Image of Hydra taken by New Horizons on 14 July 2015
|Discovered by||Hubble Space Telescope
Pluto Companion Search Team
|Discovery date||15 June 2005|
|(340) Pluto III 134|
|Dimensions||(2D) 43 km × 33 km|
|±0.5 km (projected 2D) 20.5|
|Albedo||0.51 average geometric|
Hydra is the outermost known natural satellite 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. Observed within Hydra's bright regions is a darker circular structure with a diameter of approximately 10 kilometers (5 miles). Hydra's reflectivity (the percentage of incident light reflected from the surface) is intermediate between those of Pluto and Charon.
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).
The name Hydra was announced on 21 June 2006, in IAU Circular 8723, along with the formal designation Pluto III. The name is that of the Hydra, the nine-headed serpent that 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.
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 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). 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 likely the other small Plutonian moons, 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 current rotational period is a mere 10 hours. 
Hydra is irregular in shape, 55 × 40 km 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, 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 historic 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. More images and information will be downlinked from the spacecraft from 2015 to late 2016.
- 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.
- The Pluto system: Initial results from its exploration by New Horizons
- 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)
- NASA (15 July 2015). "Hydra Emerges from the Shadows". Retrieved 16 July 2015.
- NASA/APL (2008). "Pluto's Moon Hydra".
- IAU Circular No. 8625 describing the discovery
- IAU Circular No. 8723 naming the moons
- Neil deGrasse Tyson, 2009, The Pluto Files
- Witze, Alexandra (2015). "Pluto’s moons move in synchrony". Nature. doi:10.1038/nature.2015.17681.
- "New Horizons 'Captures' Two of Pluto's Smaller Moons". Johns Hopkins University Applied Physics Laboratory. 21 July 2015. Retrieved 22 July 2015.
- Steffl, A. J.; Mutchler, M. J.; Weaver, H. A.; Stern, S. A.; Durda, D. D.; Terrell, D.; Merline, W. J.; Young, L. A.; Young, E. F.; Buie, M. W. & Spencer, J. R. (2006). "New Constraints on Additional Satellites of the Pluto System". The Astronomical Journal 132 (2): 614–619. arXiv:astro-ph/0511837. Bibcode:2006AJ....132..614S. doi:10.1086/505424.(Final preprint)
|Wikimedia Commons has media related to Hydra (moon).|
- Hydra Profile by NASA's Solar System Exploration
- NASA's Hubble Reveals Possible New Moons Around Pluto – Hubble press release
- Two More Moons Discovered Orbiting Pluto (SPACE.com)
- Pluto's Newest Moons Named Hydra and Nix (SPACE.com)
- Hydra at ESA/Hubble