Hydra is one of Pluto’s moons. It was discovered in June 2005 by astronomers using the Hubble Space Telescope.

Diameter: 60 to 168km
Mass: 4.2 x 1017kg
Composition: Unknown
Orbit: 38.2 Earth days

Hydra is the outermost of Pluto’s five moons. It was photographed by Hubble astronomers in 2005 together with another Pluto moon, Nix. These photographs confirmed earlier observations of the two moons in 2002. Hydra was provisionally designated as s/2005 P1 prior to acquiring its present name. The name derives from Greek mythology – Hydra was a monster with a serpent’s body and nine heads and guarded the underworld.

Hydra orbits Pluto at a distance of 65,000 kilometres in the same plane as Charon and Nix. But unlike Charon, P4 and P5, Hydra’s orbit is almost circular, as is the orbit of Nix. At the time of its discovery, Hydra appeared to be about 25% brighter than Nix and 10% larger. However, recent investigations indicate that Hydra may just be slightly brighter than Nix. Hydra and Nix orbit at the centre of mass of the Pluto-Charon orbital system. Neither moon changes in brightness as it revolves around the system, leading astronomers to believe that both have nearly circular shapes.

The orbits of Hydra and Nix are stable and periodic within the Charon-Pluto orbital system. In celestial mechanics, this phenomenon is called orbital resonance. This stability, together with the fact that Hydra is just one ten thousandth the mass of Charon, have led astronomers to assume that this moon was formed during the same collision between Pluto and another body that formed Charon. But if orbital resonance is discounted, Hydra may have originated outside of the Pluto-Charon system.

Little is known about Hydra’s surface. It seems to display a grey colour similar to the surface of Charon.

The American astronomer Clyde William Tonbaugh discovered Pluto in 1930. Between 1930 and 2006, astronomers described Pluto as the tenth-largest planet to orbit the Sun. Its status changed in 2006 when the International Astronomical Union reclassified it as a dwarf planet. This description places Pluto within the Kuiper Belt, a band of icy bodies located beyond the orbit of Neptune.

Astronomers believed that such icy bodies did not have the gravitational force to trap satellites but the discovery of Pluto’s moons has disproved this. Further information about the environment of Pluto will come from NASA’s New Horizons mission, which is expected to fly by the dwarf planet in July 2015.

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