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There are three causes of meteors or "shooting stars." However, there are only two causes of meteorites, the remains of meteors found on Earth.

Why the difference? Simple. Space debris of human origin - bits and pieces of rocket stages, tools dropped by spacewalking astronauts, decommissioned satellites and so on - occasionally fall back into the Earth's atmosphere. In doing so, they become superheated due to the friction of air rushing past at tens of thousands of miles per hour, and glow brightly as they streak across the sky. But if such debris survives the fall and is found, no one will mistake a screwdriver for a meteorite!

Naturally occurring meteors come from comets, asteroids, and other planets or moons. Planets or moons? Yes, I really said that. Numerous meteors found on Earth have been identified as having come from Mars! They were ejected into space by an asteroid or comet impacting Mars, floated around, and eventually wound up falling onto Earth. So there are actually bits of at least one other planet littering ours.

It's possible that the same thing has happened to the Moon, but since the Moon itself formed from rock thrown into space by Earth colliding with something in the early days of the Solar System, it would be difficult to distinguish rock flung out of the Earth to the Moon, then knocked out of the Moon and plunged back to Earth, from rock that had been part of Earth all along.

People more often think of comets, and to a lesser extent asteroids, as sources of meteors and meteorites. Comets and asteroids are very similar, distinguished only by the fact that most comets contain water or other volatile materials that escape in a "tail" when heated by the Sun, and most asteroids don't. Of course, recent research by astronomers Henry Hsieh and David Jewitt is showing that even in the asteroid belt, there are objects containing water, so the distinction asteroids and comets is fading.

If two comets or asteroids collide, pieces are likely to be knocked loose into space, just as they would be in a collision with a planet or moon. This can be the cause of stray meteors and meteorites, just as with other collisions.

Our annual Meteor showers, though, have a very specific cause. When comets approach the sun, the volatile compounds in them liquefy or vaporize, and stream out in the form of a "tail" pointing away from the sun. Bits and pieces of the comet literally break off during this process, leaving behind a trail.

If that trail crosses a point in the Earth's orbit, on the night each year where Earth reaches that point in its orbit, those bits and pieces - some only the size of grains of sand strike - the atmosphere at tens of thousands of miles per hour, and burn white-hot due to friction as they fall.

So to summarize, meteors can be caused by impacts (either between asteroids or comets, or between asteroids or comets and planets or moons), by comets leaving trails of debris that cross the Earth's orbit (the cause of meteor showers), or by "space junk." Naturally occurring meteorites, though, can only come from the first two causes.

Learn more about this author, Dan Birchall.
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