While watching a meteor shower (like the Perseids last August or the Leonids coming up in November), perhaps you have asked yourself this question: Would I be more likely to be hit by a meteor, a meteoroid, or a meteorite? Why do scientists care about any of these?
Meteor, meteoroid, meteorite what is the difference? What distinguishes these three is primarily location.
Meteors are objects falling through the atmosphere. Most are small bits of rock or metal that burn up as friction with the air heats them, producing the streaks of light in the sky we call "falling stars" or “shooting stars”. Originally the term “meteor” was more general, referring to anything in the atmosphere, including clouds, wind, and rain. The science of “meteor”ology today, however, focuses more on weather and climate than on those objects that light up the night sky. There are a few cases of people being hit by meteors, but the chances are extremely small. Meteors are of special interest because they tell us about the way comets break up or lose material. Many meteor showers are caused by clouds of material left from comets. These particles follow the orbits of the comets around the Sun, and when the Earth passes through one of these clouds the sky lights up with meteors. The November Leonids shower is associated with comet Tempel-Tuttle.
Meteorites are those meteors that hit the ground. Although tons of meteorites accumulate on the Earth every day, most of the material is such tiny particles (“micrometeorites”) that it is lost among the dust, pollen, and other substances on the ground and in the oceans. Enough pebble-sized meteorites have been found, though, that there is a commercial market for them. Some meteorites are much larger yet, weighing tons. You would not be likely to be hit by a meteorite (since it has already hit the ground) unless someone threw one at you. The scientific interest in meteorites comes from the fact that these are materials from other parts of the solar system. Some meteorites are identified as having come from the Moon and others from Mars.
Meteoroids are objects outside the Earth's atmosphere that would become meteors if they entered the atmosphere. Astronauts are the only ones who have to worry about being hit by these.Like meteors and meteorites, meteoroids come in all sizes. Space scientists (including the astronauts) must be concerned about meteoroids (and their human-made equivalent, space debris) because they are moving so fast (10 km/sec or faster) that even tiny micrometeoroids are like bullets that can damage things we launch into space. Space suits, the Shuttle, and satellites have to build in protection against meteoroids and space debris. A research group at Johnson Space Center specializes in building and testing shields against these threats.
Further reading on the Web:
Definitions and history of the terms: http://www.bartleby.com/64/C004/035.html
Meteors and meteorites: http://seds.lpl.arizona.edu/nineplanets/nineplanets/meteorites.html
The Leonids Meteor shower: http://leonid.arc.nasa.gov/
People
and things being hit by meteors: http://www.space.com/scienceastronomy/astronomy/
perseids_shower_sidebar_000809.html
The threat and protection from space debris and meteoroids: http://hitf.jsc.nasa.gov/hitfpub/main/index.html
Dave Thompson is an astrophysicist who studies gamma rays in the Laboratory for High Energy Astrophysics. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2006. His particular scientific interest is gamma-ray pulsars.