Comets

Comets • Comets are small bodies made out of dust and ices ("dirty snowballs"). • The term "comet" derives from the Greek aster kometes, which means "long-haired star"---a reference to the tail. • Mostly are on long elliptical orbits (perhaps parabolic in some cases) that take them from the outer reaches of the Solar System to the vicinity of the Sun. • If they come near the Sun they are heated and emit gases and dust that are swept by the Solar Wind into the characteristic tail that consequently always points away from the Sun. • Although a dozen or so comets pass through the inner Solar System each year, comets easily visible to the naked eye occur only every decade

Asteroids • Asteroids, sometimes called minor planets, are rocky and metallic objects too small to be considered planets. • They range in size from a diameter of about 1000 km, down to a few centimeters or less. • The name asteroids, meaning "star-like", derives from the fact that, compared with comets, they are star-like in appearance because since they are rocky they do not emit the gases and dust that give comets their fuzzy appearance.

• Asteroids on a collision course with Earth are called meteoroids. • If this meteoroid burns up because of frictional heating when it strikes our atmosphere, we term it a meteor (colloquially, a "shooting star"). • If the meteoroid doesn't burn up completely and strikes the Earth we call it a meteorite. • Asteroids represent material left over from the formation of the solar system. • it is more likely that they represent material in the solar system between the orbit of Jupiter and Mars that never coalesced into a planet.

Meteorite • Small pieces of space debris (usually parts of comets or asteroids) that are on a collision course with the Earth are called meteoroids. • When meteoroids enter the Earth's atmosphere they are called meteors. • Most meteors burn up in the atmosphere, but if they survive the frictional heating and strike the surface of the Earth they are called meteorites. • The Earth has been struck by many meteorites, • The Barringer Crater in Arizona is 1.2 kilometers across and 200 meters deep, and was formed about 49,000 years ago by the impact of a 50 meter nickel/iron meteorite travelling at a speed of 11 kilometers per second.