From: Zardoz ® 04/11/2001 19:38:38 Subject: re: COSMOLOGY FAQ post id: 484075 Kuiper Belt and Comets Dr Karl S. Kruszelnicki speaks…….. Over the last decade we've had a few visitors from outer space - Halley's Comet in '85/86, Shoemaker-Levy that slammed into Jupiter in '94, and most recently, Hale-Bopp in '97. A comet is a ball of ice and dust. Halley's is about 15 kilometres long, while Hale-Bopp is about 40 kilometres across. Now here's one big mystery about comets. Halley's Comet swings by every 76 years or so. Every time it zips past the Sun, it flares up and throws away, as a beautiful tail, about one-ten thousandth of its weight. In other words, it should survive only ten thousand orbits - maybe 750,000 years. But according to our best theories of the Origin of the Solar System, it seems that comets were made when the Sun and the rest of the Solar System popped into existence, about 4,600 million years ago, ... so where has Halley's Comet been hiding out? It seems that there are two main homes for comets. The Universe is a big place, and distances are huge, so I'll call the distance between the Sun and Earth (about 150 million kilometres), one Astronomical Unit, or one AU. Jupiter (the first of the giant gas planets) is about 5 AU from the Sun, while Neptune (the last of the giant gas planets) is about 30 AU out. We think that one home for comets-in-waiting is the Oort Cloud, named after Jan Hendrik Oort. This enormous spherical cloud is a long way out - about 10,000 to 100,000 AU from the Sun, or about one fifth the way to the nearest star. We haven't actually seen comets in the Oort Cloud with our telescopes, but the mathematicians tell us it has to be the home of the comets that come into the Inner Solar System from any direction. These comets take a long time to fall in, and then they take a long time to come back and visit us again. We think that Hale-Bopp is one of these comets. The other home for comets is the Kuiper Belt, named after the American astronomer, Gerard P. Kuiper. Back in 1951, he proposed that the comets which come around in less than 200 years (like Halley's or Shoemaker-Levy) have to come from much closer in. These comets also tend to come in much closer to the plane of the planets, rather than from any old direction. He said that there should be a flattened belt or disc of comets, beginning just outside the orbit of Neptune (30 AU), and reaching out to about 1,000 AU. But finding a comet past Neptune, is like trying to see a 100-watt light bulb at 20 times the distance of the Moon! http://abc.net.au/science/k2/moments/kuiper.htm Kuiper Belt and Comets