Moons of Pluto 3048284 222830959 2008-07-01T09:32:17Z Thijs!bot 1392310 robot Adding: [[zh:冥王星的衛星]] [[Image:Pluto system 2006.jpg|thumb|250px|Hubble image of the Plutonian system]] [[Pluto]] has three known [[Natural satellite|moons]]. The largest, [[Charon (moon)|Charon]], is proportionally larger, compared to its primary, than any other satellite of a known planet or dwarf planet in the [[solar system]]. The other two moons, [[Nix (moon)|Nix]] and [[Hydra (moon)|Hydra]], are much smaller.<ref>{{cite news |url=http://sciencenow.sciencemag.org/cgi/content/full/2006/620/1 |title=Pluto's Twins Get Their Names |first=Govert |last=Schilling |publisher=ScienceNOW Daily News |date=20 June 2006 |accessdate=2006-06-21}}</ref> ==Pluto's satellite system== [[Image:Pluto system.svg|right|thumb|250px|The orbits of the Plutonian system as seen from Earth, calculated from the discovery images of the outer moons.]] [[Image:TheKuiperBelt Orbits PlutoQuadSystem.svg|right|thumb|250px|The orbits of the Plutonian system as seen from the pole of Charon's orbit.]] The innermost moon, [[Charon (moon)|Charon]], was discovered by [[James Christy]] on [[1978]] [[June 22]], nearly half a century after Pluto. Two outer moons were imaged by the [[Hubble Space Telescope]] Pluto Companion Search Team on [[2005]] [[May 15]], and [[precovery|precovered]] from Hubble images taken in June 2002. With the orbits confirmed, the moons have been given definitive names: [[Hydra (moon)|Hydra]] (Pluto III, formerly S/2005&nbsp;P&nbsp;1) and [[Nix (moon)|Nix]] (Pluto II, formerly S/2005&nbsp;P&nbsp;2). The names were chosen in part because the initials (NH) allude to the ''[[New Horizons]]'' mission [http://www.comcast.net/news/science/index.jsp?cat=SCIENCE&fn=/2006/06/22/419501.html]. Further Hubble observations were made in February and March 2006. Given the sensitivity of the Hubble images, and the fact that the entire region of space dominated by Pluto's gravitational field has been imaged, Pluto is not expected to have any other moons larger than approximately 12&nbsp;km in diameter with the albedo of Charon, or possibly up to 40&nbsp;km if they are as dark as the darkest [[Kuiper Belt object]]s. The possibility of [[planetary ring|rings]] created by impacts on the smaller moons will be investigated by the ''[[New Horizons]]'' probe. The Plutonian system is highly compact: The three known satellites orbit within the inner 3% of the region where [[prograde and retrograde motion|prograde]] orbits would be stable. Pluto and Charon have been called a [[double planet]] because Charon is larger compared to Pluto (half its diameter and an eighth its mass) than any other moon is to a planet; indeed Charon is massive enough that, despite their proximity, Pluto orbits the system's [[barycenter]] at a point outside its surface.<ref>See {{cite web |url=http://wwwsa.boulder.swri.edu/P1P2_motion.avi |title=P1P2_motion.avi |format=AVI}} and [[barycenter]] for animations</ref> Charon and Pluto are also [[tidal locking|tidally locked]], so that they always present the same face toward each other. Following Buie and Grundy's recent re-calculations taking into account older images, the orbits of the moons are confirmed to be circular and coplanar, with inclinations differing less than 0.4° and eccentricities less than 0.005. The inclinations are roughly 96° to the [[ecliptic]] (so technically the moons' movements are [[Prograde and retrograde motion|retrograde]]). The diagram on the right shows the view from the axis of the moons' orbits ([[declination]] 0°, [[right ascension]] 133°), aligned with the [[Hubble Space Telescope| HST]] diagram above it. As seen from Earth, these circular orbits appear foreshortened into ellipses depending on Pluto's position.