If you've spent much time looking up at the night sky, you've probably seen some spectacular meteors and meteor showers. One of the most amazing things about these displays is that the majority of the space dust that causes visible meteors is tiny -- between the size of a grain of sand and the size of a small pebble.
Discussing meteor activity can be tricky because the terminology is confusing. The term meteor actually refers to the streak of light caused by a piece of space debris burning up in the atmosphere. The pieces of debris are called meteoroids, and remnants of the debris that reach Earth's surface (or another planet's) are called meteorites.
Meteoroids have a pretty big size range. They include any space debris bigger than a molecule and smaller than about 330 feet (100 meters) -- space debris bigger than this is considered an asteroid. But most of the debris the Earth comes in contact with is "dust" shed by comets traveling through the solar system. This dust tends to be made up of small particles.
So how can we see a meteor caused by such a small bit of matter? It turns out that what these meteoroids lack in mass they make up for in speed, and this is what causes the flash of light in the sky. Meteoroids enter the atmosphere at extremely high speeds -- 7 to 45 miles per second (11 to 72 kilometers per second). They can travel at this rate very easily in the vacuum of space because there's nothing to stop them. Earth's atmosphere, on the other hand, is full of matter, which creates a great deal of friction on a traveling object. This friction generates enough heat (up to 3,000 degrees Fahrenheit, or 1,649 degrees Celsius) to raise the meteoroid's surface to its boiling point, so the meteoroid is vaporized, layer by layer.
The friction breaks the molecules of both the meteoroid material and the atmosphere into glowing ionized particles, which then recombine, releasing light energy to form a bright "tail." A meteor tail caused by a grain-sized meteoroid is a few feet wide (about a meter) but, because of the high speed of the debris, may be many miles long.
So how big does a meteoroid have to be to make it to the surface of the Earth? Surprisingly, most of the meteoroids that reach the ground are especially small -- from microscopic debris to dust-particle-size pieces. They don't get vaporized because they are light enough that they slow down very easily. Moving about 1 inch (2.5 centimeters) per second through the atmosphere, they don't experience the intense friction that larger meteoroids do. In this sense, most all meteoroids that enter the atmosphere make it to the ground, in the form of microscopic dust.
As for meteoroids big enough to form visible meteors, estimates for the minimum size vary. This is because there are factors other than size involved. Most notably, a meteoroid's entry speed affects its chances of reaching the surface, because it determines the amount of friction the meteoroid experiences. Typically, though, a meteoroid would have to be about the size of a marble for a portion of it to reach the Earth's surface. Smaller particles burn up in the atmosphere about 50 to 75 miles (80 to 120 kilometers) above the Earth.
The meteorites a person is likely to find on the ground probably came from significantly larger meteoroids -- pieces of debris at least the size of a basketball, typically, since larger meteoroids usually break up into smaller chunks as they travel through the atmosphere.
You can actually find and collect tiny meteorites that have made it through the Earth's atmosphere with a simple experiment -- Put a pan on your back porch or deck to catch them!
To learn more about objects in space and how they work, look over the links on the next page.