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Hydra is the outermost known moon of Pluto. It was discovered along with Nix in June 2005, and was visited along with Pluto by New Horizons in July 2015. Hydra's surface is probably covered with water ice. It is irregularly shaped, and measures 50 kilometers (31 miles) along its longest dimension[5]. Hydra's reflectivity (the percentage of incident light reflected from the surface) is intermediate between those of Pluto and Charon.[6][7]

Hydra Enhanced Color.jpg
Enhanced color image of Hydra, taken by New Horizons.
Discovered byHubble Space Telescope
Pluto Companion Search Team
Discovery date15 June 2005
Named after
Lernaean Hydra
(134340) Pluto III
Orbital characteristics[1]
64738±3 km
38.20177±0.00003 d
Satellite ofPluto
Physical characteristics
Dimensions≈ 65 km × 45 km × 25 km[2]
Mass(4.8±4.2)×1016 kg[3]
0.4295 ± 0.0008 d (chaotic)[2]
Albedo0.83 ± 0.08 (geometric)[2]
Temperature33–55 K
22.9–23.3 (measured)[4]




Discovery images of Nix and Hydra

Hydra was discovered by the Hubble Space Telescope's "Pluto Companion Search Team", consisting of Hal A. Weaver, 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 15 May 2005, and 18 May 2005; Nix and Hydra were independently discovered by Max J. Mutchler on 15 June 2005, and 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 (Hydra) and S/2005 P 2 (Nix).[8][9]


The name Hydra was announced on 21 June 2006, in IAU Circular 8723,[9] along with the formal designation Pluto III. The name is that of the Hydra, the nine-headed serpent that battled Heracles in Greek mythology. The nine heads of Hydra are a reference to Pluto's tenure as the ninth planet; its initial, H, refers to the Hubble Telescope, which discovered Hydra and, together with Nix, to the New Horizons mission whose safe passage was the motivation for taking the Hubble images.[10]

Orbital propertiesEdit

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

Hydra orbits the barycenter of the system in the same plane as Charon and Nix, at a distance of about 65,000 km. Its eccentricity of 0.0059 is small, but significantly non-zero, and the largest of those of Pluto's small moons (slightly larger than that of Styx).

Hydra is in a 2:3 orbital resonance with Nix, and a 6:11 resonance with Styx (the ratios represent numbers of orbits completed per unit time; the period ratios are the inverses).[1][11] As a result of this "Laplace-like" 3-body resonance, it has conjunctions with Styx and Nix in a 5:3 ratio.

Its orbital period of 38.2 days is also close to a 1:6 orbital resonance with Charon, with the timing discrepancy being 0.3%. A hypothesis to explain the 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 5% in the Pluto–Charon gravitational field strength.

Like Saturn's moon Hyperion, Nix, and the other small moons of Pluto, Hydra rotates chaotically; its day length and rotational axis vary quickly over astronomical timescales, to the point that it regularly flips over. This is largely due to the aforementioned fluctuation of the Pluto–Charon gravitational field, as well as its irregular shape. Hydra's rotational period at the time of the New Horizons flyby was a mere 10 hours.[12]

Physical propertiesEdit

Hydra is irregular in shape, 55 by 40 kilometers (34.2 mi × 24.9 mi)[13] in cross-section from one side. It is spectrally neutral like Charon and most of Nix (whereas Pluto is reddish) and is composed primarily of water ice,[14] probably ice XI.


The only detailed photographs of Hydra were taken by the New Horizons spacecraft from approximately 640,000 kilometres (400,000 mi) away during its flyby of the Pluto–Charon system on 14 July 2015. It also studied Hydra's surface composition, reflectivity, and other basic physical properties. Hydra's reflectivity is estimated to be about 45 percent.


  1. ^ a b Showalter, M. R.; Hamilton, D. P. (3 June 2015). "Resonant interactions and chaotic rotation of Pluto's small moons". Nature. 522 (7554): 45–49. Bibcode:2015Natur.522...45S. doi:10.1038/nature14469. PMID 26040889.
  2. ^ a b c "2016 Lunar & Planetary Science Conference by National Institute of Aerospace".
  3. ^ "The Pluto system: Initial results from its exploration by New Horizons".
  4. ^ 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)
  5. ^ Keeter, Bill (2016-05-05). "Pluto's Icy Moon Hydra". NASA. Retrieved 2018-02-20.
  6. ^ NASA (15 July 2015). "Hydra Emerges from the Shadows". Retrieved 16 July 2015.
  7. ^ NASA/APL (2008). "Pluto's Moon Hydra".
  8. ^ IAU Circular No. 8625 describing the discovery
  9. ^ a b IAU Circular No. 8723 naming the moons
  10. ^ Neil deGrasse Tyson, 2009, The Pluto Files
  11. ^ Witze, Alexandra (2015). "Pluto's moons move in synchrony". Nature. doi:10.1038/nature.2015.17681.
  12. ^ "DPS 2015: Pluto's small moons Styx, Nix, Kerberos, and Hydra [UPDATED]".
  13. ^ "New Horizons 'Captures' Two of Pluto's Smaller Moons". Johns Hopkins University Applied Physics Laboratory. 21 July 2015. Retrieved 22 July 2015.
  14. ^ "NASA New Horizons on Twitter".


External linksEdit