Hydra (moon) 3047717 225613760 2008-07-14T15:44:09Z 67.41.162.58 {{otheruses|Hydra}} {| class="toccolours" style="margin-left: 1em; float: right; border: 1px solid #CCC;" |+ <big>'''Hydra'''</big> |- | bgcolor="#000000" colspan="2" align="center"| [[Image:Plutonian system.jpg|275px]]<br /> <font color="white"><small>Artist conception of Hydra (foreground),<br />Pluto and Charon (background),<br />and Nix (bright dot center left)</small></font> |- ! bgcolor="#a0ffa0" colspan="2" | Discovery |- ! align="left" | Discovered by | [[Hubble Space Telescope]]<br />Pluto Companion Search Team |- ! align="left" | Discovered in | June [[2005]] |- ! bgcolor="#a0ffa0" colspan="2" | [[Orbit]]al characteristics<ref name="Buie06">{{cite journal |author=Buie, M. W. |coauthors=[[William M. Grundy|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])</ref> |- ! align="left" | [[Semi-major axis]]<sup>a</sup> | 64&nbsp;780 ± 90 km |- ! align="left" | [[Eccentricity (orbit)|Eccentricity]] | 0.005 ± 0.001 |- ! align="left" | [[Orbital period]] | 38.206 ± 0.001 d |- ! align="left" | [[Inclination]]<sup>b</sup> | 0.22° ± 0.12° |- ! align="left" | Is a [[natural satellite|satellite]] of | [[Pluto]] |- ! bgcolor="#a0ffa0" colspan="2" | Physical characteristics |- ! align="left" | Mean [[diameter]] | 61 − 167 km<ref name="Weaver2006"> {{cite 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}}</ref> |- ! align="left" | [[Mass]] | 5{{e|16}} − 2{{e|18}} kg<ref> Based on the range of diameters from Buie ''et al.'' (2006), and densities ranging from 1 g/cm³ (ice) to 2 g/cm³ (Pluto).</ref> |- ! align="left" | Mean [[density]] | ''unknown'' |- ! align="left" | [[Sideral day|Rotation period]] | ''unknown'' |- ! align="left" | [[Axial tilt]] | ''unknown'' |- ! align="left" | [[Albedo]] | 0.04 − 0.35 (assumed)<ref name="Stern06b"/> |- ! align="left" | [[Apparent Magnitude]] | 22.9 to 23.3 (measured)<ref name="Stern06b"/> |- ! align="left" | Surface [[Temperature|temp.]] | 33-55 [[Kelvin|K]] |- ! align="left" | [[Celestial body atmosphere|Atmosphere]] | none |- |colspan=2| ---- |- |colspan=2| {{smaller| <sup>a</sup> Relative to the Pluto-Charon [[barycenter]].<br> <sup>b</sup> Relative to Pluto's equator (also Charon's orbital plane).}} |} '''Hydra''' ({{pronEng|ˈhaɪdrə}} ''<small>HYE</small>-drə,'' Greek ''Ύδρα)'' is the outer-most [[natural satellite]] of [[Pluto]]. It was discovered along with [[Nix (moon)|Nix]] in June, 2005 by the [[Hubble Space Telescope|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 [[precovery|precoveries]] from 2002. The moons were [[provisional designation in astronomy|provisionally designated]] '''S/2005 P 1''' (Hydra) and '''S/2005 P 2''' (Nix).<ref name="IAUC8625">[http://cfa-www.harvard.edu/iau/special/08625.pdf IAU Circular No. 8625] describing the discovery</ref><ref name="IAUC8723"/> The satellite orbits the [[barycenter]] of the system in the same plane as [[Charon (moon)|Charon]] and [[Nix (moon)|Nix]], at a distance of about 65,000 km. Unlike other satellites of Pluto, its orbit is only nearly circular; its [[eccentricity (orbit)|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.<!--- The instantaneous force at Hydra's distance in the Pluto-Charon-Hydra alignment case is 4.62% larger than in the quadrature case (where Hydra is 90° from the Pluto-Charon axis); the Charon-Pluto-Hydra case is almost exactly halfway between those values. In Buie ''et al.'', the quote is "The gravitational force exerted by Pluto on either P1 or P2 varies by roughly 15% (peak-to-peak)." Pluto's gravitational pull, ''by itself'', varies by 18% for Nix and 13% for Hydra ---> [[Image:Pluto system 2005 discovery images.jpg|thumb|250px|left|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 [[albedo|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).<ref name="Weaver2006"/> 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.<ref name=newsats>{{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) |url=http://www.boulder.swri.edu/plutomoons/ |accessdate=2007-11-10 }}</ref> Pre-discovery data from Hubble observations in 2002-3 implied that Nix was the brighter moon.<ref name="Buie06"/> However, Hubble observations in 2005-6, specifically targeting the dim moons, once again showed Hydra to be a little brighter.<ref name="Stern06b">{{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])</ref> 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,<ref name="IAUC8723">[http://www-int.stsci.edu/~mutchler/documents/IAU_Circular_8723.pdf IAU Circular No. 8723] naming the moons</ref> along with the formal designation '''Pluto III'''. It was named after [[Lernaean Hydra|Hydra]], the serpent who guarded the waters of the [[Greek underworld|underworld]] in [[Greek mythology|Greco-Roman Mythology]]. ==References== {{reflist}} <div class="references-small"> #<li value="8"> {{cite journal |journal=The Astronomical Journal |author=Steffl, A. J. |coauthors=Mutchler, M. J.; Weaver, H. A.; Stern, S. A.; [[Daniel David Durda|Durda, D. D.]]; [[Dirk Terrell|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]) </div> ==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] {{Moons of dwarf planets}} {{Trans-Neptunian objects}} [[Category:Pluto's moons]] [[ar:هيدرا (قمر)]] [[be-x-old:Гідра (спадарожнік)]] [[br:Hydra (loarenn)]] [[bg:Хидра (спътник)]] [[ca:Hydra (satèl·lit)]] [[cs:Hydra (měsíc)]] [[da:Hydra (måne)]] [[de:Hydra (Mond)]] [[es:Hidra (luna)]] [[fr:Hydra (lune)]] [[ga:Hydra]] [[gv:Hydra (fo-phlannad)]] [[ko:히드라 (위성)]] [[hr:Hidra (mjesec)]] [[io:S/2005 P 1]] [[it:Idra (astronomia)]] [[lt:Hidra (palydovas)]] [[hu:Hydra (hold)]] [[nl:Hydra (maan)]] [[ja:ヒドラ (衛星)]] [[no:Hydra (måne)]] [[nn:Plutomånen Hydra]] [[nds:Hydra (Maand)]] [[pl:Hydra (księżyc)]] [[pt:Hidra (satélite)]] [[ru:Гидра (спутник)]] [[simple:Hydra (moon)]] [[sk:Hydra (mesiac)]] [[sl:Hidra (luna)]] [[fi:Hydra (kuu)]] [[sv:Hydra (måne)]] [[zh:冥卫三]]