Styx is a small natural satellite of Pluto whose discovery was announced on 11 July 2012. It was discovered by use of the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...

. It is the second smallest moon of

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...

after Kerberos. It was imaged along with Pluto and Pluto's other moons by the ''

New Horizons ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a ...

'' spacecraft in July 2015, albeit poorly with only a single image of Styx obtained. Styx is the second-closest known satellite to Pluto, and the fifth discovered. It was discovered one year after Kerberos. Styx is approximately across its longest dimension, and its orbital period is 20.1 days.

Discovery and observations

Pluto moon P5 discovery with moons' orbits

Styx was discovered by a team led by astronomer

Mark R. Showalter Mark Robert Showalter (born December 5, 1957) is a Senior Research Scientist at the SETI Institute. He is the discoverer of six moons and three planetary rings. He is the Principal Investigator of NASA's Planetary Data System Rings Node, a co-inv ...

, using fourteen sets of images taken between 26 June and 9 July 2012 by the Wide Field Camera 3 fitted to the Hubble. 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. It was designated S/2012 (134340) 1, and informally referred to as P5. The survey work leading to the discovery of Styx was in preparation for the mission of the unmanned ''

'' 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 or

planetary ring A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known ...

as it traversed the system at a speed of over 13 km/s. Tiny moons, such as Saturn's moon Pallene, tend to be associated with tenuous rings or arcs, because their gravity is unable to hold on to material ejected by

meteoroid A meteoroid () is a small rocky or metallic body in outer space. Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as mi ...

impacts and such diffuse material represents the chief navigational hazard. However, the ''New Horizons'' spacecraft did not detect any smaller moons or rings and it passed through the Pluto system safely.

Origin

The unexpectedly complex moon system around Pluto may be the result of a collision between Pluto and another sizable

Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 tim ...

object in the distant past. Pluto's moons may have coalesced from the debris from such an event, similar to the early giant impact thought to have created the Earth's

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

. The orbital resonances may have acted as "ruts" to gather material from the collision.

Physical characteristics

Styx was originally estimated to have a diameter of between . These figures were inferred from the

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

of Styx and by using an estimated

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...

of 0.35 and 0.04 for the lower and upper bounds, respectively. After measurements made by ''New Horizons'', it turns out that Styx is (albeit not surprisingly) very irregularly shaped, measuring approximately . It is thought to have formed from the debris lofted by a collision, which would have led to losses of the more volatile ices, such as those of

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

and

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...

, in the composition of the impactors. This process is expected to have created a body consisting mainly of water ice.

Orbit

Styx orbits the Pluto–Charon

barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...

at a distance of 42,656 km, putting it between the orbits of Charon and

Nix Nix or NIX may refer to: Places * Nix, Alabama, an unincorporated community, United States * Nix, Texas, a ghost town in southwestern Lampasas County, Texas, United States * Nix (moon), a moon of Pluto People * Nix (surname), listing people wit ...

. All of Pluto'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 like Russian dolls". It is in an 11:6

orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...

with Hydra, and an 11:9 resonance with Nix (the ratios represent numbers of orbits completed per unit time; the period ratios are the inverses). 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 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 of near resonances. In contrast to the orbit, Stygian rotation is chaotic; 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 the ''New Horizons'' flyby).

Naming

Before any of Pluto's moons were discovered, author

Edmond Hamilton Edmond Moore Hamilton (October 21, 1904 – February 1, 1977) was an American writer of science fiction during the mid-twentieth century. Early life Born in Youngstown, Ohio, he was raised there and in nearby New Castle, Pennsylvania. So ...

referred to three moons of Pluto in his 1940

science fiction Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel uni ...

novel ''Calling Captain Future'', naming them , , and . Upon discovery, Styx received the

minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur, trans-Neptunian object and dwarf planet but not comet). Such designation always features a leading number (catalog ...

S/2012 (134340) 1 because it was the first satellite (S) discovered orbiting the

minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...

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. The convention for naming Plutonian moons is to use names associated with the god

classical mythology Classical mythology, Greco-Roman mythology, or Greek and Roman mythology is both the body of and the study of myths from the ancient Greeks and ancient Romans as they are used or transformed by cultural reception. Along with philosophy and poli ...

. To decide on names for P4 and P5,

Mark Showalter Mark Robert Showalter (born December 5, 1957) is a Senior Research Scientist at the SETI Institute. He is the discoverer of six moons and three planetary rings. He is the Principal Investigator of NASA's Planetary Data System Rings Node, a co-in ...

and the

SETI Institute The SETI Institute is a not-for-profit research organization incorporated in 1984 whose mission is to explore, understand, and explain the origin and nature of life in the universe, and to use this knowledge to inspire and guide present and futu ...

, 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. After the initial announcement,

William Shatner William Shatner (born March 22, 1931) is a Canadian actor. In a career spanning seven decades, he is best known for his portrayal of James T. Kirk in the ''Star Trek'' franchise, from his 1965 debut as the captain of the starship ''Enterpris ...

, the actor who plays Captain James T. Kirk in the ''

Star Trek ''Star Trek'' is an American science fiction media franchise created by Gene Roddenberry, which began with the eponymous 1960s television series and quickly became a worldwide pop-culture phenomenon. The franchise has expanded into vari ...

'' franchise, proposed the names Vulcan and Romulus, ostensibly referring to the fire god Vulcan (a nephew of Pluto), and to

Romulus Romulus () was the legendary founder and first king of Rome. Various traditions attribute the establishment of many of Rome's oldest legal, political, religious, and social institutions to Romulus and his contemporaries. Although many of these ...

the founder of Rome, but also alluding to the fictional planets of Vulcan and

in the ''Star Trek'' universe. The 'Romulus' suggestion was discounted, as there is already an asteroid moon of that name, but Vulcan won the poll after Shatner tweeted about it, with

Cerberus In Greek mythology, Cerberus (; grc-gre, Κέρβερος ''Kérberos'' ), often referred to as the hound of Hades, is a multi-headed dog that guards the gates of the Underworld to prevent the dead from leaving. He was the offspring of the ...

(the dog that guards Pluto's underworld) coming second and

Styx In Greek mythology, Styx (; grc, Στύξ ) is a river that forms the boundary between Earth (Gaia) and the Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the underworld on a great marsh, ...

(the goddess of the river of the same name in the underworld) coming third. The winning names were submitted to the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

. However, ''Vulcan'' was unacceptable to the IAU because it was not the name of an underworld figure and had already been used for a hypothetical planet inside the orbit of Mercury, as well as having given its name to the

vulcanoid The vulcanoids are a hypothetical population of asteroids that orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the hypothetical planet Vulcan, which was proposed on the basis of irregularit ...

s. On 2 July 2013, the IAU announced that it had formally approved the names ''

'' for P5 and ''Kerberos'' for P4.

References

External links

* Showalter, Mark
Hubble Press Release: Hubble Discovers a Fifth Moon Orbiting Pluto
11 July 2012. * ZME Scienc
New moon discovered around Pluto – the fifth
11 July 2012. {{DEFAULTSORT:S 2012 (134340) 1 Moons of Pluto Discoveries using the Hubble Space Telescope 20120626 Moons with a prograde orbit