The visibility of meteors is a consequence of the high velocity of meteoroids in interplanetary space. Before entering the region of the Earth's gravitational influence, their velocities range from a few kilometres per second up to as high as 72 kilometres per second. As they approach the Earth, within a few Earth radii, they are accelerated to even higher velocities by the planet's gravitational field. As a consequence, the minimum velocity with which a meteoroid can enter the atmosphere is equal to the Earth's escape velocity of 11 kilometres per second. Even at this minimum velocity, the kinetic energy of a meteoroid would be 6x104 joules per gram of its mass. This can be compared with the energy of about 4x103 joules per gram produced by chemical explosives, such as TNT. As the meteoroid is slowed down by friction with atmospheric gas molecules, this kinetic energy is converted into heat. Even at the low atmospheric density at altitudes of 100 kilometres (6x10-10 gram per cubic centimetre compared with 10-3 gram per cubic centimetre at sea level), this heat is sufficient to vaporize and ionize the surface material of the meteoroid and dissociate and ionize the surrounding atmospheric gas as well. Electronic transitions effected by this excitation of atmospheric and meteoroidal atoms produce a luminous region, which travels with the meteoroid and greatly exceeds its dimensions. At deeper levels in the atmosphere, a shock wave may be produced in the air ahead of the meteoroid. This shock wave interacts with the solid meteoroid and its vapour in a complex way. About 0.1 to 1 percent of the original kinetic energy of the meteoroid is transformed into visible light.

This great release of energy destroys meteoroids of small mass--particularly those with relatively high velocities--very quickly. This destruction is the result both of ablation (the loss of mass from the surface of the meteoroid by vaporization or as molten droplets) and of fragmentation caused by aerodynamic pressure that exceeds the crushing strength of the meteoroid. For these reasons, numerous meteors end their observed flight at altitudes above 80 kilometres, and penetration to as low as 50 kilometres is unusual. Nevertheless, some meteoroids survive to much lower altitudes owing to a combination of relatively low entry velocity (< 25 kilometres per second), large mass (>100 grams), and fairly high crushing strength (>107 dynes per square centimetre). Those that are recoverable as meteorites lose their kinetic energy before the meteoroid is completely destroyed. They are effectively stopped by the atmosphere at altitudes of 5 to 25 kilometres. Following this atmospheric braking, they begin to cool, their luminosity fades, and they fall to the Earth at low terminal velocities of 100 to 200 metres per second. This "dark flight" of the meteoroid may be several minutes in duration, in contrast to the few seconds of visible flight.

The passage of meteoroids through the atmosphere produces atmospheric shock waves that penetrate to the ground. The penetration of a meteoroid in the kilogram range to altitudes of about 40 kilometres can thereby produce sounds on the ground similar to sonic booms or thunder. These sound waves can be intense enough to become coupled to the ground and recorded by seismometers.

The effect of the final impact with the ground of meteorites in the kilogram mass range could be considered an anticlimax. The fall can go unnoticed by those near the impact site, the impact being signaled only by a whistling sound and a thud. For this reason, many meteorites are recovered only because at least one fragment of the meteoroid strikes a house, drawing the attention of the residents to an unusual event.

Excerpt from the Encyclopedia Britannica without permission.