WHAT ARE THE CENTAURS?

(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 February 2017, there are 97 known Centaurean bodies, counting Chiron.)

DEFINITION OF CENTAUR

What is the astronomical definition of a Centaurean body? First I went to an astronomical glossary, and found the following:

"CENTAUR
An ``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.

Currently (December 2008) there are three different definitions of Centaur.

1. The Minor Planet Center (MPC) defines centaurs as having a semi-major axis of less than 30.066 AU, the semi-major axis of Neptune.
2. NASA's Jet Propulsion Laboratories (JPL) defines centaurs as small bodies with orbits between Jupiter and Neptune (5.5 AU < a < 30.1 AU).
3. The Deep Ecliptic Survey (DES) defines centaurs using a dynamical classification scheme, based on the behavior of orbital integrations over 10 million years. The DES defines centaurs as nonresonant objects whose osculating perihelia are less than the osculating semimajor axis of Neptune at any time during the integration.