2060 Chiron

• 1. Introduction

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  2060 Chiron started its modern life as a small smudge on photographic plates taken at Palomar Observatory on the 18th of October 1977. Charles Kowal spotted it on the 1st of November that year. He had found something nobody had seen before in that part of the Solar System, and at the time, the press even floated the idea that it might be the tenth planet.

  It was not a planet. It was not quite an asteroid either. And as the decades passed and the instruments grew more powerful, Chiron revealed itself to be stranger and stranger. What exactly is it? Where does it come from? And why does a body barely 200 kilometers across have rings?

• 2. Discovery and Naming

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  Palomar Observatory gave Kowal the images he needed, but Chiron itself had been crossing the sky long before anyone noticed. Once researchers went looking through archival plates, they traced it back to April 1895, a full eighty-two years before the official discovery. That April image was taken just one month after Chiron had passed perihelion in March 1895, its closest point to the Sun. A perihelion passage also occurred in 1945, but no one caught it then. Searches at the time were not sensitive enough to pick up slow-moving objects at that distance.

  At the moment Kowal found it, Chiron was near aphelion, the far end of its orbit, making it the most distant known minor planet at the time. Its official designation came quickly: 2060 Chiron. The name was published by the Minor Planet Center on the 1st of April 1978, drawn from Greek mythology. Chiron was the son of the Titan Cronus and the nymph Philyra, half-human and half-horse, and regarded as the wisest and most just of all centaurs, serving as a teacher to the Greek heroes. The name also carried a practical suggestion: that future objects found in similar orbits should be named after other centaurs from mythology.

  A symbol was later devised for Chiron by Al H. Morrison. It is used mostly in astrology rather than astronomy, shaped like a key and also readable as an OK monogram standing for Object Kowal.

• 3. An Orbit Unlike Any Other

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  Chiron travels a highly eccentric path, with an eccentricity of 0.37. Its perihelion falls just inside Saturn's orbit, while its aphelion reaches just outside the perihelion of Uranus, though it never closes in on Uranus's average distance from the Sun. This places it squarely in a zone controlled by two of the Solar System's giants.

  The consequences of that position are significant. Centaurs, as a class, do not hold stable orbits. The gravitational influence of the giant planets will eventually pull Chiron away from its current path, either shifting it into a new orbit or ejecting it from the Solar System altogether, over a period of millions of years. The best current estimate is that Chiron likely originated in the Kuiper belt and will probably evolve into a short-period comet in roughly a million years.

  Chiron's closest recorded approach to Saturn in modern times occurred around May of the year 720, when it came within 30.5 million kilometers of the planet. Saturn's gravity during that passage shrank Chiron's semi-major axis from 14.55 AU to 13.7 AU. Chiron reached perihelion in 1996 and swung out to aphelion in May 2021.

• 4. A Comet in Disguise

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  In February 1988, while Chiron sat 12 AU from the Sun, it abruptly brightened by 75 percent. Asteroids do not do that. Comets do. Follow-up observations in April 1989 confirmed what astronomers had begun to suspect: Chiron had developed a cometary coma, a cloud of gas and dust surrounding its nucleus. A tail was detected in 1993.

  What drives Chiron's activity is not water. The body sits too far from the Sun for water ice to sublimate. Instead, in 1995, carbon monoxide was detected in very small amounts, and the derived production rate was calculated to be sufficient to explain the observed coma. Cyanide had also turned up in Chiron's spectrum in 1991. The James Webb Space Telescope later revealed a rich inventory: absorption bands of carbon dioxide, carbon monoxide, ethane, propane, and acetylene, along with water ice in its amorphous state. A key finding was fluorescent methane emissions, providing what the authors described as the first evidence of a gas coma rich in methane. Methane desorption was linked to a density phase transition of amorphous water ice at 61 Kelvin, a process previously studied in the laboratory.

