The Thing Called "Chiron"

Jay Bitterman

Like a snowball rolling downhill, some things just take off and get bigger as they go. That's sort of the case with the story about the object called Chiron, which I first came across when looking up some information about the astronomer Charles Kowal. (See the December, 1998, edition of NightTimes.) I mentioned that Kowal had discovered an object, Chiron, that might be either a comet or an asteroid. I was curious and decided to follow up with some library research. Here's what I found.

On October 18, 1977, Kowal was using the 48-inch Schmidt telescope at Mount Palomar when he came across a previously unknown object. Assuming that it was an asteroid, it was given the preliminary name "1977 UB". But it was a slowly moving object with an orbit that extended far beyond the orbits of known asteroids -- out between Saturn and Uranus. Further research showed that this same object had been recorded on old photographic plates going as far back as 1895. Its orbital period was determined to be 49 years, but unlike a comet, its orbit appeared to be very stable.

Brian Marsden of the Smithsonian Astrophysical Observatory said: "We really don't know what it is. It could be an asteroid or a comet or a small planet. We think it may be a lump of cometary material, but for now we really have to call it a minor planet until we can prove observationally that it is or isn't a comet."

Then, at a conference in 1978, Kowal suggested that the object he had found might really be a comet. It had been previously predicted that thousands of comets were orbiting the sun at about the same distance as 1977 UB. Kowal proposed that the object be called "Chiron", since Chiron is a mythical centaur, half man and half horse. Since then, such questionable objects are said to belong to the so-called Centaur family.

Jumping ahead to 1988, the object 1977 UB was finally observed with a coma and tail. It appeared as though Chiron really is a comet, but one with a diameter much larger than that of any other known comet. The official designation is now that of a comet -- "95/P Chiron". In February 1996, Chiron passed its perihelion (the nearest it comes to the sun) for the first time since it was discovered. There's a web site with info about Chiron:

Published in the February 1999 issue of the NightTimes