Depth 12 · Captured moon

Triton

Triton is Neptune's largest moon and the only large moon in the solar system that orbits backward, against its planet's spin. Scientists think it began as a Kuiper Belt object that Neptune captured, which makes it a close cousin of Pluto.

Fact sheet

Type

Moon

Moon of

Neptune

Distance

About 354,760 km (220,440 miles) from Neptune; Neptune orbits about 4.5 billion km (2.8 billion miles), or 30 AU, from the Sun

Size

Diameter about 2,700 km (1,680 miles)

Discovered

Discovered 10 October 1846 by William Lassell

On the voyage

Depth 12, in the Outer Sol section of the route, between Neptune and Pluto

Things to know

• Triton circles Neptune in the opposite direction to the planet's rotation, completing one backward orbit in about 5.9 days. NASA NSSDC
• Voyager 2 measured surface temperatures of about -235°C (-391°F), among the coldest found anywhere in the solar system. NASA
• Voyager 2 caught active geysers on Triton in 1989, showing it is one of the few geologically active moons. NASA
• Its surface is a crust of frozen nitrogen that reflects about 70 percent of the sunlight that reaches it. NASA
• William Lassell found Triton on 10 October 1846, just 17 days after Neptune itself was discovered. NASA

Missions

Voyager 2 is the only spacecraft to have visited Triton, photographing about two-thirds of its surface during the August 1989 Neptune flyby.

Sources

• NASA – Triton
• NSSDC – Neptunian Satellite Fact Sheet
• NASA – Voyager 2