No. 60: Nov-Dec 1988
Even though Chiron is large (about 200 kilometers in diameter) and a bit dark for a rocky asteroid, astronomers have been quite comfortable with calling it an asteroid. True, its orbit between Saturn and Uranus is unusual, but what else could it be but an asteroid? A comet, that's what! Recently, the brightness of Chiron has doubled, suggesting that it has expelled considerable gas and dust -- a characteristic of comets, according to mainstream thinking. Another peculiar aspect of the phenomenon is that Chiron is now located 12 A.U. from the sun (12 earth orbits out). Conventional wisdom has it that solar heating is too weak at that distance to vaporize cometary ices. However, other comets, such as Schwassmann-Wachmann 1, have displayed comas even farther away from the sun. Thus, we have two possible anomalies here:
(1) The existence of a huge cometlike asteoid in a peculiar orbit (2) A mechanism that expels gas and dust from comets at great distances from the sun.
The blurring of the distinctions between asteroids and comets is aggravated by the recognition that some other asteroids produce streams of particles that create meteor showers; that is, some asteroids are not merely associated with meteor streams, they actually create them, just as comets expel ice and rocky debris. Some bold astronomers now ask whether asteroids are all burntout comets.
(Kerr, Richard A.; "Another Asteroid Has Turned Comet," Science, 241:1161, 1988.)
Comment. Taking the matter a step further, could not the four inner, terrestrial planets also be the cores of huge burnt-out comets? See AVO8 in our catalog: The Moon and the Planets for observations of comet-like phenomena associated with Venus! This book is described here.