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Discovery images of Hydra (and Nix)
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Discovery
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| Discovered by | Hubble Space Telescope Pluto Companion Search Team |
| Discovery date | June 2005 |
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Designations
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| Pronunciation | /ˈhaɪdrə/[lower-alpha 1] |
| Named after | Lernaean Hydra |
| Alternate name(s) | (134340) Pluto III |
| Adjective | Hydrian |
| Semi-major axis | 64 749 km |
| Eccentricity | 0.0051 |
| Orbital period | 38.206 ± 0.001 d |
| Inclination | 0.212° |
| Satellite of | Pluto |
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Physical characteristics
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| Mean radius | 30 − 84 km[2] |
| Mass | 4.2×1017 kg[3] |
| Mean density | (unknown) |
| Rotation period | (unknown) |
| Axial tilt | (unknown) |
| Albedo | 0.04 − 0.35 (assumed)[4] |
| Temperature | 33–55 K |
| Apparent magnitude | 22.9 to 23.3 (measured)[4] |
Hydra is the second 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.
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Hydra was found by the Hubble Space Telescope's Pluto Companion Search Team, which is composed 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; the moons 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. The moons were provisionally designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix).[5][6]
The name Hydra was announced on June 21, 2006, in IAU Circular 8723,[6] along with the formal designation Pluto III. It was named after Hydra, the nine-headed serpent who battled Hercules in Greco-Roman Mythology.
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 a true resonance awaits more detailed determinations of its orbit, in particular its rate of precession. If there is no true resonance, a hypothesis to explain the near-resonance is that it originated before the outward migration of Charon following the formation of all four known moons, and is maintained by the periodic local fluctuation of 5% in the Pluto–Charon gravitational field strength.
Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 61 km, if its geometric albedo is similar to Charon's 35 percent, and about 167 km, if it has a reflectivity of 4 percent like the darkest Kuiper belt objects (KBOs).[2] 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.[7] Pre-discovery data from Hubble observations in 2002-3 implied that Nix was the brighter moon.[1] However, Hubble observations in 2005-6, 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, though Pluto is reddish.
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