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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.
The name Hydra was announced on June 21, 2006, in IAU Circular 8723, 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.
Orbital properties
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. Unlike other satellites of Pluto, its orbit is only nearly circular; its eccentricity of 0.0052 is small, but significantly non-zero. Its orbital period of 38.2 days is close to a 1:6 orbital resonance with Charon, with the timing discrepancy being 0.3%. Whether this is an actual resonance awaits more detailed determinations of its orbit, in particular its rate of precession. If there is no actual resonance, 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
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. 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. Pre-discovery data from Hubble observations in 2002–03 implied that Nix was the brighter moon. However, Hubble observations in 2005–06, specifically targeting the dim moons, once again showed Hydra to be a little brighter. Hydra appears to be spectrally neutral like Charon and Nix, whereas Pluto is reddish.
References
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Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi: 10.1086/504422 , please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi= 10.1086/504422 instead.. a, i, e per JPL (site updated 2008 Aug 25)
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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. PMID16495991. {{cite journal}}: Unknown parameter |lastauthoramp= ignored (|name-list-style= suggested) (help)
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.
^ 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)
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. {{cite web}}: Unknown parameter |lastauthoramp= ignored (|name-list-style= suggested) (help)
Discovery images of Hydra (and Nix).jpg)
