Comets are basically “snowy dirtballs” or “dirty snowballs”—collections of rocky material, dust, and frozen water, methane, and ammonia that move through the solar system in long, highly elliptical orbits around the Sun.
When they are far away from the Sun, comets are simple, solid bodies; but when they get closer to the Sun, they warm up, causing the ice in the comets’ outer surface to vaporize. This creates a cloudy “coma” that forms around the solid part of the comet, called the “nucleus.”
The loosened comet vapor forms long “tails” that can grow to millions of miles in length.
The English astronomer Edmund Halley (1656–1742) calculated the paths travelled by 24 comets recorded by astronomers over the years. Among these, he found that three—one visible in 1531, one in 1607, and one that Halley himself had observed in 1682—had nearly identical flight paths across the sky. This discovery led him to the conclusion that comets follow in an orbit around the sun, and thus can reappear periodically.
In 1695 Halley had predicted that a comet which has been seen thrice would return 76 years after its last sighting, in the year 1758. Unfortunately, Halley died before he could see that he was, indeed, correct. The comet was named in his honor, and to this day Halley ’s Comet remains the best-known comet in the world. It last passed by Earth in 1986, and will return again in 2062.
Origin of Comets
Most of the comets that orbit the Sun originate in the Kuiper Belt or the Oort Cloud, two major zones in our solar system beyond the orbit of Neptune.
“Shortperiod comets” usually originate in the Kuiper Belt.
Some comets and comet-like objects, however, have even smaller orbits; they may have once come from the Kuiper Belt and Oort Cloud, but have had their orbital paths altered by gravitational interactions with Jupiter and the other planets.
Comet Shoemaker-Levy 9
The encounter between Comet Shoemaker-Levy 9 and the planet Jupiter was the first collision between solar system bodies ever directly observed by humans.
As the comet approached Jupiter in 1994, it broke up into a long chain of fragments. Astronomers observed with amazement in July 1994 as these fragments crashed, one by one, into the gas giant’s thick atmosphere.