In the years following 1977, both the astronomical and astrological world had been going under the assumption that Chiron was a very unique body, totally one of a kind. So when Damocles was found Feb 18, 1991, and Pholus the next year (Jan 9, 1992), once again we had to rethink our conception of the solar system. And when 1992 QB1 (now designated 15760 Albion) was discovered (Aug. 30, 1992), out past Neptune --- the first of over 80 Transneptunians now known, the question arose: Could Chiron be an escapee from the Kuiper Belt?
(It was at this point that Pluto's status as a planet began to be seriously questioned by astronomers. Since 15760, more than 100 similar objects have been found past Pluto, many of them in a 3:2 resonance with Neptune. And Pluto also has such a 3:2 resonance with Neptune. Was Pluto simply the largest of these bodies?)
On April 26, 1993 this was further complicated by the discovery of yet another new body, 1993 HA2. The more I studied this one, the more I began to see connections with its influence and the mythological stories of the Centaur Nessus. Soon I began to think about it by that name. Then I discovered something truly remarkable: several other astrologers, in totally independent studies, were coming to the same conclusion --- 1993 HA2 had to be Nessus. Marianne Alexander, for one. In her book, "The Centaur Pholus", published in June 1996, speaking of the next Centaur to be named, she wrote, "I believe this one will be Nessus".
Meanwhile, four additional bodies were discovered in the same area of space that Chiron and Pholus travel in. (As of early 2020 there are 200 known Centaurean bodies, counting Chiron.)
What is the astronomical definition of a Centaurean body? It has changed over time, and even today there are differences of opinion among different astronomers.
"CENTAURAn ``outer planet crosser''. A minor body whose heliocentric orbit is between Jupiter and Neptune and typically crosses the orbits of one of the other outer giant planets (Saturn, Uranus, Neptune). The orbits of the Centaurs are dynamically unstable due to interactions with the giant planets, so they must be transition objects from a larger reservoir of small bodies to potentially active inner solar system objects. The Kuiper belt is believed to be this source reservoir."
Next, I wrote to Dr. Brian Marsden, asking if an official definition of "Centaur" existed. His reply (October 1999) was:
"I tend to think of a centaur as something with perihelion distance 5-6 AU and up to about 20 AU, and aphelion distance from 9-10 AU up to about 40 AU. There is no exact definition, and I think it premature to make one. Does one include objects like comets 39P/Oterma, and even 29P/Schwassmann- Wachmann 1, with their aphelia inside Saturn? And when does a centaur become a scattered-disk object, with perihelion near Neptune and aphelion way out further?"
Then on March 9, 2000, it was discovered that the list of Centaurs http://www.minorplanetcenter.net/iau/lists/Centaurs.html, which was overseen by Dr. Marsden, was now expanded to also include the Scattered-Disk Objects, such as 1999 TD10 and 1996 TL66. Immediately, I wrote to Dr. Marsden again, questioning this new development. He replied back the same day:
"For some years now, it has been evident that some of the objects we were attributing to the Transneptunian Belt (a.k.a. the Kuiper Belt, the Edgeworth-Kuiper Belt, etc.) really didn't belong in that category, which should apply to objects in rather stable orbits, i.e., they cannot pass anywhere near Neptune. So the genuine transneptunians include what some call the "classical" members (though I prefer to call them "cubewanos", after their prototype 1992 QB1) and Pluto and the other "plutinos" in 2:3 resonance with Neptune, as well as the smaller number of other resonators. In contrast to these stable objects, we have also had, beginning in 1977, Chiron and the other centaurs, with rather unstable orbits in the general Jupiter-Neptune region. When we discovered 1996 TL66, called by some a "scattered-disk object", I realized immediately that it was basically just an extension of the centaur theme, and as time has gone by we have filled in the gaps, with objects like 1999 TD10, which goes from 12 AU at perihelion to 190 AU at aphelion, but which until now was an classifiable, because we didn't know whether to call it a centaur or a member of the Transneptunian Belt. But now we have categories that have more meaning. Of course, what we expect is that plutinos (or maybe some of the other "geniuine" transneptunians, eventually pass just too close to Neptune and get transformed into a centaur or scattered-disk object. So there is a connection, even though this process takes a long time; and eventually they can become short-period comets with their aphelia near Jupiter."
So it appears that the "scattered-disk objects" are, as Dr. Marsden puts it, " an extension of the centaur theme ". The basic difference between them, and the Centaurs, is that the SDO's go out much farther than the Centaurs.
As of December 2008 there were three different definitions of Centaur.
Unusual Minor Planets
Centaur objects have perihelia beyond the orbit of Jupiter and semimajor axes inside the orbit of Neptune.
Trans-Jovian Planets crossing or approaching the orbit of a giant Planet but not in a stabilizing resonance (so called Centaurs) are named for centaurs.
NASA'S JPL has the following definition at Orbit Classification, Centaur
Objects with orbits between Jupiter and Neptune (5.5 au < a < 30.1 au).
Another more elaborate definition can found at Swineburne University of Technology site, COSMOS, The SAO Encylopedia of Astronomy
Centaurs are icy planetesimals located between Jupiter and Neptune. They cross the orbits of one or more of the giant planets in their journey around the Sun, and interactions with these outer planets cause the orbits of Centaurs to be inherently unstable. For this reason, they are thought to be transitory objects that have been scattered out of the Kuiper Belt (perhaps more specifically the scattered disk) and will soon be ejected from the Solar System, or are in the process of transitioning from Kuiper Belt objects (KBOs) to Jupiter-family comets.
One of the most interesting things about Centaur, aside from their orbits, is their range of colors. True, many of them are a dull grey, but Chiron is blue or blue-grey, there are some that are blue-green, and Pholus is rust-red.
What is the reason for the variation in colors? While it was originally thought that all the Centaurs were escapees from the Kuiper Belt, there is growing evidence that not all of them came from the same place. Some of them may actually have been ejected from the gas giant planets. As you may know, Neptune has a greenish color, Uranus blue, Saturn pale yellow and Jupiter a rusty red. More and more, it is looking like the color of many Centaurs point to their birth from one of the other planets in our solar system. And there is even some evidence that Saturn's moon Phoebe may once have been a Centaur that was captured by Saturn's gravitational pull.
Another reason for the color differences may be that the Centaurs are made up of different materials, or at least, their surface chemicals are different. Thus Pholus' red color may be due to a high percentage of organic material.
Here is a grid showing the range of Centaur colors, matching them up against the colors of Mars, and two of Saturn's Moons:
