Hydra, a moon of Pluto
40,227 miles
31 miles
Hydra is the outermost and second-largest of Pluto’s small moons. It was discovered, alongside Nix, in June 2005 by the Hubble Space Telescope during observations aimed at refining Pluto’s orbit ahead of the New Horizons mission. Hydra is an irregular, elongated moon with an estimated diameter of around 50 kilometres (31 miles). It orbits the Pluto-Charon system at a distance of about 64,700 kilometres (40,200 miles), making it the most distant member of Pluto’s little moon family.
Despite its small size, Hydra has a surprisingly bright surface, suggesting it reflects a large amount of sunlight. Observations indicate that its surface is rich in water ice, giving it a reflective, icy appearance. Like Pluto’s other small moons, Hydra travels around the Pluto–Charon system in a complex gravitational environment. Pluto and Charon behave like a double dwarf planet, orbiting a shared centre of mass between them known as the barycentre, and it is this shifting gravitational pull that influences Hydra’s motion.
When New Horizons passed through the Pluto system in July 2015, it captured the first close-up images of Hydra. These revealed a lumpy, elongated world with a cratered surface, hinting at a long history of impacts. Hydra’s bright patches likely represent areas of fresh water ice exposed by collisions.
One of Hydra’s most intriguing characteristics is the way it rotates. Although its orbit around the Pluto-Charon barycentre is almost perfectly circular, Hydra itself doesn’t spin in a steady, predictable way. Instead, it tumbles chaotically through space. This wild rotation is driven by the constantly shifting gravitational pull from both Pluto and Charon, combined with Hydra’s irregular shape. Scientists believe that if Pluto had only one major moon, Hydra’s rotation might have been more typical. But in this double-planet system, Hydra is kept in an unpredictable wobble, making it one of the most oddly behaving moons in the Solar System.
Hydra is named after the Lernaean Hydra, a multi-headed serpent monster from Greek mythology. According to legend, cutting off one of the Hydra’s heads caused two more to grow back in its place. The name ties into Pluto’s mythological theme as ruler of the Underworld, and also reflects the moon’s discovery alongside Nix, the goddess of the night. Hydra’s name was formally approved by the International Astronomical Union in 2006.
