Meteors and Meteorites
Upon entering Earth's upper atmosphere, a meteoroid begins to collide with an ever-increasing number of air molecules. These collisions will both slow the meteoroid down and heat its surface layers. Some of the meteoroid's lost energy is transformed into light; it is this light we observe as a meteor. As the meteoroid continues its journey through the atmosphere, its surface layers become so hot that vaporization begins. Continued heating causes more and more surface mass loss in a process known as ablation, and ultimately the meteoroid is completely vaporized.
The amount of surface heating that a meteoroid experiences is proportional to its surface area, and consequently very small meteoroids are not fully vaporized in the atmosphere. The size limit below which vaporization is no longer important is about 0.0004 in (0.01 mm). The smallest of meteoroids can safely pass through Earth's atmosphere without much physical alteration, and they may be collected as micrometeorites at Earth's surface. It is estimated that 22,000 tons (20,000 metric tons) of micrometeoritic material falls to Earth every year.
Visual meteors (shooting stars) are produced through the vaporization of millimeter-sized meteoroids. The speed with which meteoroids enter Earth's atmosphere varies from a minimum of 7 mi/sec (11 km/sec) to a maximum of 45 mi/sec (72 km/sec). The meteoroid ablation process typically begins at heights between 62-71 mi (100-115 km) above the Earth's surface, and the whole meteoroid is usually vaporized by the time it has descended to a height of 43.5 mi (70 km).
Astronomers have found that the visually observed meteors are derived from two meteoroid populations; a continuously active, but sporadic, background and a number of specific sources called meteoroid streams.