Despite the fact that comets are probably the most numerous astronomical bodies in the solar system aside from small meteor fragments and the asteroids (小行星), they are largely a mystery. Scientists don’t know exactly what comets are or where they come from. Educated guesses are the best we have in hand.
Considering the role of comets in lore, legend, and the memory of man, it is remarkable that we still know so little, relatively, about them. The most famous comet of all, Halley’s Comet (named for the man who predicted its return), was first sighted by the Chinese in 240 B.C., and it has returned to terrify the people of the world on a regular basis ever since then (last scheduled return: 1986). The ancients considered it an object of ill omen. By mysterious coincidence, the arrival of Halley’s Comet coincided with such events as the battle of Hastings in 1066, the Jewish revolt of 66 A.D., and the last battle of Attila the Hun against the Romans. Nor is it the only comet to fill man with awe, but merely the most famous in a rich aristocracy of blood-freezers.
Comets are even more fascinating to amateur astronomers than to professionals, because this is one area where amateurs can (and do) make major discoveries. Comet Ikeya Seki, one of the brightest comets to appear in the last century was discovered in 1965 by a pair of Japanese amateurs, Ikeya and Seki. The person who discovers a new comet gets his (or her) name put on it. And amateurs have a head start in the race to discover new comets; the shorter focal lengths on their smaller telescopes give them a positive advantage over the huge telescope such as Mount Wilson which is built to scan for galaxies, not comparatively of short distances.
Most scientists tend to agree with the astronomer Fred L. Whipple that a comet is really a large mushy snowball of frozen ices and gases (ammonia, methane, possibly carbon dioxide) with a few bits of solid particles stuck inside. But no one is sure how comets are created in the first place.
Scientists believe that comets don’t exhibit their characteristic tail while they lurk far out in space away from the warmth of the sun but, rather, wander in the form of frozen lumps, like icebergs. This is the core of the comet. Only when the comet approaches the heat of the sun, does the ice begin to melt and stream away in the form of visible gases. The tails of the comet stream out behind for, literally, astronomical distances. Halley’s Comet had a tail of 94 million miles long when it visited here in 1910. The Great Comet of 1843 had a tail of 186 million miles long. Halley’s Comet is mentioned in Paragraph 2