|Date of discovery||Date of discovery::May 15, 2005|
|Name of discoverer||Discoverer::Hubble Space Telescope Pluto Companion Search Team|
|Name origin||Name origin::Greek ''Nyx'' goddess of night and mother of Charon (Egyptian spelling); first initial of "New Horizons" mission|
|Celestial class||Member of::Moon|
|Order from primary||Order::3|
|Semi-major axis||Semi-major axis::48,675 km|
|Orbital eccentricity||Orbital eccentricity::0.002|
|Sidereal month||Sidereal period::24.8562 da|
|Inclination||Inclination::0.04° to Pluto's equator|
|Mass||5 * 1016 kg|
|Equatorial radius||Equatorial radius::17.5 km|
|Mean temperature||Mean temperature::44 K|
The Hubble Space Telescope Pluto Companion Search Team examined Pluto and its already-known moon Charon using the Hubble Space Telescope's Advanced Camera for Surveys. The stated discovery date of May 15, 2005 is the date that the discovery images were taken; the discovery was announced only after confirmatory comparison with previous images of the Plutonian system.
The name Nix is the Egyptian spelling of the Greek goddess Nyx, whose name literally means "night" in Greek. Nyx is the mother of Charon, the ferryman of the River Styx. The names given to Nix and the outermost moon Hydra are also the initials of the NASA mission New Horizons, launched in 2006.
Nix and Hydra, like Charon, revolve around Pluto in the direction of Pluto's own rotation about its axis. Nix' orbit is very nearly circular, and its sidereal month is almost, but not quite, four times that of Charon.
The Hubble Space Telescope's instruments have not been able to resolve Nix sufficiently to measure its diameter. The diameter has a calculated range of between 46 km and 137 km, depending on the albedo—and current instruments cannot adequately resolve the albedo, either. Nor has any instrument been able to resolve its sidereal day. Nix is somewhat less bright than is Hydra, and so might be smaller.
The New Horizons space probe, launched in 2006, will visit Nix and its companions in February of 2015. This will represent the earliest opportunity to study Nix in detail.
The very existence of such a complex system as the plutonian system is difficult to explain, primarily on account of Pluto's small size. This has led one member of the discovery team to speculate that a giant impact (similar to that which most astronomers now favor for the origin of the Moon of Earth) on Pluto formed Charon, and that Nix and Hydra are two pieces of debris from that same impact. While the lead investigator doubts that Nix and Hydra are captured objects, Pluto is no longer considered a planet precisely on account of its failure to "clear its neighborhood" of other objects. Nix and Hydra could, therefore, be two of many Trans-Neptunian Objects (TNOs) that, instead of crashing into Pluto, fell into orbit around it as Pluto passed.
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- An assumed value
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