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Triton drawn as a smiling character

Moon of Neptune

Triton

Neptune's captured giant moon, orbiting backwards with icy geysers at minus 235 degrees

"My dark spots are storms with secrets untold,
And Triton spins backwards, icy and cold."From Hugo's Journey Through Space
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Key facts

Diameter
About 1,680 miles / 2,700 km (a little smaller than our Moon)
Distance from Neptune
About 220,000 miles / 354,800 km on average
One orbit
About 5.88 Earth days, travelling backwards (retrograde)
One day (rotation)
Same as its orbit: one side always faces Neptune
Discovered
10 October 1846, by William Lassell
Named after
Triton, son of the sea god Poseidon in Greek myth
Surface
Frozen nitrogen and water ice, with active geysers
Surface temperature
About minus 235 degrees Celsius
Origin
Probably a captured Kuiper Belt object
Neptune's known moons
16, as of August 2026 (Triton is the largest)
Visited by
Voyager 2 only, flying past on 25 August 1989

About Triton

Triton is a moon. It goes round the planet Neptune. Neptune is very far from the Sun. So Triton is very, very cold. It is covered in ice. It is one of the coldest places we know. Triton goes round Neptune the wrong way. Most moons go the other way. Triton has icy geysers. They shoot dark dust up high. Long ago, Triton was not a moon. It was a small icy world. Neptune's pull caught it. Now Triton is Neptune's biggest moon. One spacecraft has seen it up close. Its name is Voyager 2. It flew past long before you were born.

Triton is the largest moon of Neptune, the farthest planet from the Sun. It is about 2,700 kilometres across, a little smaller than our own Moon. Triton is strange because it orbits Neptune backwards, against the planet's spin. Astronomers call this a retrograde orbit, and no other large moon does it.

This is a clue that Triton was not born beside Neptune at all. It probably formed in the Kuiper Belt, far out beyond the planets. Long ago Neptune's gravity captured it as it drifted past. Triton is one of the coldest worlds we know, at about minus 235 degrees Celsius.

Its surface is covered in frozen nitrogen, water ice and other frozen gases. Even so, Triton is active, with geysers that blast dark dust into the sky. It has a very thin atmosphere made mostly of nitrogen. Triton takes just under six days to complete one orbit of Neptune.

It rotates once in exactly the same time, so one side always faces the planet. Only one spacecraft has ever visited, when Voyager 2 flew past in August 1989. It photographed less than half of the surface, so much of Triton remains a mystery.

Triton is Neptune's largest moon by a wide margin and one of the most unusual satellites in the solar system. William Lassell discovered it on 10 October 1846, just 17 days after Neptune itself was found. NASA gives its diameter as about 2,700 km, which makes it slightly smaller than our Moon; JPL's current figure for its mean radius is 1,352.6 km. It orbits Neptune at an average distance of about 354,800 km, completing one nearly circular orbit every 5.88 days, and it rotates synchronously, always presenting the same face to the planet.

What sets Triton apart is the direction of that orbit. Its inclination of 157.3 degrees means it travels retrograde, against Neptune's rotation. It is the only large moon in the solar system to do so, and this is the strongest evidence that Triton did not form alongside Neptune. It is generally thought to be a captured Kuiper Belt object, a cousin of Pluto, that was pulled into orbit by Neptune's gravity. Its density, about twice that of water, is consistent with a mixed rock and ice body of that origin.

Voyager 2 remains the only spacecraft to have visited. It made its closest approach to Neptune on 25 August 1989, passing about 4,800 km above the cloud tops, and imaged roughly 40 per cent of Triton's surface. It found a surface of frozen nitrogen, water ice and other ices reflecting about 70 per cent of the sunlight that reaches it, with a temperature of about minus 235 degrees Celsius, among the coldest measured on any body in the solar system. Voyager also found a thin nitrogen atmosphere with traces of methane, and, unexpectedly, active geysers lofting dark material high above the surface. The famous cantaloupe terrain, smooth volcanic plains and a scarcity of craters suggest a surface that may be only around 10 million years old, so Triton is geologically active today.

A common misstatement is that Triton is the coldest object in the solar system. It is one of the coldest measured surfaces, but permanently shadowed craters near the Moon's poles are colder. Scientists suspect Triton may hide a subsurface ocean; a proposed NASA mission concept, Trident, aimed to test this but was not chosen in NASA's 2021 Discovery selection, so Triton has no confirmed follow-up visit.

Did you know?

Neighbours around Neptune

Questions children ask

Why does Triton go round Neptune the wrong way?

Moons that form alongside their planet orbit in the same direction the planet spins. Triton goes the opposite way, so scientists think it formed elsewhere, probably in the Kuiper Belt, and was captured by Neptune's gravity long ago.

Is Triton the coldest place in the solar system?

It is one of the coldest surfaces ever measured, at about minus 235 degrees Celsius. But it is not the very coldest: permanently shadowed craters near the poles of our own Moon get colder still.

Has any spacecraft been to Triton?

Only one. Voyager 2 flew past Neptune on 25 August 1989 and photographed about 40 per cent of Triton's surface. It found geysers, a thin nitrogen atmosphere and strange cantaloupe-like terrain. No other mission has visited since.

What are Triton's geysers made of?

Voyager 2 saw dark plumes rising from Triton's surface. They are thought to be nitrogen gas bursting up through the ice, carrying dark dust that falls back as streaks. Triton is one of the few geologically active moons known.

Quick quiz

Three questions for space cadets. Nothing is recorded.

  1. Which planet does Triton orbit?
  2. What is unusual about Triton's orbit?
  3. Which spacecraft is the only one to have visited Triton?

Where next?

The MoonPlutoNeptune

Written by Anthony (A.Z. Storyworks). Facts checked against NASA Science and JPL, September 2026. Moon counts as of September 2026.

Sources