Styx is a small natural satellite of Pluto whose discovery was announced on 11 July 2012. It is the fifth confirmed satellite of Pluto and was found approximately one year after Kerberos, Pluto’s fourth confirmed satellite.
Styx was discovered by a team led by astronomer Mark R. Showalter, using fourteen sets of images taken between 26 June and 9 July 2012 by the Wide Field Camera 3 fitted to the Hubble Space Telescope.
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 New Horizons spaceprobe, currently en route to the Pluto system with flyby scheduled for 14 July 2015.
The unexpectedly complex moon system around Pluto may be the result of a collision between Pluto and another sizable Kuiper belt 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 Earth’s Moon. The orbital resonances may have acted as “ruts” to gather material from the smashup.
Styx is estimated to have a diameter of between 10 and 25 kilometers (6 and 16 mi). These figures are inferred from the apparent magnitude of Styx and by using an estimated albedo of 0.35 and 0.04 for the lower and upper bounds, respectively. Because of its small size, Styx is likely to be irregular in shape. 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 and methane, in the composition of the impactors. This process is expected to have created a body consisting mainly of water ice.
Styx orbits the Pluto–Charon barycenter at a distance of about 42,000 km, putting it between the orbits of Charon and Nix. All of Pluto’s moons appear to travel in orbits that are very nearly circular and coplanar, described by Showalter as “neatly nested … a bit like Russian dolls”. Its orbital period is estimated to be 20.2 days. This is a value about 5.4% 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 a remarkable 1:3:4:5:6 sequence of near resonances.
Upon discovery, Styx received the minor planet designation S/2012 (134340) 1 because it was the first satellite (S) discovered orbiting minor planet (134340) in 2012. It is known informally as “P5”, meaning the fifth Plutonian moon to be discovered.
To decide on names for P4 and P5, Mark Showalter and the SETI Institute, on behalf of the discovery team, conducted a non-binding internet poll in 2013. The public could choose from a selection of Greek mythological names related to the god Pluto, or could propose their own names. On 2 July 2013, the IAU announced that it had formally approved the names Styx for P5 and Kerberos for P4.