Styx, formal designation(134340) Pluto V, is a smallnatural satellite of Pluto whose discovery was announced on 11 July 2012. It was discovered by use of theHubble Space Telescope, and is the smallest of the five known moons ofPluto. It was imaged along with Pluto and Pluto's other moons by theNew Horizons spacecraft in July 2015, albeit poorly with only a single image of Styx obtained.[5]
Styx is the second-closest known satellite to Pluto, and the fifth discovered. It was discovered one year afterKerberos. Styx is approximately 16 km (9.9 mi) across its longest dimension,[5] and its orbital period is 20.1 days.
Hubble image of Styx (circled), with the outer moons' orbits shown
Styx was discovered by a team led by astronomerMark R. Showalter, using fourteen sets of images taken between 26 June and 9 July 2012 by theWide Field Camera 3 fitted to theHubble.[10] The discovery was announced on 11 July 2012. Styx is about half as bright as the dimmest previously known object in the system, Kerberos, and about one hundred thousandth as bright as Pluto.[11] It was designated S/2012 (134340) 1,[12] and informally referred to as P5.[11][13][14]
The survey work leading to the discovery of Styx was in preparation for the mission of the uncrewedNew Horizons spacecraft, which flew by the Pluto system on 14 July 2015. The discovery of another small Plutonian moon heightened concerns that this region of space may harbor more bodies too small to be detected, and that the spacecraft could be damaged by an uncharted body orplanetary ring as it traversed the system at a speed of over 13 km/s.[8][15] Tiny moons, such as Saturn's moonPallene, tend to be associated with tenuous rings or arcs, because their gravity is unable to hold on to material ejected bymeteoroid impacts and such diffuse material represents the chief navigational hazard.[16] However, theNew Horizons spacecraft did not detect any smaller moons or rings and it passed through the Pluto system safely.
The unexpectedly complex moon system around Pluto may be the result of a collision between Pluto and another sizableKuiper belt object in the distant past.[17] Pluto's moons may havecoalesced from the debris from such an event, similar to theearly giant impact thought to have created the Earth'sMoon. The orbital resonances may have acted as "ruts" to gather material from the collision.[11]
Styx was originally estimated to have a diameter of between 10 and 25 km (6.2 and 15.5 mi).[18][19] These figures were inferred from theapparent magnitude of Styx and by using an estimatedalbedo of 0.35 and 0.04 for the lower and upper bounds, respectively.[8] Measurements made byNew Horizons found an elongated shape with dimensions measuring approximately 16 km × 9 km × 8 km (9.9 mi × 5.6 mi × 5.0 mi).[5] It is thought to have formed from the debris lofted by a collision, which would have led to losses of the morevolatile ices, such as those ofnitrogen andmethane, in the composition of the impactors. This process is expected to have created a body consisting mainly ofwater ice.[20]
Styx orbits the Pluto–Charonbarycenter at a distance of 42,656 km,[4] putting it between the orbits of Charon andNix. All ofPluto's moons appear to travel in orbits that are very nearly circular and coplanar, described by Styx's discoverer Mark Showalter as "neatly nested ... a bit likeRussian dolls".[17]
It is in an 11:6orbital resonance withHydra, and an 11:9 resonance with Nix (the ratios represent numbers of orbits completed per unit time; the period ratios are the inverses).[4][21] As a result of this "Laplace-like" 3-body resonance, it has conjunctions with Nix and Hydra in a 2:5 ratio.
Its orbital period of 20.16155 days[4] is about 5.0% from a 1:3 mean-motion resonance with the Charon–Pluto orbital period of 6.387 days. With the other moons Nix,Kerberos, and Hydra, it forms part of an unusual 1:3:4:5:6 (period ratio) sequence ofnear resonances.[11] In contrast to the orbit, Stygian rotation ischaotic; like the other small Plutonian moons, Styx is not tidally locked, and its rotation varies over short timescales (at a rate of about 3.239 days at the time of theNew Horizons flyby).[7]
Before any of Pluto's moons were discovered, authorEdmond Hamilton referred to three moons of Pluto in his 1940science fiction novelCalling Captain Future, naming themCharon,Styx, andCerberus.[22]
Upon discovery, Styx received theminor planet designation S/2012 (134340) 1 because it was the first satellite (S) discovered orbiting theminor planet 134340 Pluto in 2012. It is known informally as "P5", meaning the fifth Plutonian moon to be discovered. The designation S/2012 P 1 has sometimes been used, following the pre-2006 format where Pluto was still considered a planet and given its own single-letter abbreviation.[2]
The convention for naming Plutonian moons is to use names associated with the godPluto inclassical mythology. To decide on names for P4 and P5,Mark Showalter and theSETI Institute, on behalf of the discovery team, conducted a non-binding internet poll in 2013, in which the general public was invited to vote for their favorite names. The public could choose from a selection of Greek mythological names related to the god Pluto, or could propose their own names.[23] After the initial announcement,William Shatner, the actor who playsCaptain James T. Kirk in theStar Trek franchise, proposed the names Vulcan and Romulus, ostensibly referring to the fire godVulcan (a nephew of Pluto), and toRomulus the founder of Rome, but also alluding to the fictional planets ofVulcan andRomulus in theStar Trek universe.[24][25] The 'Romulus' suggestion was discounted, as there is alreadyan asteroid moon of that name,[26] but Vulcan won the poll after Shatner tweeted about it, withCerberus (the dog that guards Pluto's underworld) coming second andStyx (the goddess of the river of the same name in the underworld) coming third. The winning names were submitted to theInternational Astronomical Union.[25]However,Vulcan was unacceptable to the IAU because it was not the name of an underworld figure and had already been used for ahypothetical planet inside the orbit of Mercury, as well as having given its name to thevulcanoids.[24][27][28]
On 2 July 2013, the IAU announced that it had formally approved the namesStyx for P5 andKerberos for P4.[29][30]
^Showalter, M.R.; Weaver, H. A.; Stern, S. A.; Steffl, A. J.; Buie, M. W.; Merline, W. J.; Mutchler, M. J.; Soummer, R.; Throop, H. B. (2012). "New Satellite of (134340) Pluto: S/2012 (134340) 1".IAU Circular.9253: 1.Bibcode:2012IAUC.9253....1S.
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