Hydra (moon)

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Pluto system 2005 discovery images.jpg
Discovery images of Hydra (and Nix)
Discovered by Hubble Space Telescope
Pluto Companion Search Team
Discovery date June 2005
Pronunciation /ˈhdrə/
Named after
Lernaean Hydra
(134340) Pluto III
Adjectives Hydrian
Orbital characteristics[1]
64738±3 km
Eccentricity 0.005862±0.000025
38.20177±0.00003 d
Inclination 0.242°±0.005° 
Satellite of Pluto
Physical characteristics
Dimensions 58 km × 34 km ± ? [2]
Mass 4.2×1017 kg[3][dubious ]
Mean density
Albedo 0.04–0.35 (assumed)[4]
Temperature 33–55 K
22.9–23.3 (measured)[4]

Hydra is the outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.[5]


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 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).[6][7]


The name Hydra was announced on June 21, 2006, in IAU Circular 8723,[7] along with the formal designation Pluto III. The name is that of the Hydra, the nine-headed serpent which battled Hercules 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.[8]

Orbital properties[edit]

Labeled image of Hydra released upon IAU name approval

The satellite 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 greater 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).[9][1] 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.

Physical properties[edit]

Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 61 and 167 kilometers (38 and 104 mi). To meet the lower end of the range, its geometric albedo would have to be similar to Charon's 35%. For it to be the at the higher end of the range, it would have to have a reflectivity of 4%, like the darkest Kuiper belt objects.[10] At the time of discovery, Hydra was about 25 percent brighter than its sister moon Nix, which led to the assumption that its diameter was some 10 percent larger.[11] Pre-discovery data from Hubble observations in 2002–03 implied that Nix was the brighter moon.[12] However, Hubble observations in 2005–06, specifically targeting the dim moons, once again showed Hydra to be a little brighter.[4] Hydra appears to be spectrally neutral like Charon and Nix, whereas Pluto is reddish.


  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. doi:10.1038/nature14469. 
  2. ^ NASA's Hubble finds Pluto's moons tumbling in absolute chaos
  3. ^ Tholen, David J.; M. W. Buie; W. M. Grundy (October 2010). "Improved Masses of Nix and Hydra". AAS DPS Meeting #42 42: 984. Bibcode:2010DPS....42.2008T. 
  4. ^ 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)
  5. ^ Cain, Fraser (2008). "Pluto's Moon Hydra". 
  6. ^ IAU Circular No. 8625 describing the discovery
  7. ^ a b IAU Circular No. 8723 naming the moons
  8. ^ Neil deGrasse Tyson, 2009, The Pluto Files
  9. ^ Witze, Alexandra (2015). "Pluto’s moons move in synchrony". Nature. doi:10.1038/nature.2015.17681. 
  10. ^ H. A. Weaver; S. A. Stern; M. J. Mutchler; A. J. Steffl; M. W. Buie; W. J. Merline; J. R. Spencer; E. F. Young & L. A. Young (23 February 2006). "Discovery of two new satellites of Pluto". Nature 439 (7079): 943–945. arXiv:astro-ph/0601018. Bibcode:2006Natur.439..943W. doi:10.1038/nature04547. PMID 16495991. 
  11. ^ Stern, Alan; Hal Weaver (JHU APL); Max Mutchler (STScI); Andrew Steffl (SwRI); Bill Merline (SwRI); Marc Buie (Lowell Observatory); John Spencer (SwRI); Eliot Young (SwRI) & Leslie Young (SwRI) (15 May 2005). "Background Information Regarding Our Two Newly Discovered Satellites of Pluto". Planetary Science Directorate (Boulder Office). Retrieved 2007-11-10. 
  12. ^ 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)

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