There are two small worlds orbiting among the realm of Saturn and Uranus, over a billion miles away, that are scientific wonderlands awaiting exploration. These small worlds are Chiron and Chariklo, just two of hundreds of known objects classified as Centaurs that orbit between Jupiter and Neptune.

Chiron, discovered by Charlie Kowal on November 1, 1977, was thought to be a distant asteroid over 100 miles in diameter. It was such an oddity that it was even thought to be the core of a giant comet, but scientists now know that Chiron and the Centaurs are a population of recently dislodged Kuiper Belt Objects (KBO) that are constantly resupplied by the Kuiper Belt. They must be resupplied because the orbits of the Centaurs cannot last longer than a few million years due to the gravitational tugs of the giant planets disturbing their orbits, eventually sending them on a collision course to impact a planet or ejecting them from the Solar System forever.

The Centaurs are easier to study than the Kuiper Belt because they are so much closer to Earth and therefore brighter. They range in size from only a few miles across to the largest, Chariklo and Chiron at 150 and 135 miles respectively. A few of the Centaurs, including Chiron, are active with fuzzy, comet-like atmospheres and discrete jets of gas and dust. Their surface compositions vary from water ice to organic compounds and display a wide range of color from neutral gray to extremely red. A few have moons and some even have rings.

Chiron was initially considered an asteroid and classified as a minor planet with a designation of 2060 Chiron, but something strange was discovered in 1989. Chiron was behaving like a comet and is now also known by the cometary designation of 95P/Chiron. A series of stellar occultation events through the 2010s and early 2020s revealed that Chiron has rings, one of only four minor planets and the only known comet to have rings. Chiron’s orbit is highly eccentric with perihelion just inside the orbit of Saturn and aphelion just outside the orbit of Uranus. There is no danger of it colliding with either planet, but a close encounter with Saturn of about 18 million miles around May 720 CE decreased the size and period of Chiron’s orbit slightly. Analysis of Chiron’s light curves yielded an accurate rotation of 5.9 hours with only a slight brightness variation which indicated it is roundish. The diameter of Chiron was more challenging to determine because of its cometary activity and fuzzy coma, but was determined to be about 135 miles, very large for a comet, thus adding to its oddity. In April 1989 Chiron developed a cometary coma and in 1993 a tail was detected. Chiron was different from other comets in that water could not be the major component of the coma since it is too far from the Sun for water ice to sublimate. Carbon monoxide and cyanide were discovered in Chiron’s spectrum and the James Webb Space Telescope has discovered carbon dioxide and methane. The composition of Chiron is similar to C-type carbonaceous asteroids which are volatile-rich and contain a large amount of carbon, in addition to rocks and minerals like Halley’s Comet.

Chiron has rings. But it is a comet! Beyond all imagination rings were discovered around Chiron on November 7, 1993, during an occultation with a star and was further verified on March 9, 1994, and November 29, 2011. They were initially thought to be jets of cometary activity but were sharply defined and uniform on both sides of Chiron. The rings are water ice, thin and evolving. Chiron had less ring material during a stellar occultation on November 28, 2018, than in 2011, but developed a partial third ring that was observed during another stellar occultation on December 15, 2022. The cometary outgassing may be strong and productive enough to replenish the rings. It is possible that Chiron might have one or more tiny moons that could shepherd the rings.

Chariklo was discovered by James V. Scotti on February 15, 1997, and is the largest known Centaur at 150 miles in diameter, although suspected to be shaped like a potato. It is in a more stable orbit between Saturn and Uranus than Chiron with less chance of its orbit being disrupted since it is in resonance with Uranus. Chariklo is very dim as seen from Earth shining most of the time around magnitude +18, and Chiron at +15 making them challenging to observe and study. A stellar occultation in 2013 revealed that Chariklo has two rings that are nearly threads, the inner one about 4 miles thick and the outer barely 2 miles, separated by a gap of about 6 miles. They are composed of water ice. Ring systems around minor planets and comets were unexpected because it was thought that rings could only be stable around much more massive worlds. Chariklo’s rings should disperse over a period of a few million years, so they are either very young or contained by shepherd moons that are yet to be discovered. Chariklo’s rapid rotation of 7 hours and its potato shape can clear material in an equatorial disc through resonance and maintain the rings. There is no detected cometary activity to replenish them. The James Webb Space Telescope was used to further define the rings using light curves to refine the thickness, the sizes and color of the ring particles, and if there are fainter rings or shepherd moons. The rings are probably composed of small particles of water ice mixed with dark material debris from a possible collision with another icy body in the past. Chariklo is too small and too far away for Webb to image the gap between rings and their separation from the main body, so stellar occultations remain the only tool to characterize the rings. The light spectra from Webb detected crystalline ice for the first time. This means that Chariklo experiences continuous micron collisions that either expose pristine material or triggers crystalline processes. The James Webb Space Telescope is proving to be a powerful tool for the study of small objects in the outer Solar System.

The Centaurs are a crucial part of understanding the Solar System as they are from the Kuiper Belt, some of the original building blocks. They are so scientifically important that missions have been proposed to explore them, and Chiron is the main target. Chiron possesses all three of the most interesting characteristics of the known Centaurs: an atmosphere (coma), surface activity (cometary jets), and rings, making for an exciting trifecta of exploration and discovery. The most logical mission is a multiple flyby of three or more Centaurs on the way to Chiron, sampling the diversity of the Centaur population in color, composition, size, activity, and ring systems. The spacecraft would include color and panchromatic visible imaging cameras that would carry out geological mapping, geophysics, satellite and ring studies. An infrared composition mapping spectrometer would determine surface composition while an ultraviolet mapping spectrometer would measure the composition of the atmosphere. A radio science package would study the density and thermal properties of the Centaurs. An impactor could also slam into one of the Centaurs with the resulting crater and ejecta being studied. Launch opportunities for the Centaurs open as soon as 2026 and last for several years, but given the current state of the budget, exploration may be decades away, although doable with the potential of inspiring a whole new generation of scientists, engineers, and explorers.