Hydra (moon)
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Image of Hydra taken by New Horizons on 14 July 2015
|
|
| Discovery | |
|---|---|
| Discovered by |
Hubble Space Telescope Pluto Companion Search Team |
| Discovery date | 15 June 2005 |
| Designations | |
| Pronunciation | /ˈhaɪdrə/ |
|
Named after
|
Lernaean Hydra |
| (134340) Pluto III | |
| Adjectives | Hydrian |
| Orbital characteristics | |
| 64738±3 km | |
| Eccentricity | 0.005862±0.000025 |
| 38.20177±0.00003 d | |
| Inclination | 0.242°±0.005° |
| Satellite of | Pluto |
| Physical characteristics | |
| Dimensions | ≈ 65 km × 45 km × 25 km |
| Mass | (4.8±4.2)×1016 kg |
| 0.4295 ± 0.0008 d (chaotic) | |
| variable | |
| Albedo | 0.83 ± 0.08 (geometric) |
| Temperature | 33–55 K |
| 22.9–23.3 (measured) | |
Hydra is the outermost known moon of Pluto. It was discovered along with Nix in June 2005, and was visited along with Pluto by New Horizons in July 2015. Hydra's surface is probably covered with water ice. Observed within Hydra's bright regions is a darker circular structure with a diameter of approximately 10 kilometers (5 miles). Hydra's reflectivity (the percentage of incident light reflected from the surface) is intermediate between those of Pluto and Charon.
Hydra was discovered by the Hubble Space Telescope's "Pluto Companion Search Team", consisting 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 15 May 2005, and 18 May 2005; Nix and Hydra were independently discovered by Max J. Mutchler on 15 June 2005, and Andrew J. Steffl on 15 August 2005. The discoveries were announced on 31 October 2005, after confirmation by precoveries from 2002. They were provisionally designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix).
The name Hydra was announced on 21 June 2006, in IAU Circular 8723, along with the formal designation Pluto III. The name is that of the Hydra, the nine-headed serpent that battled Heracles in Greek mythology. The nine heads of Hydra are a reference to Pluto's tenure as the ninth planet; its initial, H, refers to the Hubble Telescope, which discovered Hydra and, together with Nix, to the New Horizons mission whose safe passage was the motivation for taking the Hubble images.
Hydra orbits the barycenter of the system in the same plane as Charon and Nix, at a distance of about 65,000 km. Its eccentricity of 0.0059 is small, but significantly non-zero, and the largest of those of Pluto's small moons (slightly larger than that of Styx).
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