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

Diameter: 23 to 68km
Mass: 5.0 x 1016 to 2.0 x 1018kg
Composition: Unknown
Orbit: 24.8 Earth days

Nix is the second outermost of Pluto’s five moons. It was photographed by Hubble astronomers in 2005 together with another Pluto moon, Hydra. These photographs confirmed earlier observations of the two moons in 2002. Nix was provisionally designated as s/2005 P2 prior to acquiring its present name. The name derives from Greek mythology and is that of the Greek goddess of darkness and light. The original spelling of Nyx was changed to Nix to avoid confusion with the asteroid 3908 Nyx.

Nix orbits Pluto at a distance of 48,708 kilometres in the same plane as Charon and Hydra. But unlike P4 and P5, Nix’s orbit is almost circular, as is the orbit of Hydra. At the time of its discovery, Nix appeared to be darker than Hydra and smaller. Recent investigations indicate that Hydra may just be slightly brighter than Nix. Nix and Hydra 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 orbital plane of Nix, Hydra and Charon lies at an angle of 57.5 degrees to the orbital plane of Pluto around the Sun. The orbits of Nix and Hydra are stable and periodic within the Charon-Pluto orbital system. In celestial mechanics, this phenomenon is called orbital resonance. This stability and the fact that the average assumed mass of Nix is just one ten thousandth the mass of Charon, leads astronomers to assume that Nix was formed during the same collision between Pluto and another body that formed Charon. But if orbital resonance is discounted, Nix 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.

Pluto was discovered in 1930. Its status as a planet was changed in 2006 by the International Astronomical Union and it is now referred to as a dwarf planet. Dwarf planets are icy bodies in the Kuiper Belt beyond the orbit of Neptune. Prior to the discovery of Pluto’s moons, astronomers thought that such bodies could not have a complex system of satellites.

NASA’s New Horizons mission will fly by the dwarf planet in July 2015. The discovery of five moons around Pluto means that the environment for spacecraft may be more dangerous than astronomers had thought.

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