<ref>{{cite web |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/19/image/h |title=Orbits of 4 Bodies in Pluto System about Barycenter as Seen from Earth |publisher=Hubblesite |accessdate=2006-06-21}}</ref> The Plutonian system has not been visited by spacecraft, but a flyby by the ''[[New Horizons]]'' mission is due in July 2015. ==Table of known moons== The Plutonian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have [[gravitational collapse|collapsed]] into a [[spheroid]] are highlighted in light purple. Pluto has been added for comparison, for it orbits a point outside itself. {| class="wikitable" |+ align=bottom style="text-align:left; padding:0.5em;" | <small>'''Notes:'''<br>Data from [http://arxiv.org/abs/astro-ph?papernum=0511837 Buie & Grundy]</small> |- style="background:#efefef;" ! colspan="3" | Name<br>([[Help:pronunciation|pronunciation]]) ! Image ! Mean diameter (km) ! Mass (×10<sup>21</sup>&nbsp;kg) ! Semi-major<br>axis (km) ! Orbital period (days) ! Eccentricity ! Inclination<br>(to Pluto's equator) ! Discovery date |- style="text-align:center; background:#ccccff" | colspan=3|'''Pluto''' || bgcolor=black| [[Image:Pluto.jpg|50px|center]] || 2306 ± 20 || 13.05 ± 0.07 || 2390 || 6.387230 || 0 || &mdash; || 1930 |- style="text-align:center; background:#eeeeff" | Pluto I || '''[[Charon (moon)|Charon]]''' || {{IPA|/ˈʃɛərən/}},<ref>Many astronomers use this idiosyncratic pronunciation, rather than the classical {{IPAEng|ˈkɛərən}}, but both are acceptable.</ref> {{IPA|/ˈkɛərən/}} || bgcolor=black| [[Image:Charon plutoface.png|50px|center]] || 1207 ± 3 || 1.52 ± 0.06 || 19&nbsp;571 ± 4 || 6.387230 || 0 (0.000% ± 0.007%) || 0.00° ± 0.014° || 1978 |- style="text-align:center; background:#ffffff;" | Pluto II || [[Nix (moon)|Nix]] || {{IPA|/ˈnɪks/}} || || 46-137 || < 0.002 || 48&nbsp;675 ± 120 || 24.856 ± 0.001 || ~0 (0.2% ± 0.2%) || 0.04° ± 0.22° || 2005 |- style="text-align:center; background:#ffffff;" | Pluto III || [[Hydra (moon)|Hydra]] || {{IPA|/ˈhaɪdrə/}} || || 61-167 || < 0.002 || 64&nbsp;780 ± 90 || 38.206 ± 0.001 || 0.5% ± 0.1%|| 0.22° ± 0.12° || 2005 |} [[Image:Plutonian system size.svg|thumb|275px|right|Relative sizes and colors of the Plutonian system.]] When discovered, Hydra was somewhat brighter than Nix, and therefore thought to be larger by 20%, but follow-up observations found them to be nearly identical. It is likely that the change in brightness is due to the lightcurve of Hydra, but whether this is due to an irregular shape or to a variation in surface brightness ([[albedo]]) is unknown. The diameters of objects can be estimated from their assumed albedos; the estimates above correspond to a 35% albedo like Charon, but the moons could be as large as 130&nbsp;km if they have the 4% albedo of the darkest [[Kuiper belt object|KBO]]s. However, given their color and suspected chemical similarities to Charon, it is likely that their albedos are similar as well and that the diameters are closer to the lower estimates. ==Resonances and the origin of the Plutonian system== It is suspected that the Plutonian satellite system was created by a massive collision, similar to the [[Giant impact hypothesis|"Big Whack"]] believed to have created the Earth's moon. In both cases it may be that the high [[Angular momentum|angular momenta]] of the moons can only be explained by such a scenario. The nearly circular orbits of the smaller moons suggests that they were also formed in this collision, rather than being captured Kuiper Belt objects. This and their near [[orbital resonance]]s with Charon (see below) suggest that they formed even closer to Pluto than they are at present, and that they migrated outward as Charon achieved its current orbit. If Hydra and Nix turn out to be tidally locked, as Charon is, that will settle the issue, as tidal forces are insufficient to damp their rotations in their present orbits. Both are a Lunar grey like Charon,<ref>{{cite web |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/15/ |title=Hubble's Latest Look at Pluto's Moons Supports a Common Birth |publisher=Hubblesite |accessdate=2006-06-21}}</ref> which is consistent with a common origin. Their difference in color from Pluto, one of the reddest bodies in the Solar system due to the effects of sunlight on the nitrogen and methane ices of its surface, may be due to a loss of such volatiles during the impact or subsequent coalescence, leaving the surfaces of the moons dominated by water ice. Such an impact would be expected to create additional debris (more moons), but these must be relatively small to have avoided detection by Hubble. It is possible that there are also undiscovered [[irregular satellite]]s, which are captured [[Kuiper Belt objects]]. Nix and Hydra are very close to a 1:4:6 [[orbital resonance]] with the Charon-Pluto orbital period: Nix is within 2.7% of resonance, while Hydra is within 0.3%, though neither are in an exact resonance. It may be that these orbits originated as forced resonances when Charon was tidally boosted into its current geosynchronous orbit, and then released from resonance as Charon's orbital eccentricity was tidally damped. Today the Pluto-Charon pair continue to produce strong tidal forces, with the gravitational field at the outer moons varying by 15% peak to peak. At the lower estimated size range, Nix should have no significant [[precession]], while Hydra should have a precession period of 15 years. However, at their maximum projected masses (assuming an albedo of 4%), the two moons may be in a 3:2 orbital resonance with each other, with [[libration]] periods of 400 to 450 days, though this may already be ruled out by the low eccentricity of Charon.