Triton is, with a diameter of 2,700 km, the largest of Neptune’s 13 moons. It is the only large moon in the Solar System with a retrograde orbit (an orbit in the opposite direction to its planet’s rotation), which cannot have formed out of the same region as Neptune, so it must have been captured from elsewhere. Because of its retrograde orbit and composition similar to Pluto’s, Triton is thought to have been captured from the Kuiper belt.
This color photo of Neptune's largest moon Triton was obtained by NASA's Voyager 2 probe on Aug. 24, 1989, from 330,000 miles away. The resolution is about 6.2 miles, sufficient to begin to show topographic detail. CREDIT: NASA/JPL
Triton : Moon of Neptune - This seventh-largest moon in the Solar System is the only large moon to have a retrograde orbit, moving opposite the direction of its planet's rotation. Geologically active, it has a surface of mostly frozen nitrogen, a mostly water ice crust, an icy mantle and a substantial core of rock and metal (two-thirds of the total mass). It is 1,700 miles (2,700 km) in diameter
While living on Neptune's moon Triton, you'd be subjected to the coldest temperatures in the solar system, incredibly weak gravity and retrograde sunrises and sunsets. This image of Triton was taken by NASA's Voyager 2 spacecraft in 1989. - Credit: NASA/JPL