About 500,000 meteoroids of millimeter-size and up enter the earth's atmosphere daily. Those large enough to reach the earth's surface intact are then called meteorites, and meteors are the glowing streaks that meteoroids leave as they pass through the atmosphere. Still more meteoroids are smaller than 1 mm; these are small enough to be slowed by the atmosphere without burning up and can reach the surface as micrometeorites. The total accumulation of meteoroids by the earth is about 22,000 tons per year or 60 tons per day.

Most meteoroids are vaporized not by friction but by ram pressure. The fast-moving meteoroid compresses the air in front of it to the point where the air becomes superheated and ionized. This heat then vaporizes the meteoroid. The lingering streak in the sky is mostly from excited molecules of air and vaporized meteoroid. Even if a meteoroid is large enough to avoid vaporization, it may still disintegrate as it encounters denser air, which can decelerate it sharply enough that it literally breaks apart or even explodes in midair (such as appears to have happened in Siberia in 1908).
Source(s):
http://www.enotes.com/earth-science/meteoroids-meteorites
http://articles.adsabs.harvard.edu//full/2005ESASP.590...33V/0000033.000.ht...
http://en.wikipedia.org/wiki/Meteor_shower

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