<ref>{{cite web |url=http://arxiv.org/abs/astro-ph?papernum=0603214 |format=PDF |accessdate=2006-06-21 |title=On the Orbits and Masses of the Satellites of the Pluto-Charon System |first=Man Hoi |last=Lee |coauthors=S. J. Peale |year=May 9 2006 |publisher=arXiv.org}}</ref> Thus accurate orbital data can help resolve the sizes of these moons. However, it was recently calculated that a resonance with Charon could boost either Nix or Hydra into its current orbit, but not both: boosting Hydra would have required a near-zero Charonian eccentricity of 0.024, while boosting Nix would have required a larger eccentricity of at least 0.05. They suggest instead that Nix and Hydra were captured and migrated inward until they were trapped in resonance with Charon.<ref name=Lithwick2007>{{cite journal|author=Y. Lithwick & Y. Wu|journal=American Astronomical Society, DDA meeting #38, #3.05|title=On the Origin of Pluto's Minor Moons, Nix and Hydra|year=2007|url=http://adsabs.harvard.edu/abs/2007DDA....38.0305L}}</ref> [[Image:PLUTOMOONFROM4.jpg|thumb|250px|Creation of Pluto's Moons. Number 1 shows a [[Kuiper Belt Object]] nearing [[Pluto]]. Number 2 shows a [[KBO]] is impacting Pluto. Number 3 shows a [[dust ring]] forming around Pluto. Number 4 shows the [[Debris]] clumping together to form Charon. Number 5 shows Pluto and [[Charon]] becoming a spherical body and Charon orbiting Pluto ]] ==See also== * Natural satellites of [[Moons of Mars|Mars]]{{·}} [[Moons of Jupiter|Jupiter]]{{·}} [[Moons of Saturn|Saturn]]{{·}} [[Moons of Uranus|Uranus]]{{·}} [[Moons of Neptune|Neptune]] * [[Solar System]] * [[Timeline of discovery of Solar System planets and their natural satellites]] * [[Naming of natural satellites]] ==Notes== {{reflist}} ==References== * S.A. Stern, H.A. Weaver, A.J. Steffl, M.J. Mutchler, W.J. Merline, M.W. Buie, E.F. Young, L.A. Young, & J.R. Spencer (2006), ''Characteristics and Origin of the Quadruple System at Pluto,'' Nature, submitted ([http://arxiv.org/abs/astro-ph/0512599 preprint]) * 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. (2005), ''New Constraints on Additional Satellites of the Pluto System'', Astronomical Journal, submitted ([http://arxiv.org/abs/astro-ph/0511837 preprint]) * Buie M.W., Grundy W.M., Young, E.F., Young L.A., Stern S.A. (2005), ''Orbits and photometry of Pluto's satellites: Charon, S/2005 P1 and S/2005 P2'', submitted ([http://arxiv.org/abs/astro-ph/0512491 preprint]) * [http://cfa-www.harvard.edu/iau/special/08625.pdf IAU Circular No. 8625 describing the discovery] * [http://cfa-www.harvard.edu/iau/special/08686.pdf IAU Circular No. 8686] which reports a much more neutral color for P2 * IAU Circular No. 8723 announcing the names of Nix and Hydra * [http://www.boulder.swri.edu/plutonews/ Background Information Regarding Our Two Newly Discovered Satellites of Pluto] – The discoverers' website ==External links== *[http://wwwsa.boulder.swri.edu/P1P2_motion.avi Animation of the Plutonian system] * [http://www.nasa.gov/vision/universe/solarsystem/hubble_pluto.html Hubble Spots Possible New Moons Around Pluto] (NASA) * [http://www.space.com/scienceastronomy/051031_pluto_moons.html Two More Moons Discovered Orbiting Pluto] (SPACE.com) {{-}} {{Moons of dwarf planets}} {{Pluto}} {{Solar System moons (compact)}} {{Solar System}} <!--Categories--> [[Category:Pluto's moons| ]] <!--Other languages--> [[be-x-old:Спадарожнікі Плютона]] [[cs:Měsíce Pluta]] [[es:Lunas de Plutón]] [[fr:Système plutonien]] [[io:Sateliti di Plutono]] [[it:Satelliti naturali di Plutone]] [[lt:Plutono palydovai]] [[nah:Mictlāntēuccītlalli īmētz]] [[ja:冥王星の衛星]] [[nn:Plutomånane]] [[pl:Księżyce Plutona]] [[ru:Спутники Плутона]] [[sk:Mesiace Pluta]] [[sl:Plutonovi naravni sateliti]] [[sv:Plutos naturliga satelliter]] [[tl:Mga buwan ng Pluto]] [[zh:冥王星的衛星]]