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The main difference is that asteroids tend to be dense chunks of solid rock, iron and organic material, whereas comets are less dense and more loose lumps of ice, dirt, rock and organic matter - "dirty snowballs." Asteroids seem to be dead, inert chunks of solid rock, but the ice in comets means that when they get close to the Sun, some of that ice starts to melt and evaporate into space, making the comet grow a fuzzy, thin atmosphere called a coma, which ultimately streams out behind the comet (pushed out by the Solar Wind, a stream of charged particles flowing from the Sun into space) and grows into a tail.

Most asteroids are held in a belt between Mars and Jupiter, where they follow roughly circular, planet-like orbits. Comets, on the other hand, tend to come from much further out in the Solar System - either from the Kuiper Belt beyond Neptune, or the Oort Cloud much further out - and have very long, thin, highly eccentric and elliptical orbits.

Recently though scientists have found the distinction between asteroids and comets isn't all that clear cut. Objects called centaurs, which travel between the orbits of Saturn and Neptune, look like asteroids and have asteroid-like orbits, yet are made of significant amounts of ice and often get fuzzy when they get relatively close to the Sun. Many asteroids in the outer part of the main asteroid belt contain a lot of ice. And many asteroids we can point to seem to be ancient ex-comets whose ice has all melted and evaporated, leaving behind the dust and rock that has compacted into a single mass - an asteroid.

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14y ago

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