How big is Triton?

Triton is the largest moon of Neptune, about 2,707 kilometers (1,682 miles) across. It is the only large moon in the Solar System that orbits in the opposite direction to its planet's rotation. Scientists think it began as an icy world in the Kuiper Belt, the zone of small icy bodies beyond Neptune, and was captured by Neptune's gravity.

Size

NASA gives Triton's radius, the distance from its center to its surface, as 1,353.4 kilometers, which makes it 2,707 kilometers across. The list kept by NASA's Jet Propulsion Laboratory gives 1,352.6 kilometers, give or take 2.4 kilometers. The two figures differ by less than 1 kilometer.

Triton's density, the amount of mass packed into each cubic centimeter, is about twice that of water. That is higher than that of almost any other moon of the outer planets. That means Triton holds more rock than the icy moons of Saturn and Uranus. A crust of frozen nitrogen covers an icy mantle, which is thought to surround a core of rock and metal.

Size comparisons

Triton is a little more than three-quarters as wide as Earth's Moon, which is 3,475 kilometers across, and about one-fifth as wide as Earth.
The dwarf planet Pluto, a world much like Triton, is 2,377 kilometers across. Triton is about 330 kilometers wider.
Jupiter's moon Europa is 3,122 kilometers across, so Triton is about seven-eighths as wide.
If Neptune were the size of a basketball, Triton would be a bead about 1.3 centimeters (half an inch) across, orbiting 1.7 meters (5.7 feet) from the ball's center.

A moon that orbits backward

The British astronomer William Lassell discovered Triton on October 10, 1846, 17 days after Neptune itself was found. Most large moons travel around their planets in the same direction as the planet spins. Triton goes the other way, in what astronomers call a retrograde orbit.

Triton circles Neptune every 5.9 days, 354,760 kilometers from the planet's center, a little closer than the Moon is to Earth. Like the Moon, it always keeps the same side facing its planet. Its orbit is steeply tilted, so each of its poles takes a turn facing the Sun. Scientists think Neptune captured Triton from the Kuiper Belt millions of years ago. Triton shares many features with Pluto.

What Voyager 2 found

Voyager 2 is the only spacecraft to have flown past Neptune and Triton. During its flyby in 1989, it measured surface temperatures of −235 °C (−391 °F), making Triton one of the coldest objects in the Solar System. Most of Triton's nitrogen is frozen as frost, and its icy surface reflects 70 percent of the sunlight that hits it.

Voyager 2 also found active geysers, so Triton is one of the few moons known to be geologically active. Its images show few craters, but smooth volcanic plains, mounds and round pits formed by flows of icy lava. A thin atmosphere, mostly nitrogen with a little methane, surrounds the moon.