Hydra (moon) facts for kids

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Hydra
Plutonian system.jpg
Artist conception of Hydra (foreground),
Pluto and Charon (background),
and Nix (bright dot center left)
Discovery
Discovered by Hubble Space Telescope
Pluto Companion Search Team
Discovered in June 2005
Orbital characteristics
Semi-major axisa 64 780 ± 90 km
Eccentricity 0.005 ± 0.001
Orbital period 38.206 ± 0.001 d
Inclinationb 0.22° ± 0.12°
Is a moon of Pluto
Physical characteristics
Mean diameter 61 − 167 km
Mass 5×1016 − 2×1018 kg
Mean density unknown
Rotation period unknown
Axial tilt unknown
Albedo 0.04 − 0.35 (assumed)
Apparent magnitude 22.9 to 23.3 (measured)
Surface temp. 33-55 K
Atmosphere none

a Relative to the Pluto-Charon barycenter.
b Relative to Pluto's equator (also Charon's orbital plane).

Hydra is the farthest moon of Pluto. It was found along with Nix 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 May 15, 2005 and May 18, 2005; the moons were independently found 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. The moons were provisionally designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix).

The moon orbits the barycenter of the system in the same plane as Charon and Nix, at a distance of about 65,000 km. Unlike other moons of Pluto, its orbit is only nearly circular; its eccentricity of 0.0052 is small, but significantly non-zero. Its orbital period of 38.2 days.

Pluto system 2005 discovery images
Discovery images of Hydra.

Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 61 km, if its reflectivity 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 (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 bigger. Pre-discovery data from Hubble observations in 2002-3 inferred 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 grey like Charon and Nix, though Pluto is reddish.

Hydra is to be visited along with Pluto by the New Horizons mission in 2015.

The name Hydra was announced on June 21, 2006, in IAU Circular 8723, along with the formal designation Pluto III. It was named after Hydra, the monster who guarded the waters of Pluto/Hades's underworld in Greco-Roman Mythology.


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