  Chiron is officially classified as both a minor planet and a comet, carrying the cometary designation 95P/Chiron alongside 2060 Chiron. The term proto-comet has also been applied. At roughly 220 kilometers in diameter, it is unusually large for a comet nucleus. Its dual classification reflects a genuine ambiguity in how astronomers draw the line between the two categories of object. Chiron was the first member of what is now recognized as the family of Chiron-type comets, a grouping that also includes 39P/Oterma, 165P/LINEAR, 166P/NEAT, and 167P/CINEOS.

• 5. Rings Around a Comet

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  On the 7th of November 1993 and the 9th of March 1994, astronomers watching Chiron pass in front of background stars noticed unexpected dips in starlight that no one could immediately explain. A third such event on the 29th of November 2011 added weight to the puzzle. These signals had initially been attributed to jets from Chiron's comet-like activity. A re-analysis proposed something more surprising: Chiron has rings, proposed to sit at a radius of 324 units, sharply defined.

  Once the ring hypothesis was taken seriously, a lot of earlier puzzles fell into place. The long-term variation in Chiron's brightness, which had complicated size and albedo estimates for decades, could be largely explained by the changing viewing angle of the rings. The disappearance of infrared water-ice absorption bands in 2001 matched the moment when the rings were edge-on to observers on Earth.

  Further occultations on the 28th of November 2018 and the 15th of December 2022 showed that the rings are not static. In 2018, there was less material than in 2011, but signs of a partial third ring beginning to develop. By 2022, there was more material than in either earlier observation, and the third ring had become fully formed. J.L. Ortiz speculated that an outburst observed in 2021 may have fed material into orbit around Chiron, driving the third ring's growth. The ring structure is expected to continue cycling in this way.

  Chiron's rings share a striking resemblance to those of 10199 Chariklo. Until the 2018 observation, the width, separation, and optical depth of the two ring systems were nearly identical, suggesting a common structural origin. Both ring systems also sit within their respective Roche limits, the zone inside which tidal forces prevent material from clumping into a moon. Chiron's newly developed third ring may reach outside that boundary, depending on its density.

• 6. Size, Shape, and Spectrum

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  Pinning down Chiron's size has proved surprisingly difficult. In 1984, Larry Lebofsky estimated a diameter of around 180 kilometers. Estimates through the 1990s came in closer to 150 kilometers. The Spitzer Space Telescope in 2007 suggested a figure of 233 kilometers, and the Herschel Space Observatory weighed in at 218 kilometers in 2013. A 2023 study by Sickafoose returned to 196 kilometers. The uncertainty is not just a matter of measurement precision; Chiron's highly variable cometary activity makes it hard to determine its nucleus's true absolute magnitude, which in turn affects every size calculation.

  Four rotational light curves taken between 1989 and 1997 produced a well-defined rotation period of 5.918 hours. The brightness variation was small, between 0.05 and 0.09 magnitudes, suggesting the body is close to spheroidal rather than elongated or irregular.

  Chiron's visible and near-infrared spectrum is neutral, similar to C-type asteroids and the nucleus of Halley's Comet. Near-infrared observations show no water ice on its surface, though the James Webb Space Telescope found water ice in its amorphous form in the coma. Complex carbon-bearing molecules resulting from the combined irradiation of CH and CO groups were not detected, which researchers suggest may point to a physical or temporal separation between Chiron's methane and carbon dioxide reservoirs.

• 7. Missions Proposed

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  The Chiron Orbiter Mission was published in May 2010 as a proposal for NASA's New Frontiers or Flagship program. It outlined an orbiter mission to Chiron with a potential launch window stretching from 2023 at the earliest to 2025 at the latest, depending on budget and choice of propulsion.

  A separate proposal emerged under the Discovery Program, named Centaurus. If approved, it would have launched between 2026 and 2029 and conducted a flyby of 2060 Chiron and one other centaur in the 2030s. Neither mission has been selected as of the time of this documentary, but the evolving ring structure documented through the 2022 occultation gives any future mission a dynamic and changing target to study.