What's the Difference Between an Asteroid and a Comet?

In the spring of 1996, Comet Hyakutake inspired Arizona photographers Rick Scott and Joe Orman to take this picture showing faint stars near the constellation Ursa Minor (the Little Dipper) shining through the comet's long, graceful tail. Comet tails generally point away from the sun, since they are blown by solar wind View full size image

An asteroid is a body of rock smaller than 600 miles (1,000 kilometers) in diameter — usually composed of carbon or metals — that orbits the sun. Most of our solar system’s asteroids live in the asteroid belt between Mars and Jupiter. Although there are millions of asteroids in the belt, and several hundred with a diameter greater than 60 miles (100 kilometers), the mass of all of them clumped together would be less than five percent of our moon.

Comets, such as Comet Harley 2 , are balls of dirt and ice that are formed in the Kuiper Belt or the Oort Cloud. Comets also orbit the sun, but have much larger orbits than asteroids, which are generally more elliptical. As comets near the sun, solar energy begins to vaporize the ice, which creates their signature tail.

There are also substantial differences between meteoroids, meteors, and meteorites. A meteoroid, according to the International Astronomical Union, is a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom. In most cases, a meteor is debris from an asteroid collision. A meteor is simply a meteoroid that has entered Earth’s atmosphere. (These typically burn up and create streaks through the sky, which earned meteors their “shooting star” nickname.) And a meteorite is a meteor that survives the atmosphere’s burn and lands on the planet.

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