© by Zane B. Stein
(7968) Elst-Pizzaro = 133P/Elst-Pizzaro
Discovered in 1996 and reported as a comet, it was later linked by Dr. Marsden with a previously known (but
not named or numbered) asteroid, 1979 OW7.
(4015) Wilson-Harrington = 107P/Wilson-Harrington
In 1992, images of minor planet (4015) 1979 VA were found on plates taken in. Dr. Marsden realised that this object was comet Wilson-Harrington (1949 III), an object that had been reported in 1949, but that had been poorly followed and was lost.
What Is A Comet?
You probably know that a comet is like a dirty snowball of various frozen gases, such as methane or ammonia, and water. But it may interest you to know how a comet may be formed.
Imagine in some distant past, far out beyond Pluto, there is a tiny grain of dust. The temperatures out there are unbelievably cold, and because of that, any molecule that happens to bump into the grain of dust, such as a molecule of water, will stick. As millions of years pass, through one molecule or another hitting it, this grows, and a crystal lattice is formed. All sorts of molecules are trapped in the lattice, which is kind of like a hexagonal cage. Here, a bit of Methane. There, a bit of Hydrogen Cyanaide. And it grows larger, and larger.
The technical name for this type of water ice lattice, where a trapped molecule is imprisoned, yet not chemnically combined with the ice, is a CLATHRATE. A more familiar word, possibly, is MATRIX, which is a substance (such as a crystal) in which another substance (such as a fossil) is embedded.
The following is from "Comet" by Carl Sagan and Ann Druyan, copyright 1985, Random House, Inc.:
These molecules, on a comet, go off into space.....they change from ice to vapor without ever going through the liquid state. And this is the coma, and then the tail, of the comet. Because different chemicals have different melting points, different ices vaporize at different temperatures. Again, from "Comet":
However, Chiron is a bit unusual, compared to a typical comet.
Chiron's diameter is estimated at being between 148 to
208 km...as I mentioned earlier, over 50,000 times the volume of a typical comet. This means Chiron has a much greater gravity than a regular comet. What does this high gravity mean?
Dr. James L. Elliot, a planetary scientist at the Mass. Institute of Technology, says that Chiron "is the only known comet large enough for gravity to hold dust in bound orbits for periods ranging from tens of days to a year." In other words,
Chiron frequently has a dust atmosphere in the inner 1200 km of the coma. And this creates a strange set of circumstances: some of the gas never leaves Chiron....rather, when Chiron heads back away from the Sun, part of the gases refreeze and drop back down to the surface again! Yet, in spite of this, the coma's diameter has been measured to reach almost 2 million km!
And there are still more unusual things about this unique comet. A team of astronomers found pictures of Chiron on older photographic plates taken 1969-1972 (pre-discovery), and found that Chiron was brighter back then than it has been since. Yet at that time, Chiron was around its aphelion...it's FURTHEST from the Sun! This is NOT the time one would expect a comet, or any type of body, to be at its brightest.
Here is a bit more astronomical info on Chiron:
Chiron has a very elliptical orbit, 50.7 years. This takes it
from a perihelion (closest to the Sun) of 8.46 AU to an aphelion
(furthest from the Sun) of about 19 AU. This means several very interesting things.
First, it actually crosses Saturn's orbit and gets closer to us than Saturn can ever get, (Saturn at perihelion is 9.54 AU.)
It crossed that line, bringing it closer to us than Saturn,
around Jan. 1992! (Remember...that was when Pholus was found.)
And, it once again moved out further than Saturn in January
1999....about a month before Pluto once again moves out past
Second, it gets almost as far out as Uranus' mean distance (19.19126393 AU.)
Third....when Uranus is at perihelion, Chiron can actually be further from us than Uranus! On September 16, 1965, Chiron reached 18,284 AU and crossed Uranus' orbit. From then until March 13, 1975, (when Chiron again crossed Uranus' orbit at 18,469 AU), Chiron was further away from us than Uranus. (Thank you Robert for those date calculations.) Uranus was near its perihelion at that time.....the exact perihelion was in May 1966, at 18,254 AU.
Speaking of perihelions, Chiron, recently reached perihelion, on February 14, 1996.
Chiron and the other Centaurs may all have escaped from something
called the Kuiper Belt, a disk of objects orbiting beyond
Neptune. Pholus, especially, fits the nature of a large group
of such Kuiper Belt objects. According to a recent report, there are two classes of objects there, and one of these contains objects that are said to be "the reddest objects of the Solar System".
The report, entitled THE ASTONISHING REDNESS OF KUIPER-BELT OBJECTS, theorizes that these bodies could be an "indirect indication of extraterrestrial microbiology in the outer solar system." Possibly, there is some type of biological activity occurring on these objects even in the unbelievably cold Kuiper Belt!
Our Chiron is not the first solar system object to be named Chiron, by the way. According to Paul Schlyter, "In April 1861, Hermann Goldschmidt announced the discovery of a ninth moon of Saturn, which orbited the planet between Titan and Hyperion. He named that moon Chiron. However, the discovery was never confirmed -- no one ever saw this satellite "Chiron" again."
That "Chiron" may never be found, because it may not have truly been a satellite. And 37 years later, when they actually did
verify a ninth moon orbiting Saturn, it was called "Phoebe",