lie near Neptune's equatorial plane. These moons are probably synchronous rotators; that is, their rotational and orbital periods are the same.

In 2002-03, five additional tiny moons, estimated to be about 15-30 km (9-18 miles) in radius, were discovered in Earth-based observations. These are irregular, having highly eccentric orbits that are inclined at large angles to the planet's equator; three also orbit in the retrograde direction. Their mean distances from Neptune lie roughly between 15 million and 48 million km (9 million and 30 million miles), well outside the orbit of Nereid.

Each of Neptune's moons are named after figures in Greek mythology usually connected with Poseidon (the Roman god Neptune) or with water. Of Voyager's six discoveries, all but Proteus orbit Neptune in less time than it takes the planet to rotate. Hence, to an observer positioned near Neptune's cloud tops, these five would appear to rise in the west and set in the east. Voyager observed two of its discoveries, Proteus and Larissa, closely enough to detect both their size and approximate shape. Both bodies are irregular in shape and appear to have heavily cratered surfaces. The sizes of the other four are estimated from a combination of distant images and their brightnesses, based on the assumption that they reflect about as much light as Proteus and Larissa—about 7 percent. The other five moons are much smaller, each having a mean radius of less than 100 km (60 miles).

Following are slightly more in-depth profiles of Neptune's three largest moons: Triton, Proteus, and Nereid.

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