Moons of Neptune
Written by Fraser Cain
At current count, Neptune has a total of 13 known moons. Most of these are just small irregular icy objects, but the largest, Triton has enough gravity to pull itself into a sphere – it's the 7th largest moon in the Solar System.
Neptune was discovered in 1846. And just 17 days later, astronomer William Lassell discovered its first moon, Triton. Triton has a diameter of 2707 km, and it has enough mass to pull itself into a sphere – astronomers call this hydrostatic equilibrium. In fact, if Triton was orbiting the Sun, and not Neptune, it would be considered a dwarf planet in its own right. Triton is one of the coldest places in the Solar System, with a surface temperature of just 38 Kelvin. It may be freezing cold, but Triton is actually very active. During its 1989 flyby, NASA's Voyager 2 spacecraft discovered geysers blasting nitrogen onto the surface of Triton.
It took more than 100 years until the discovery of Neptune's second moon Nereid in 1949. Nereid is much smaller, measuring just 340 km across.
The third moon to be discovered around Neptune was Larissa, found in 1981. It was actually a lucky observation, when the tiny moon passed in front of a star, dimming it briefly. Follow on observations by Voyager 2 during its 1989 flyby confirmed that Larissa was the only object orbiting in that trajectory around Neptune.
When Voyager 2 arrived at Neptune in 1989, it turned up 5 additional tiny moons: Naiad, Thalassa, Despina, Galatea and Proteus.
The last moons discovered around Neptune were turned up during a survey made by the Hubble Space Telescope in 2002 and 2003. The largest of these moons is just 60 km across, so it's a testimony to the power of Hubble to make such a sensitive observation at such a great distance. These new moons were titled: Halimede, Sao, Laemedeia, Psamathe and Neso.
And so, this brings the total count of Moons around Neptune to 13. There are probably more, but they will need new telescopes or missions to find them.
We have written many articles about Neptune on Universe Today, including the discovery of new moons around Neptune. Here's an article announcing the discovery of 3 additional moons, and another article about how Neptune might have captured Triton.
Want more information about Neptune? Here's NASA's World Book on Neptune, and here's NASA's Solar System Exploration Guide.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.
Filed under: Astronomy
