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Comets Oort Cloud • The Oort cloud surrounds the Sun beyond the Kuiper Belt. – 10,000 to 100,000 AU – 1/5 the way to the nearest star – Spherical shell of icy objects Cometary Orbits • Comets that are pulled toward the sun enter new orbits. – Nudged by gravity – Most pass only once – never return – A few enter elliptical orbits Comet Nuclei • Comet Halley has a nucleus with a lot of carbon soot and grains of sand mixed with ice. • Comet Borrelly shows gases coming from the surface. Tails • • • • As a comet is heated near the sun the ice vaporizes. The vapors form a coma around the nucleus. Part of the gas streams away from the sun and forms a tail. Comet tails can be millions of kilometers long. Meteor Showers • Many times each year meteors will arrive in a group. – Up to hundreds per hour – Same direction from space • This is called a meteor shower. Gary W. Cronks Comets and Meteor Showers Radiant • In the 1800’s astronomers realized that meteors from a single shower came from a single point among the stars. • The point origin is called the radiant. – Named for the constellation in which it appears to originate Repeating Showers • Some meteor showers recur at regular intervals. – Leonids match comet Tempel-Tuttle • Astronomers recognized that this was consistent with a predictable orbit. Orbit of the Leonids Meteor Storms • As comets pass the sun they can leave a large amount of debris. • If the earth hits this debris it can generate 1000’s of meteors per hour. • This is called a meteor storm.