Neptune Moons: Facts

Global Color Mosaic of Triton
This global color mosaic of Triton was taken in 1989 by Voyager 2 during its flyby of the Neptune system. - More about this image

We don't know if William Lassell had a celebratory beverage after his discovery of Neptune's moon, Triton, but beer helped make the finding possible. Lassell was one of 19th century England's grand amateur astronomers. He used the fortune he made in the brewery business to finance his telescopes.

Lassell spotted Triton on Oct. 10, 1846 – just 17 days after a Berlin observatory discovered Neptune. Scientists using telescopes and spacecraft have since discovered more moons bringing the total to 16 moons orbiting the distant world.

Curiously, a week before he found the satellite, Lassell thought he saw a ring around the planet. That turned out to be a distortion caused by his telescope. But when NASA's Voyager 2 visited Neptune in 1989, it revealed that the gas giant does have rings, though they're far too faint for Lassell to have seen them.


Triton (not to be confused with Saturn's moon, Titan), is far and away the largest of Neptune's satellites. Dutch-American astronomer Gerard Kuiper (for whom the Kuiper Belt was named) found Neptune's third-largest moon, Nereid, in 1949. He missed Proteus, the second-largest because it's too dark and too close to Neptune for telescopes of that era. Proteus is a slightly non-spherical moon, and it is thought to be right at the limit of how massive an object can be before its gravity pulls it into a sphere.

Proteus and five other moons had to wait for Voyager 2 to make themselves known. All six are among the darker objects found in the solar system. Astronomers using improved ground-based telescopes found more satellites in 2002 and 2003.

Voyager 2 revealed fascinating details about Triton. Part of its surface resembles the rind of a cantaloupe. Ice volcanoes spout what is probably a mixture of liquid nitrogen, methane, and dust, which instantly freezes and then snows back down to the surface. Voyager 2 took an image showing a frosty plume shooting 5 miles (8 kilometers) into the sky and drifting 87 miles (140 kilometers) downwind.

Triton's icy surface reflects so much of what little sunlight reaches it that the moon is one of the coldest objects in the solar system, about -400 degrees Fahrenheit (-240 degrees Celsius).

Triton is the only large moon in the solar system that circles its planet in a direction opposite to the planet's rotation (a retrograde orbit), which suggests that it may once have been an independent object that Neptune captured. The disruptive effect this would have had on other satellites could help to explain why Nereid has the most eccentric orbit of any known moon – it's almost seven times as far from Neptune at one end of its orbit as at the other end.

Neptune's gravity acts as a drag on the counter-orbiting Triton, slowing it down and making it drop closer and closer to the planet. Millions of years from now, Triton will come close enough for gravitational forces to break it apart – possibly forming a ring around Neptune bright enough for Lassell to have seen with his telescope.

How Neptune's Moons Got Their Names

Since Neptune was named for the Roman god of the sea, its moons were named for various lesser sea gods and nymphs in Greek mythology.