Hydra (moon)

Hydra is the fifth nearest and second- largest known moon of the dwarf planet Pluto. Its diameter is estimated to be 61-167 kilometers.

Discovery and designation

On 21 June 2006, the Moon of the International Astronomical Union ( IAU) officially Hydra (Greek for " water-snake" ) was named to the nine-headed sea monster of Greek mythology. If it lost a head, grew in its place two new ones. The Hydra was the daughter of Echidna and Typhon as well as the sister of Cerberus, the Chimera and the Sphinx.

In addition, the names Nix and Hydra correspond to the initials of the spacecraft New Horizons, which is to explore the system in July 2015.

Web properties

Hydra orbits the common center of mass of the Pluto -Charon system in a prograde, slightly elliptical orbit at an average distance of 62,745 km ( 64,780 km distance from Pluto's center, about 56.18 Pluto radii ). The orbital eccentricity is 0.0052, the orbit is inclined 0.25 ° relative to the equator of Pluto. The web is the most eccentric of the Pluto system.

The orbit of the next inner moon Kerberos has an approximately 7000 km radius less; Nix and Charon orbiting Pluto on even narrower lanes.

The orbital period is close to a 1:6 orbital resonance with Charon, the deviation is 0.3 %. Whether this is a genuine resonance, can only be safely resolved with a more accurate orbit determination, in particular the determination of the precession. If it is not a real resonance, there is a hypothesis that originated such a near- resonance before the current outward migration of Charon. Maintaining it is therefore by the periodic local fluctuation of 5 % in the gravitational field strength through the circulation of Pluto and Charon around each other.

Physical Properties

The exact value of the diameter could not be determined so far as the albedo (albedo ) of the surface is unknown. If the albedo equal to that of Charon ( 0.372 ) may be, this would amount to 61 km, with an albedo of 0.04, corresponding to the darkest Kuiper Belt objects, 167 km.

Hydra is lighter by 25 % than Nix, suggesting a 10 - 15% indicating larger diameter, assuming an equal albedo of the two moons. The Hubble images from 2002 to 2003 implied that Nix the larger moon. The Hubble observations were matched to 2006 2005, for observing the faint moons, again proved Hydra brighter than the moon. For spectral investigations Hydra was in contrast to the reddish Pluto in a similarly neutral gray color as Charon and Nix.

Hydra is likely analogous to the theory on the origin of the Moon, the product of large collision of a predecessor of Pluto with another pluto large body of the Kuiper Belt, which formed the moon, Charon, while debris fell into outer orbits around Pluto, this follows the moon Hydra formed.

Research

After the discovery in the summer of 2005, with images from the Hubble Space Telescope, the two newly discovered moons were observed in September 2005 by the Keck and the Gemini Observatory in Hawaii and by the ESO VLT telescope in Chile to confirm the discovery, but this failed, because the conditions for observation of the Pluto system were unfavorable at this time. On October 24, Marc W. Buie and Eliot F. Young could make out the moons in the pictures of 2002.

It is planned to have the space probe launched on January 19, 2006 New Horizons fly by on July 14, 2015, 27,000 km distance to Charon and 9600 km distance to Pluto. Since the discovery of Hydra and Nix and the fourth moon Kerberos was not confirmed at the start of the probe, they were not planned in the mission. Hydra and Nix, however, were included in the monitoring program afterwards. You should be localizable 90 days prior to the passage and you will then record their tracks more closely to ensure their position in the flyby. It will then both moons capture with a resolution of better than 200 m / pixel, only slightly worse than Charon (200 m / pixel). Under the current simulations, Hydra is on the same side of Pluto as Charon, Nix, however, on the other side, but closer to the probe.

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