dipper@utastro.UUCP (Debbie Byrd) (02/01/86)
Comets are thought to originate in a huge shell surrounding the solar system. More -- after this. February 1 The Origin of Comets Comets visit our sky -- then go away. That's because they orbit the sun, much as Earth does -- but their orbits are long and narrow. A comet will visit our neighborhood of space -- round the sun as Comet Halley is about to do -- then head back out to the outer solar system. The comet may take thousands of years to return -- or, like Halley, it may return in relatively few years. But where do comets come from -- and where do they go? Early theories said that comets originate in the space between stars -- or that they might be shot like bullets from the giant planets in our own solar system. But the best theory we have nowadays was proposed in 1950 by an astronomer named Jan Oort. He proposed that a great cloud of comets surrounds the solar system -- extending about halfway to the nearest stars. These comets would have formed from material left over from the birth of the planets and sun. In honor of the astronomer who proposed it, this theoretical cloud is called the Oort cloud. The comets are thought to move slowly within the cloud -- until something jolts them loose -- perhaps a passing star -- or a near collision with another comet in the cloud -- or something else. Then they leave the cloud and plunge in toward the sun. According to some estimates, there may be as many as one or two trillion comets in the cloud. But each one is so insubstantial that the total mass of the cloud may be only about eight times that of Earth. Script by Deborah Byrd. (c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin