Hydra (moon)

+ Hydra 275px Artist conception of Hydra (foreground),Pluto and Charon (background),and Nix (bright dot center left) Discovery Discovered by Hubble Space TelescopePluto Companion Search Team Discovered in June 2005 Orbital characteristics(Final preprint) Semi-major axisa 64 780 ± 90 km Eccentricity 0.005 ± 0.001 Orbital period 38.206 ± 0.001 d Inclinationb 0.22° ± 0.12°  Is a moon of Pluto Physical characteristics Mean diameter 61 − 167 km Mass 5 − 2 kgBased on the range of diameters from Buie et al. (2006), and densities ranging from 1 g/cm³ (ice) to 2 g/cm³ (Pluto). Mean density unknown Rotation period unknown Axial tilt unknown Albedo 0.04 − 0.35 (assumed) Apparent magnitude 22.9 to 23.3 (measured) Surface temp. 33-55 K Atmosphere none 
Hydra is the farthest moon of Pluto. It was found 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 found 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).IAU Circular No. 8625 describing the discovery

The moon orbits the barycenter of the system in the same plane as Charon and Nix, at a distance of about 65,000 km. Unlike other moons 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.

Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 61 km, if its reflectivity is similar to Charon's 35 percent, and about 167 km, if it has a reflectivity of 4 percent like the darkest 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 bigger.  Pre-discovery data from Hubble observations in 2002-3 inferred that Nix was the brighter moon.  However, Hubble observations in 2005-6, specifically targeting the dim moons, once again showed Hydra to be a little brighter. (Final preprint)  Hydra appears to be grey like Charon and Nix, though Pluto is reddish.
The New Horizons mission visited the area of Pluto and photographed its satellites in 2015. Hydra was on the far side of Pluto and the pictures were less good than of the others.

The name Hydra was announced on June 21, 2006, in IAU Circular 8723,IAU Circular No. 8723 naming the moons 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.

References

 (Final preprint)

Other websites
 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 

Category:Pluto's moons