Nix (moon)

• 1. Introduction

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  Nix is a tiny, tumbling moon of Pluto, so faint that it is about 5,000 times dimmer than the planet it orbits. When astronomers first suspected it might exist, they had no direct evidence. They had only a hunch: if Pluto and its large moon Charon formed from a catastrophic ancient collision, perhaps other fragments had survived, quietly circling in the dark.

  On the 15th of May 2005, Max Mutchler and Andrew Steffl, working with the Hubble Space Telescope, captured images that confirmed the hunch. What they found was not just a moon. It was a spinning, tumbling, jelly-bean-shaped world with a mysterious red scar on its surface and a rotation so chaotic it can occasionally flip its entire axis.

  This is the story of how Nix was found, named, and finally seen up close. And it raises questions that astronomers are still working to answer.

• 2. The Discovery

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  Max Mutchler and Andrew Steffl made the find independently, both members of the Pluto Companion Search Team. Their hunt was driven by a specific suspicion held by the New Horizons mission team: that Pluto and Charon were not alone. The team used Hubble to scan the region around Pluto, and because Nix is so faint relative to Pluto, they needed long-exposure images to pull it out of the darkness.

  The critical images were captured on the 15th of May 2005 and again on the 18th of May 2005. But the discovery was not announced immediately. Astronomers first checked archival Hubble images of Pluto from 2002, a process called precovery, to confirm that the object had been there all along. Once confirmed, the announcement came on the 31st of October 2005.

  At that point, the two new moons had only provisional designations. Nix was called S/2005 P 2, while Hydra was S/2005 P 1. The discovery team simply called them P2 and P1. The naming would come later, and it would carry a hidden tribute.

• 3. A Name With Hidden Meaning

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  On the 21st of June 2006, the International Astronomical Union announced the official names of the two new moons in IAU Circular 8723. Nix takes its name from Nyx, the Greek goddess of darkness and night, and the mother of Charon, the ferryman of the underworld in Greek mythology. The familial connection to Charon, already the name of Pluto's largest moon, made Nyx a natural choice.

  But there was a complication. An asteroid already carried the classical spelling: 3908 Nyx. To avoid confusion in the astronomical catalogue, the IAU shifted to the Coptic spelling, Nix. The adjectival form of the name became Nictian.

  The naming also concealed an intentional tribute. The initial letters of the two new moons, N and H for Nix and Hydra, honor the New Horizons spacecraft sent to explore the Pluto system. This was a deliberate echo of an older tradition: the first two letters of Pluto's own name honor Percival Lowell, the astronomer who championed the search for a ninth planet. Features on Nix itself follow a related rule: they must be named after deities of the night drawn from literature, mythology, and history.

• 4. A Chaotic Spin

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  At the time of the New Horizons flyby in July 2015, Nix was rotating with a period of 43.9 hours. It was spinning retrograde, meaning backwards relative to its orbit around Pluto, with an axial tilt of 132 degrees. That already sounds unusual. What makes Nix stranger is that these numbers are not stable.

  Nix tumbles chaotically, as do all of Pluto's smaller moons. Its axial tilt and rotation period shift significantly over short timescales, and the moon can occasionally flip its entire rotational axis. Two forces drive this instability. First, the gravitational pull of Pluto and Charon varies as they orbit their shared center of mass, a point called the barycenter, and that varying pull tugs at Nix in irregular ways. Second, Nix's elongated shape amplifies the problem. An elongated body experiences torques that a sphere would not, and those torques feed the chaotic tumbling.

  Between the discovery of Nix in 2005 and the New Horizons flyby in 2015, the rotation rate of Nix had already increased by 10 percent. That measurable change over a single decade hints at just how dynamic the environment around Pluto's barycenter truly is.

• 5. The Red Scar

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  Early measurements of Nix's surface produced contradictory results. Some studies found the surface reddish in color. Others found it spectrally neutral, similar to Pluto's other small moons, with water ice as the dominant surface material. Researchers also noted that Nix appeared to vary in brightness and albedo, its reflectivity, and attributed those fluctuations to patches of different reflectivity across the surface.

  The New Horizons images resolved the contradiction, at least partially. They revealed a large reddish region approximately 18 km across on an otherwise pale surface. That single feature could account for both the reddish readings and the neutral readings, depending on which part of Nix was facing the instruments at any given time.

  The leading explanation is that the red region is an impact crater. A collision would have punched through the water ice layer and thrown reddish material from beneath it onto the surface. Astronomers also consider the possibility that trace amounts of frozen methane on Nix's surface react with ultraviolet radiation from the Sun to produce tholins, a class of reddish organic compounds. Crater counting data from New Horizons puts the age of Nix's surface at a minimum of four billion years, so whatever left that mark has been sitting there for a very long time.

• 6. Named Craters

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  Nix carries two officially named craters, both approved on the 2nd of September 2021. The largest is Gleti, named after the Fon goddess of the moon. Gleti measures 13 km in diameter and is surrounded by a dark red ejecta blanket, a ring of material thrown outward by the ancient impact. The second named crater, Metztli, is roughly 10 km in diameter and takes its name from Metztli, the Aztec goddess of the moon.

  Both names follow the naming convention for Nix's features: deities of the night and moon from world mythology. The dark red ejecta around Gleti is one of the most visually striking details New Horizons returned from the entire Pluto system, a small circle of alien color on an otherwise pale and ancient surface.

• 7. Orbit and Origins

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  Nix orbits the Pluto-Charon barycenter at a distance of 48,694 km, falling between the orbits of Styx and Kerberos. Its orbit, like those of all Pluto's moons, is nearly circular and coplanar with Charon's equatorial plane. That regularity is thought to be a product of tidal evolution: Nix may originally have followed a more eccentric path, and over time Charon's gravitational influence smoothed it out by drawing away the orbital eccentricity.

  Nix's orbital period sits close to a 1:4 resonance with Charon, meaning Nix completes roughly one orbit for every four completed by Charon. The timing is off by 2.8 percent, so there is no active resonance at present. One hypothesis holds that the resonance was once exact, established before Charon migrated outward after the moons formed, and that a periodic 9 percent fluctuation in the Pluto-Charon gravitational field has preserved the near-resonance since.

  The origin of Nix itself points back to a collision. Scientists believe Pluto's smaller moons coalesced from debris thrown out when a large Kuiper Belt object struck Pluto, the same general mechanism thought to have produced Earth's own Moon. The scenario fits the coplanar, circular orbits well. What it does not easily explain is Nix's high reflectivity, a puzzle the collision hypothesis has not yet solved.

• 8. New Horizons Up Close

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  New Horizons flew through the Pluto system on the 14th of July 2015, and of Pluto's smaller moons, only Nix and Hydra were imaged at resolutions sharp enough to show surface details. Before the flyby, measurements from New Horizons' Long Range Reconnaissance Imager had placed Nix's diameter at around 35 km. The close-approach images revised that significantly.

  The first detailed pictures, taken from a distance of about 231,000 km, were received from the spacecraft on the 18th of July 2015 and released publicly on the 21st of July 2015. At a resolution of 3 km per pixel, Nix's outline was rounded and irregular enough that mission scientists compared it to a jelly bean. Enhanced color images from the Ralph MVIC instrument confirmed the reddish region and yielded a refined measurement of Nix's approximate dimensions: 42 by 36 km.

  The full measured shape of Nix, derived from those images, gives dimensions of 49.8 by 33.2 by 31.1 km, with its longest axis slightly under 50 km. That elongated, non-spherical form is itself part of why Nix tumbles so unpredictably: it is shaped exactly like something that gravity finds difficult to hold steady.