Hydra (moon)

Hydra (moon)

Infobox Planet
name = Hydra
bgcolour = #a0ffa0

caption = Artist conception of Hydra (foreground),
Pluto and Charon (background),
and Nix (bright dot center left)
discovery = yes
discoverer = Hubble Space Telescope
Pluto Companion Search Team
discovered = June 2005
alt_names = Pluto III
adjectives = Hydrean
orbit_ref = cite journal |author=Buie, M. W. |coauthors=Grundy, W. M.; Young, E. F.; Young, L. A.; and Stern, S. A. |title="Orbits and Photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2" [sic] |journal=Astronomical Journal |year=2006 |volume=132 |pages=290 |doi=10.1086/504422 ( [http://arxiv.org/abs/astro-ph/0512491v2 Final preprint] ) "a, i, e" per [http://ssd.jpl.nasa.gov/?sat_elem JPL] (site updated 2008 Aug 25)]
semimajor = 64 749 km
eccentricity = 0.0051
period = 38.206 ± 0.001 d
inclination = 0.212°
satellite_of = Pluto
physical_characteristics = yes
mean_radius = 30 − 84 kmcite journal | author = H. A. Weaver |coauthors = S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young and L. A. Young | year = 2006 | month = 23 February | title = Discovery of two new satellites of Pluto | journal = Nature | volume = 439| issue = 7079 | pages = 943–945| doi = 10.1038/nature04547 | url = http://arxiv.org/pdf/astro-ph/0601018 | accessdate = 2007-11-11 | format = subscription required]
mass = 5e|16–2e|18 kg [ Based on the range of diameters from Buie "et al." (2006), and densities ranging from 1 g/cm³ (ice) to 2 g/cm³ (Pluto).]
density = (unknown)
single_temperature = 33-55 K
rotation = (unknown)
axial_tilt = (unknown)
albedo = 0.04 − 0.35 (assumed)
magnitude = 22.9 to 23.3 (measured)

Hydra (pronEng|ˈhaɪdrə respell|HYE|drə, or as in Greek "Ύδρα)" is the outer-most natural satellite of Pluto. It was discovered 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 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. The moons were provisionally designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix). [http://cfa-www.harvard.edu/iau/special/08625.pdf IAU Circular No. 8625] describing the discovery]

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 a true resonance awaits more detailed determinations of its orbit, in particular its rate of precession. 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 three known moons, and is maintained by the periodic local fluctuation of 5% in the Pluto-Charon gravitational field strength.

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 larger.cite web
last=Stern |first=Alan
coauthors=Hal Weaver (JHU APL), Max Mutchler (STScI), Andrew Steffl (SwRI), Bill Merline (SwRI), Marc Buie (Lowell Observatory), John Spencer (SwRI), Eliot Young (SwRI), and Leslie Young (SwRI)
date=15 May 2005
title=Background Information Regarding Our Two Newly Discovered Satellites of Pluto
publisher=Planetary Science Directorate (Boulder Office)
] 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.cite journal | author=Stern, S. A. |coauthors=Mutchler, M. J.; Weaver, H. A.; and Steffl, A. J. |title="The Positions, Colors, and Photometric Variability of Pluto's Small Satellites from HST Observations 2005-2006" |journal=Astronomical Journal |year=2006 |pages=submitted ( [http://www.arxiv.org/abs/astro-ph/0605014 Final preprint] )] Hydra appears to be spectrally neutral 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, [http://www-int.stsci.edu/~mutchler/documents/IAU_Circular_8723.pdf IAU Circular No. 8723] naming the moons] along with the formal designation Pluto III. It was named after Hydra, the serpent who guarded the waters of the underworld in Greco-Roman Mythology.


  • cite journal |journal=The Astronomical Journal |author=Steffl, A. J. |coauthors=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.; and Spencer, J. R. |year=2006 |title=New Constraints on Additional Satellites of the Pluto System |volume=132 |pages=614–619 |doi=10.1086/505424 ( [http://arxiv.org/abs/astro-ph/0511837 Final preprint] )
  • External links

    * [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Plu_Hydra Hydra Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
    * [http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/19/ NASA's Hubble Reveals Possible New Moons Around Pluto] – Hubble press release
    * [http://www.space.com/scienceastronomy/051031_pluto_moons.html Two More Moons Discovered Orbiting Pluto] (SPACE.com)
    * [http://www.space.com/scienceastronomy/060621_nix_hydra.html Pluto's Newest Moons Named Hydra and Nix] (SPACE.com)
    * [http://www.spacetelescope.org/images/html/opo0609b.html Hydra at ESA/Hubble]

    io:S/2005 P 1

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