Styx (moon)

• 1. Introduction

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  Styx is a small natural satellite of Pluto whose discovery was announced on the 11th of July 2012. It measures roughly 16 km across its longest dimension and sits so far from Earth that it registers about one hundred thousandth as bright as Pluto itself. Its formal designation, (134340) Pluto V, places it fifth in the order of discovery among Pluto's known moons. What makes Styx worth a closer look is not just its tiny size, but the cascade of questions its existence raised: about the dangerous neighbourhood New Horizons would fly through, about the violent origin of Pluto's whole moon system, and about what happens when a celebrity actor crashes a scientific naming process.

• 2. Finding the Faintest Moon

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  Astronomer Mark R. Showalter led the team that found Styx by combining fourteen sets of images taken between the 26th of June and the 9th of July 2012 with the Wide Field Camera 3 on the Hubble Space Telescope. At the time of discovery Styx was about half as bright as Kerberos, itself the dimmest previously known object in the Pluto system. The survey was not idle curiosity. It was preparation for the arrival of the New Horizons spacecraft, which was already en route and would fly past the Pluto system on the 14th of July 2015. Finding yet another small moon sharpened fears that additional, undetected bodies might lurk in the region. New Horizons was travelling at over 13 km/s, and a collision with even a tiny uncharted fragment or a diffuse ring could have ended the mission. Saturn's moon Pallene was cited as a cautionary example: small moons like it tend to be surrounded by tenuous rings fed by debris that their weak gravity cannot retain, and that kind of diffuse material posed the chief navigational threat. In the end, New Horizons passed through safely and detected no hidden moons or rings, but the worry was genuine enough that its discovery shaped the way mission controllers thought about the flyby.

• 3. A Violent Beginning

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  Styx is thought to be a relic of a catastrophic collision between Pluto and another sizable Kuiper belt object long ago. The scenario is broadly similar to the giant impact believed to have formed Earth's Moon: a massive strike threw debris into orbit, and that debris gradually coalesced into the moons we see today. The orbital resonances among Pluto's moons may have acted, in the source's phrasing, as "ruts" that gathered and held material from the collision. When New Horizons measured Styx's shape, it found an elongated body with dimensions of approximately 16 by 9 by 8 km. Scientists believe the violent impact would have driven off the more volatile ices, particularly nitrogen and methane, from the impactors' composition. What remained is expected to be a body composed mainly of water ice.

• 4. An Unusual Orbit

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  Styx orbits the shared gravitational centre of Pluto and Charon at a distance of 42,656 km, placing it between the orbits of Charon and Nix. All five of Pluto's known moons travel in orbits that are very nearly circular and coplanar. Mark Showalter described the arrangement as "neatly nested... a bit like Russian dolls." Styx's orbital period is 20.16155 days, which sits about 5.0% away from a 1:3 resonance with the Pluto-Charon orbital period of 6.387 days. Together with Nix, Kerberos, and Hydra, Styx forms part of an unusual 1:3:4:5:6 near-resonance sequence. Its rotation, however, is a different story. Unlike its tidy orbit, Styx's spin is chaotic. It is not tidally locked, and at the time of the New Horizons flyby its rotation period was running at about 3.239 days, a figure that varies over short timescales.

• 5. The Name That Almost Wasn't

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  Before any of Pluto's moons were discovered, the science fiction author Edmond Hamilton gave three fictional Plutonian moons names in his 1940 novel Calling Captain Future. The convention for real Plutonian moons calls for names tied to the god Pluto in classical mythology. To pick names for both P4 and P5, Mark Showalter and the SETI Institute ran a non-binding public internet poll in 2013. Voters could choose from a pre-selected list of Greek mythological names or suggest their own. William Shatner, the actor known for playing Captain James T. Kirk in Star Trek, entered the process by proposing Vulcan and Romulus. He framed these as references to the fire god Vulcan, a nephew of Pluto, and to Romulus the founder of Rome, though both also allude to fictional planets in the Star Trek universe. Romulus was set aside because an asteroid moon already carries that name. Vulcan won the public vote after Shatner promoted it on social media, with Cerberus coming in second and Styx in third. When the winning names went to the International Astronomical Union, Vulcan was rejected. The IAU ruled it was not an underworld figure and had already been applied to a hypothetical planet once proposed inside Mercury's orbit, a body that had also given its name to the vulcanoids. On the 2nd of July 2013, the IAU formally approved Styx for P5 and Kerberos for P4, names that had finished third and second in the public vote.