Nix was found by the Hubble Space Telescope Pluto Companion Search Team, composed of Hal A. Weaver, S. Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A. Young. The discovery images were taken on May 15, 2005, and May 18, 2005; Nix and Hydra were independently discovered by Max J. Mutchler on June 15, 2005, and Andrew J. Steffl on August 15, 2005. The discoveries were announced on October 31, 2005, after confirmation by precoveries from 2002. They were provisionally designatedS/2005 P 1 (Hydra) and S/2005 P 2 (Nix).
Labeled image of Nix released upon IAU name approval.
Nix follows a circular orbit in the same plane as Charon. Its orbital period of 24.9 days is close to a 1:4 orbital resonance with Charon, but the timing discrepancy is 2.7%, which suggests that there is no active resonance. A hypothesis explaining such a near-resonance is that it originated before the outward migration of Charon following the formation of all five known moons, and is maintained by the periodic local fluctuation of 9% in the Pluto–Charon gravitational field strength.
Although its size has not been directly measured, Nix has been calculated to have a diameter of between 46 km (if its geometric albedo is similar to Charon's 35%) and 137 km (if it has a reflectivity of 4%, like the darkest Kuiper belt objects). Nix is slightly fainter than Hydra, suggesting that it is somewhat smaller in size. In the discovery image, Nix is 6,300 times fainter than Pluto.
Early research appeared to show that Nix was reddish like Pluto and unlike the other moons, but more recent reports have been that it is grey like the remaining satellites.
The formal name "Nix", from the Greek goddess of darkness and night and mother of Charon, was announced on June 21, 2006 on IAU Circular 8723, where the designation Pluto II is also given. Together with Hydra (Pluto's third moon) the initials are those of the New Horizons probe. The initial proposal was to use the classical spelling Nyx, but to avoid confusion with the asteroid3908 Nyx the spelling was changed to Nix. The USGS Gazetteer of Planetary Nomenclature states that Nix is the "Egyptian spelling", while Jürgen Blunck explains it as the "Spanish translation" of the Greek name.