Hydra (moon)

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**Hydra**
Discovery
Artist conception of Hydra (foreground), Pluto and Charon (background), and Nix (bright dot center left)
Discovered by	Hubble Space Telescope Pluto Companion Search Team
Discovered in	June 2005
Orbital characteristics ^[1]
Semi-major axis^a	64 780 ± 90 km
Eccentricity	0.005 ± 0.001
Orbital period	38.206 ± 0.001 d
Inclination^b	0.22° ± 0.12°
Is a satellite of	Pluto
Physical characteristics
Mean diameter	44 − 130 km ^[1]
Mass	5×10¹⁶ − 2×10¹⁸ kg ^[2]
Mean density	unknown
Rotation period	unknown
Axial tilt	unknown
Albedo	0.04 − 0.35 (assumed)
Surface temp.	33-55 K
Atmosphere	none

^a Relative to the Pluto-Charon barycenter. ^b Relative to Pluto's equator (also Charon's orbital plane).

Hydra is the outer-most natural satellite of Pluto. It was discovered along with Nix in June, 2005 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).^[3]

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 three known moons, and is maintained by the periodic local fluctuation of 15% in the Pluto-Charon gravitational field.

Discovery images of Hydra.

Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 40 km, if its reflectivity is similar to Charon's 35 percent, and about 130 km, if it has a reflectivity of 4 percent like the darkest Kuiper belt objects (KBOs). 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. However, in subsequent observations the two moons were about equal in brightness.^{[citation needed]} This is likely due to an oblong shape, although brightness variation over its surface may also be responsible. Hydra appears to be spectrally neutral like Charon and probably Nix, but unlike Pluto, which is reddish.

Hydra is to be visited along with Pluto by the New Horizons mission in 2015.

The name Hydra was announced on June 21, 2006, in IAU Circular 8723, along with the formal designation Pluto III. It was named after Hydra, the monster who guarded the waters of Pluto/Hades's underworld in Greco-Roman Mythology.

[edit] External links

IAU Circular No. 8625 describing the discovery
Background Information Regarding Our Two Newly Discovered Satellites of Pluto – The discoverers' website
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

[edit] References

^ ^a ^b Buie M.W., Grundy W.M., Young, E.F., Young L.A., Stern S.A. (2006). "Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2". Astronomical Journal 132: 290. (http://arxiv.org/abs/astro-ph/0511837)
^ Based on the range of diameters from Buie et al (2006), and densities ranging from 1 g/cm³ (ice) to 2 g/cm³ (Pluto).
^ IAU Circular No. 8625 describing the discovery

A.J. Steffl; M.J. Mutchler, H.A. Weaver, S.A.Stern, D.D. Durda, D. Terrell, W.J. Merline, L.A. Young, E.F. Young, M.W. Buie, J.R. Spencer (2006). "New Constraints on Additional Satellites of the Pluto System". The Astronomical Journal 132: 614-619. (Final preprint)
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 (2006). "The Discovery of Two New Satellites of Pluto". Nature 439 (7079): 943-945.