<human>:  What is the difference between an asteroid, comet and meteor?
<bot>: The main difference between an asteroid, comet and meteor is that asteroids are composed mainly of rock and metal, comets are icy bodies of rock and dust, and meteors are small rocks that are found in outer space. Asteroids and comets have both been observed orbiting the sun, while meteors usually burn up in the atmosphere before they reach the Earth's surface.