Short answers, pulled from the story.
Astronomers spotted the moon Hydra on the 15th of May 2005 using the Hubble Space Telescope. Max J. Mutchler identified one moon on the 15th of June 2005 while Andrew J. Steffl found the other on the 15th of August 2005. The team announced their findings publicly on the 31st of October 2005 after verifying archival data from 2002.
Scientists named the larger of the pair Hydra after a nine-headed serpent from Greek mythology to honor Pluto's former status as the ninth planet in our solar system. The International Astronomical Union officially approved the name on the 21st of June 2006 through IAU Circular 8723.
Hydra measures approximately 42 kilometers along its longest axis and about 30 kilometers across its shortest dimension. Water ice covers much of the terrain creating a highly reflective exterior with a geometric albedo reaching 83 percent.
Unlike most large moons that keep one face toward their planet, Hydra rotates chaotically with an approximate rotational period of 10 hours at the time of the New Horizons flyby. Its rotational axis tilted about 110 degrees relative to its orbital plane during that encounter causing the moon to tumble unpredictably over astronomical timescales.
Hydra orbits the Pluto-Charon barycenter at a distance of roughly 65,000 kilometers remaining the outermost known moon in the entire Plutonian system. An orbital period of 38.2 days keeps it synchronized with other satellites through resonances including a 2:3 orbital resonance connecting Hydra with Nix.