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Hydra (moon)

Hydra (moon)

Overview
Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix
Nix (moon)
Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

 in June 2005, and is to be visited along with Pluto by the New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

mission in July 2015.

Hydra was found by the Hubble Space Telescope's
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 Pluto Companion Search Team, which is composed of Hal A. Weaver, Alan Stern
Alan Stern
S. Alan Stern is an American planetary scientist. He is the principal investigator of the New Horizons mission to Pluto....

, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie
Marc W. Buie
Marc W. Buie is an astronomer at Lowell Observatory in Flagstaff, Arizona. He grew up in Baton Rouge, Louisiana and went on to get a B.S. in Physics from Louisiana State University in 1980. After that he switched fields and earned his Ph.D. in Planetary Science from the University of Arizona in...

, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A.
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Encyclopedia
Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix
Nix (moon)
Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

 in June 2005, and is to be visited along with Pluto by the New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

mission in July 2015.

Discovery


Hydra was found by the Hubble Space Telescope's
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 Pluto Companion Search Team, which is composed of Hal A. Weaver, Alan Stern
Alan Stern
S. Alan Stern is an American planetary scientist. He is the principal investigator of the New Horizons mission to Pluto....

, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie
Marc W. Buie
Marc W. Buie is an astronomer at Lowell Observatory in Flagstaff, Arizona. He grew up in Baton Rouge, Louisiana and went on to get a B.S. in Physics from Louisiana State University in 1980. After that he switched fields and earned his Ph.D. in Planetary Science from the University of Arizona in...

, 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
Precovery
Precovery is a term used in astronomy that describes the process of finding the image of an object in old archived images or photographic plates, for the purpose of calculating a more accurate orbit...

 from 2002. The moons were provisionally designated
Provisional designation in astronomy
Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calculated...

 S/2005 P 1 (Hydra) and S/2005 P 2 (Nix).

The name Hydra was announced on June 21, 2006, in IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 Circular 8723, along with the formal designation Pluto III. It was named after Hydra
Lernaean Hydra
In Greek mythology, the Lernaean Hydra was an ancient nameless serpent-like chthonic water beast, with reptilian traits, that possessed many heads — the poets mention more heads than the vase-painters could paint, and for each head cut off it grew two more — and poisonous breath so virulent even...

, the nine-headed serpent who battled Hercules
Hercules
Hercules is the Roman name for Greek demigod Heracles, son of Zeus , and the mortal Alcmene...

 in Greco-Roman Mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

.

Orbital properties



The satellite orbits the barycenter
Barycentric coordinates (astronomy)
In astronomy, barycentric coordinates are non-rotating coordinates with origin at the center of mass of two or more bodies.The barycenter is the point between two objects where they balance each other. For example, it is the center of mass where two or more celestial bodies orbit each other...

 of the system in the same plane as Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...

 and Nix
Nix (moon)
Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...

, at a distance of about 65,000 km. Unlike other satellites of Pluto, its orbit is only nearly circular; its eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 of 0.0052 is small, but significantly non-zero. Its orbital period of 38.2 days is close to a 1:6 orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

 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
Apsidal precession
In celestial mechanics, perihelion precession, apsidal precession or orbital precession is the precession of the orbit of a celestial body. More precisely it is the gradual rotation of the line joining the apsides of an orbit, which are the points of closest and farthest approach...

. 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 four known moons, and is maintained by the periodic local fluctuation of 5% in the Pluto–Charon gravitational field strength.

Physical properties


Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 61 km, if its geometric albedo
Geometric albedo
The geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle to that of an idealized flat, fully reflecting, diffusively scattering disk with the same cross-section....

 is similar to Charon's 35 percent, and about 167 km, if it has a reflectivity of 4 percent like the darkest Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

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. Pre-discovery data from Hubble observations in 2002-3 implied 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. Hydra appears to be spectrally neutral like Charon and Nix, though Pluto is reddish.

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