Beginner's Guide
Meteors, meteorites, meteoroids, asteroids... oh my!
A meteoroid is a bit of material larger than dust but smaller than an asteroid. Beech and Steel proposed a modified definiton of meteoroids to take into account the improved observational capabilities of modern telescopes. They propose that objects 10 meters and smaller should be called meteoroids and objects larger than 10 meters be called asteroids. On the lower end, they suggest that 100µm be the dividing line between meteoroids and dust.
A meteor, aka shooting or falling star, is the phenomenon seen in the atmosphere when a meteoroid arrives. The term, then, covers the flash of light seen, or the train of ionisation produced, and so on. Should the meteoroid survive the destructive passage through the atmosphere and hit the ground, it is called a meteorite.
But where do meteoroids come from? Asteroids and comets are the main sources.
A large number of asteroids are located in the Main Belt. Over time, asteroids have collided, breaking apart, sometimes recoalescing, but usually creating these smaller bits of material.
A comet is a ball of ice and dirt, orbiting the Sun (usually millions of miles from Earth). As the ices in the nucleus are heated and vaporized by the Sun, gas escapes, taking solid particles (dust, dirt) along with it. The particles and gas are pushed away from the Sun by the solar wind and radiation pressure, producing the comet's tail (which always points away from the Sun). Large particles (the size of a grain of sand or larger) take a long time to be pushed around, so they remain in nearly the same orbit as the comet for months or even years, forming a large "cloud" of material or meteoroid stream. (Imagine a jet contrail -- when the jet first flies by, the contrail is very obvious, but as time passes the contrail spreads out and becomes more diffuse.)
A Meteor Shower takes place when the Earth crosses one of the meteoroid streams. The duration and intensity of the shower is determined by the size and density of the cloud of particles. If the cloud is widely spread out (an "old" contrail), then the shower could be seen as a few meteors per hour for several days. On the other hand, if the cloud is very small and dense (a "new" contrail), then the shower will consist of thousands of meteors raining down in only a few minutes (which is classified as a Meteor Storm).
References:
- Beech, Martin and Steel, Duncan. On the Definition of the Term 'Meteoroid' Q.J.R. astr. Soc (1995) 36, 281-284.
- Steel, Duncan. Meteoroids Cambridge Conference Network, 25 Jan 2001.
- IAU FAQ about planets, dwarf planets, and small solar system bodies