Pluto has four known natural satellites (Charon, Nix, Hydra and S/2011 P1. (provisional name, also known as P4, identified by the Hubble Space Telescope in 2011).
Hydra is the second largest known natural satellite of Pluto. It was discovered along with Nix in June 2005.
Based on estimated size, Hydra is the 45th largest moon in the Solar System currently known.
Discovery
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.
Naming
Hydra was provisionally designated S/2005 P 1. Hydra the moon, was named after Hydra, the nine-headed serpent who battled Hercules in Greco-Roman Mythology.
Stats
Diameter (mean): ~ 114 (61 – 167) km
Semi-major axis: 64,749 km
Orbital Period: 38.21 days
Rotation Period: Unknown
Physical properties
Hydra's 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.
Exploration Status
The arrival of the New Horizons Spacecraft to Plutonian System in July 2015 is highly anticipated. The true size of Hydra and other new discoveries will be revealed.
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