This article is about the third moon of Pluto. It is not to be confused with the asteroid 3908 Nyx

Nix is a natural satellite of Pluto, 53 km (33 mi) across its longest dimension. It was discovered along with Pluto's outermost moon Hydra in June 2005. It was imaged along with Pluto and its other moons by the New Horizons spacecraft as it flew by the Pluto system in July 2015.[8] Of the four small Plutonian moons the highest resolution images are of Nix, with resolutions as high as 330 meters per pixel.[9]

Discovery [ edit ]

Discovery images of Nix (and Hydra)

Nix was discovered by a group of researchers called the Pluto Companion Search Team using the Hubble Space Telescope.[10] Since Nix's brightness is about 5,000 times fainter than Pluto, long exposure images were taken in order to find it.[11] The discovery images were taken on 15 May 2005 and 18 May 2005. Nix and Hydra were independently discovered by Max J. Mutchler on 15 June 2005 and by Andrew J. Steffl on 15 August 2005. The discoveries were announced on 31 October 2005, after confirmation by precoveries from 2002. They were provisionally designated S/2005 P 1 for Hydra and S/2005 P 2 for Nix and informally referred to as "P1" and "P2", respectively.[12][13]

Naming [ edit ]

The name Nix was approved by the International Astronomical Union (IAU) and was announced on 21 June 2006 along with the naming of Hydra in the IAU Circular 8723.[12] Nix was named after Nyx, the Greek goddess of darkness and night and mother of Charon, the ferryman of Hades in Greek mythology. The two newly named moons were intentionally named in the way their order of their initials N and H honors the New Horizons mission to Pluto.[9][14] The original proposal for the naming of Nix was to use the classical spelling Nyx, but to avoid confusion with the asteroid 3908 Nyx, the spelling was changed to Nix. Jürgen Blunck explains it as the "Spanish translation" of the Greek name.[15]

Characteristics [ edit ]

Physical characteristics [ edit ]

Enhanced color MVIC image of Nix showing its reddish hue.

Nix has an elongated shape, with its longest axis measured at 54 km (34 mi) across and its shortest axis 36 km (22 mi) across. This gives Nix the measured dimensions of 54 km × 41 km × 36 km (34 mi × 25 mi × 22 mi).

Early research appeared to show that Nix's surface is reddish in color.[16] Contrary to this, other studies show that Nix's surface is spectrally neutral, similar to the other small moons of Pluto.[7] Nix also appeared to vary in brightness and albedo.[7] During the New Horizons flyby of Pluto and its moons, the spacecraft imaged Nix, showing a large reddish area approximately 18 km (11 mi) across.[17] The reddish area is thought to be a large impact crater that had ejected dark material underneath Nix's surface.[18] Detailed images of the moon have revealed at least 6 impact craters on it, the largest of which is about 15 km (9.3 mi) across.

Nix tumbles chaotically similarly to the smaller moons of Pluto; the moon's axial tilt and day length vary greatly over short timescales.[19][20] At the time of the New Horizons flyby, Nix was rotating with a period of 43.9 hours retrograde to Pluto's equator with an inclination of 132 degrees — in other words it was rotating "backwards" in relation to its orbit around Pluto.[21] The rotation period has actually increased by 10% since Nix was discovered.[21]

Orbit [ edit ]

Animation of moons of Pluto around the barycenter of Pluto - Ecliptic plane Front view Side view Pluto · Charon · Styx · Nix · Kerberos · Hydra

Nix follows a equatorial, circular orbit around the Pluto-Charon barycenter at a distance of 48,694 km (30,257 mi).[3][22] It is in a 3:2 orbital resonance with Hydra, and a 9:11 resonance with Styx (the ratios represent numbers of orbits completed per unit time; the period ratios are the inverses).[2][23] As a result of this "Laplace-like" 3-body resonance, it has conjunctions with Styx and Hydra in a 2:3 ratio.

Its orbital period of 24.9 days is also close to a 1:4 orbital resonance with Charon, but the timing discrepancy is 2.8%; there is no active resonance.[2][16] 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.

Exploration [ edit ]

New Horizons is the only spacecraft that has flown through the Pluto system and hence the only one that has explored Nix. Until 13 July 2015, when NASA's Long Range Reconnaissance Imager on board New Horizons determined Nix's size,[24] all but its existence was unknown. Of Pluto's smaller moons, only Nix and Hydra were imaged at resolutions high enough for surface features to be visible.

Polar view of Nix imaged by New Horizons on 13 July 2015.

Nix imaged by New Horizons, released on 18 December 2015