Hydra (formerly known as S/2005 P 1) is a natural satellite of Pluto. It is one of two discovered 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 on May 15 and May 18, 2005; the moons were first spotted by Max J. Mutchler on June 15 and the discoveries were announced on October 31, once confirmed by other observations and precoveries from 2002 IAU Circular No. 8625 describing the discovery.

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. The orbital period is 38.2 days, which misses being a 1:6 orbital resonance with Charon by only 0.3 percent. It is thought that this near-resonance originated in the outward migration of Charon after the formation of all three moons, and is maintained by a periodic local fluctuation of 15 percent in the Pluto-Charon gravitational field.

Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 60 km, if its reflectivity is similar to Charon's 35 percent, and about 170 km, if it has a reflectivity of 4 percent like the darkest Kuiper belt objects (KBOs). All of Pluto's moons are spectrally neutral, however, so the lower end of the range seems likely. 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. This variation may be due either to albedo variations in Hydra's surface or to an oblong shape.

The moon is to be visited along with Pluto by the New Horizons mission in 2015. Hydra was nicknamed "Baltimore" by its discoverers. The formal name Hydra (named after Hydra, the monster that guarded Pluto's underworld in Greek mythology) was announced on June 21, 2006, in IAU Circular 8723, the designation Pluto III was also given. It should not be confused with the similarly-named asteroid 21290 Vydra.

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)

References

• Steffl A.J., Mutchler M.J., Weaver H.A., Stern S.A., Durda D.D., Terrell D., Merline W.J., Young L.A., Young E.F., Buie M.W., Spencer J.R. (2005), New Constraints on Additional Satellites of the Pluto System, Astronomical Journal, submitted (preprint)

• Buie M.W., Grundy W.M., Young, E.F., Young L.A., Stern S.A. (2005), Orbits and photometry of Pluto's satellites: Charon, S/2005 P1 and S/2005 P2, submitted (preprint)