18th June: Triton

This date in 1799 saw the birth of William Lassell, the astronomer who discovered Triton, the largest moon of Neptune, just 17 days after the discovery of Neptune itself. Here are ten things you might not know about his discovery.

1. Triton is named after the Greek sea god Triton, son of Poseidon (the Greek god comparable to the Roman Neptune). Lassell didn't name it - the name was proposed by Camille Flammarion in his 1880 book Astronomie Populaire. Lassell did, however, name Hyperion, the eighth moon of Saturn when he discovered that.
2. The first attempt to measure the diameter of Triton was made by Gerard Kuiper in 1954. Its diameter is 2,700 kilometres (1,700 mi). It is therefore the seventh-largest moon in the Solar System and the sixteenth-largest object in the Solar System, larger than the dwarf planets Pluto and Eris.
3. Triton is the only large moon in the solar system to have a retrograde orbit, that is, in the opposite direction to its planet's rotation.
4. Because of this, and the fact it is similar in composition to Pluto, scientists believe Neptune captured it from the Kuiper belt.
5. Knowledge of the surface of Triton was acquired from a distance of 40,000 km by the Voyager 2 spacecraft during a single encounter in 1989, when 40% of Triton's surface was observed.
6. The surface (as much as we've seen, possibly all of it) is covered in a thin sheet of nitrogen ice. It has an icy mantle and a core of rock and metal, which makes up two-thirds of its total mass.
7. Its atmosphere is nitrogen, thought to come from geysers on the surface that erupt nitrogen gas. A geyser eruption on Triton can last up to a year.
8. Triton keeps one face oriented toward the planet at all times, like our own Moon, although Triton's polar regions take turns facing the Sun, resulting in seasonal changes.
9. It has a unique type of terrain not found anywhere else - fissures and depressions known as "cantaloupe terrain" because it looks like the skin of a cantaloupe melon.
10. Several further explorations to the Neptune system have been proposed by NASA, with Triton as a prime target, possibly with a probe landing there - but so far priority has been given to the Jupiter and Saturn systems.