Welcome to Triton, the largest moon of Neptune! Triton is so great in size compared to the other moons of Neptune that its mass makes up 99.5% of Neptune’s moon system.

This large moon has a mass of 2.14×1022 kg, or 0.00359 times the mass of Earth, and its mean radius of 1353.4 km makes Triton the size of  0.2122 Earths.

Triton has a mean density of 2.061 g/cm3 and a gravitational constant of 0.779 m/s2, or  0.48 times the mass of Earth’s moon. The escape velocity of Triton is 1.455 km/s.


Triton was discovered by amateur British astronomer William Lassell on October 10, 1846, 17 days after Neptune was discovered. A brewer by trade, he had been crafting lenses for his telescope for years. He began looking towards Neptune when William Herschel indicated to him that there was a chance he would make a discovery.

Neptune’s largest moon was later named by Camille Flammarion for the son of Poseidon (Neptune) in 1880 in his Astronomie Populaire. The name Triton was officially recognized many years later.

Astronomie Populaire

This moon of Neptune has an average orbital speed of 4.39 km/s around a nearly circular orbit. Its orbital tilt is -23 degrees, and its axial tilt is 0 degrees. Fun fact: Triton also has a retrograde orbit!

Just like our moon, Triton experiences synchronous rotation, or tidal locking, which means that it takes the same amount of time for its axial and orbital rotations, 5.88 days. Because of this, an observer on Neptune would always see the same face of Triton.

Tide Locking

Comparatively speaking, Triton has a very high albedo (0.76). This moon reflects 60-95% of sunlight that hits it. For reference, Earth’s moon reflects only 11%; wear your moon glasses, Neptunians!

With cryovolcanic activity and tectonic terrains, Triton is of the solar system’s few geologically active moons. Triton is really cold — its surface temperature is only 38 K, or −235.1° C! This moon also has a tenuous nitrogen atmosphere less than 1/70,000 the pressure of Earth’s atmosphere at sea level.Everything we know about Triton’s surface we learned from a distance of 40,000 km when the Voyager 2 spacecraft passed by in 1989.

Triton and Pluto are strikingly similar in size and composition, which leads to the idea that they might share a common origin. It’s likely that Triton was a dwarf planet much like Pluto and was captured by Neptune from the Kuiper Belt. This would explain the eccentric orbit of another of neptune’s moons as well as a lack of more Neptunian moons. *


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