Nix (moon)

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Pluto system 2005 discovery images.jpg
Discovery images of Nix (and Hydra)
Discovered by Hubble Space Telescope
Pluto Companion Search Team
Discovery date June 2005
Pronunciation /ˈnɪks/
Named after
(134340) Pluto II[1]
Adjectives Nictian
Orbital characteristics[2]
48694±3 km
Eccentricity 0.002036±0.000050
24.85463±0.00003 d
Inclination 0.133°±0.008° 
Satellite of Pluto
Physical characteristics
Mean radius
18 × 8 ± ? miles[3]
Mass ?
Mean density
Albedo 0.04–0.35 (assumed)[5]
Temperature 33–55 K
23.38–23.7 (measured)[5]

Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be imaged along with Pluto and Charon by the New Horizons mission in July 2015.[6] Of the four small Plutonian moons, New Horizons is expected to get the best pictures of Nix.


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,[7] where the designation Pluto II is also given. Together with Hydra (Pluto's third moon) the initials are those of the unmanned New Horizons spaceprobe. The initial proposal was to use the classical spelling Nyx, but to avoid confusion with the asteroid 3908 Nyx the spelling was changed to Nix. The USGS Gazetteer of Planetary Nomenclature states that Nix is the "Egyptian spelling",[8] while Jürgen Blunck explains it as the "Spanish translation" of the Greek name.[9]

Labeled image of Nix released upon IAU name approval


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 designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix).[10][7]

Physical characteristics[edit]

Nix has been measured to be 56.3 × 25.7 km in diameter, indicating a very elongated shape, and a very high geometric albedo.[11] In the discovery image, Nix is 6,300 times fainter than Pluto.[12]

Early research appeared to show that Nix was reddish like Pluto and unlike the other moons,[13] but more recent reports have been that it is grey like the remaining satellites.[5]

Nix compared to the other moons of Pluto. (artist's concept)


Nix follows a circular orbit in the same plane as Charon. 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).[14][2] 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.[13][2] 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.

As it orbits, it flips up and down on its axis, with the north pole becoming the south pole and vice versa, periodically.[15]

Nix tumbles unpredictably, based on computer modeling.[16]


  1. ^ Jennifer Blue (2009-11-09). "Gazetteer of Planetary Nomenclature". IAU Working Group for Planetary System Nomenclature (WGPSN). Retrieved 2010-08-30. 
  2. ^ a b c Showalter, M. R.; Hamilton, D. P. (3 June 2015). "Resonant interactions and chaotic rotation of Pluto’s small moons". Nature 522 (7554): 45–49. doi:10.1038/nature14469. 
  3. ^ NASA's Hubble finds Pluto's moons tumbling in absolute chaos
  4. ^
  5. ^ a b c Stern, S. A.; Mutchler, M. J.; Weaver, H. A.; Steffl, A. J. (2006). "The Positions, Colors, and Photometric Variability of Pluto's Small Satellites from HST Observations 2005–2006". Astronomical Journal 132 (3): submitted. arXiv:astro-ph/0607507. Bibcode:2006AJ....132.1405S. doi:10.1086/506347.  (Final preprint)
  6. ^ Cain, Fraser (2008). "Pluto’s Moon Nix". 
  7. ^ a b IAU Circular No. 8723 naming the moons
  8. ^ "Planet and Satellite Names and Discoverers". Gazetteer of Planetary Nomenclature. USGS Astrogeology. July 21, 2006. Retrieved 2006-08-15. 
  9. ^ Blunck, Jürgen, Solar System Moons: Discovery and Mythology (2009), p. 129.
  10. ^ IAU Circular No. 8625 describing the discovery
  11. ^ Satellite diameters from [1]
  12. ^ Brightness Difference on 2005-05-15: (5th root of 100) ^ (Nix APmag 23.38 – Pluto APmag 13.87) = 6,368x
  13. ^ a b Buie, Marc W.; Grundy, William M.; Young, Eliot F.; Young, Leslie A.; Stern, S. Alan (2006). "Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2". The Astronomical Journal 132 (1): 290. arXiv:astro-ph/0512491. Bibcode:2006AJ....132..290B. doi:10.1086/504422.  edit. a, i, e per JPL (site updated 2008 Aug 25)
  14. ^ Witze, Alexandra (2015). "Pluto’s moons move in synchrony". Nature. doi:10.1038/nature.2015.17681. 
  15. ^ Boyle, Alan. "Pluto's Moons Raise New Puzzles for NASA's New Horizons Mission". NBC NBC News. Retrieved 3 June 2015. 
  16. ^ Chang, Kenneth (3 June 2015). "Astronomers Describe the Chaotic Dance of Pluto’s Moons". New York Times. Retrieved 4 June 2015. 

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