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Hydra

Discovery images of Hydra (and Nix)
Discovery
Discovered by	Hubble Space Telescope Pluto Companion Search Team
Discovery date	June 2005
Designations
Pronunciation	/ˈhaɪdrə/
Named after	Lernaean Hydra
Alternative names	(134340) Pluto III
Adjectives	Hydrian
Orbital characteristics
Semi-major axis	64749 km
Eccentricity	0.0051
Orbital period (sidereal)	38.206±0.001 d
Inclination	0.212°
Satellite of	Pluto
Physical characteristics
Mean radius	30–84 km
Mass	4.2×10 kg
Mean density	(unknown)
Synodic rotation period	chaotic
Axial tilt	varies
Albedo	0.04–0.35 (assumed)
Temperature	33–55 K
Apparent magnitude	22.9–23.3 (measured)

Hydra is the outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.

Discovery

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 May 15, 2005, and May 18, 2005; Nix and Hydra 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 from 2002. They were provisionally designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix).

Name

The name Hydra was announced on June 21, 2006, in IAU Circular 8723, along with the formal designation Pluto III. The name is that of the Hydra, the nine-headed serpent which battled Hercules 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.

Orbital properties

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. Its orbital period of 38.2 days is close to a 1:6 orbital resonance with Charon, with the timing discrepancy being 0.3%. Whether this is an actual resonance awaits more detailed determinations of its orbit, in particular its rate of precession. If there is no actual resonance, a hypothesis to explain the near-resonance is that it originated before the outward migration of Charon following the formation of all five 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 and 167 kilometers (38 and 104 mi). To meet the lower end of the range, its geometric albedo would have to be similar to Charon's 35%. For it to be the at the higher end of the range, it would have to have a reflectivity of 4%, like the darkest Kuiper belt objects. 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 2002–03 implied that Nix was the brighter moon. However, Hubble observations in 2005–06, specifically targeting the dim moons, once again showed Hydra to be a little brighter. Hydra appears to be spectrally neutral like Charon and Nix, whereas Pluto is reddish.

References

1. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi: 10.1086/504422 , please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi= 10.1086/504422 instead.. a, i, e per JPL (site updated 2008 Aug 25)
2. ^ H. A. Weaver; S. A. Stern; M. J. Mutchler; A. J. Steffl; M. W. Buie; W. J. Merline; J. R. Spencer; E. F. Young; L. A. Young (23 February 2006). "Discovery of two new satellites of Pluto". Nature. 439 (7079): 943–945. arXiv:astro-ph/0601018. Bibcode:2006Natur.439..943W. doi:10.1038/nature04547. PMID 16495991. {{cite journal}}: Unknown parameter |lastauthoramp= ignored (|name-list-style= suggested) (help)
3. Tholen, David J.; M. W. Buie; W. M. Grundy (October 2010). "Improved Masses of Nix and Hydra". AAS DPS Meeting #42. 42: 984. Bibcode:2010DPS....42.2008T.
4. ^ Stern, S. A.; Mutchler, M. J.; Weaver, H. A.; Steffl, A. J. (2006). "The Positions, Colors, and Photometric Variability of Pluto's Small Satellites from HST Observations 2005–2006". Astronomical Journal. 132 (3): submitted. arXiv:astro-ph/0607507. Bibcode:2006AJ....132.1405S. doi:10.1086/506347. (Final preprint)
5. Cain, Fraser (2008). "Pluto's Moon Hydra".
6. IAU Circular No. 8625 describing the discovery
7. ^ IAU Circular No. 8723 naming the moons
8. Neil deGrasse Tyson, 2009, The Pluto Files
9. Stern, Alan; Hal Weaver (JHU APL); Max Mutchler (STScI); Andrew Steffl (SwRI); Bill Merline (SwRI); Marc Buie (Lowell Observatory); John Spencer (SwRI); Eliot Young (SwRI); Leslie Young (SwRI) (15 May 2005). "Background Information Regarding Our Two Newly Discovered Satellites of Pluto". Planetary Science Directorate (Boulder Office). Retrieved 2007-11-10. {{cite web}}: Unknown parameter |lastauthoramp= ignored (|name-list-style= suggested) (help)

• 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. (2006). "New Constraints on Additional Satellites of the Pluto System". The Astronomical Journal. 132 (2): 614–619. arXiv:astro-ph/0511837. Bibcode:2006AJ....132..614S. doi:10.1086/505424. {{cite journal}}: Unknown parameter |lastauthoramp= ignored (|name-list-style= suggested) (help)(Final preprint)

External links

• Hydra Profile by NASA's Solar System Exploration
• 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)
• Hydra at ESA/Hubble

v t e Pluto
Atmosphere Climate Geology
Geography (features)	Regions "Brass Knuckles" Safronov Sharaf Harrington Vucub-Came Hun-Came Meng-pʻo Belton Cadejo Hayabusa Lowell Morgoth Pioneer Tombaugh "The Heart" Venera Viking Voyager Hills and mountains Baret Montes Challenger Colles Coleta de Dados Colles Hillary Montes Piccard Mons Tenzing Montes Wright Mons Plains Sputnik Planitia Valleys and depressions Adlivun Cavus Lineae Luna Linea Craters Burney
Moons	Charon Hydra Kerberos Nix Styx
Exploration	New Horizons
Astronomy	Official definitions Definition of planet IAU definition of planet Scientist opinions Alan Stern Michael E. Brown Neil deGrasse Tyson Discovery Planet X Lowell Observatory Percival Lowell Clyde Tombaugh Roger Putnam General Double planet Dwarf planet Planet Plutino Trans-Neptunian object Kuiper belt
Related	Fiction Mythology Venetia Burney

• Moons of Pluto
• Astronomical objects discovered in 2005