Monday, March 30, 2009

“Mommy, Where Do Comets Come From?”

The solar system has two cometary reservoirs, both named after the Dutch astronomers who discovered them. The nearer reservoir is called the Kuiper Belt. The short-period comets, those whose orbital period is less than 200 years, are believed to come from this region, which extends from the orbit of Pluto out to several 100 A.U. Comets from this region orbit peacefully unless some gravitational influence sends one into an eccentric orbit that takes it outside of the belt. Long-period comets, it is believed, originate in the Oort Cloud, a vast area (some 50,000–100,000 A.U. in radius) surrounding the solar system and consisting of comets orbiting in various planes. Oort comets are distributed in a spherical cloud instead of a disk.
The Oort Cloud is at such a great distance from the sun, that it extends about 1⁄3 of the distance to the nearest star. We don’t see the vast majority of these comets, because their orbital paths, though still bound by the sun’s gravitational pull, never approach the perimeter of the solar system. However, it is believed that the gravitational field of a passing star from time to time deflects a comet out of its orbit within the Oort Cloud, sending it on a path to the inner solar system, perhaps sealing our fate.
Once a short-period or long-period comet is kicked out of its Kuiper Belt or Oort Cloud home, it assumes its eccentric orbit indefinitely. That is, it can’t go home again. A comet will, each time it passes close to the sun, lose a bit of its mass as it is boiled away. A typical comet loses about 1⁄100 of its mass each time it passes the sun, and so, after 100 passages, will typically fragment and continue to orbit or coalesce with the sun as a collection of debris. As the earth passes through these orbital paths, we experience meteor showers.

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