All About Meteorites

They are often called “shooting stars,” but meteorites have a less than stellar origin. Actually, they are space debris, mostly fragments of asteroids, but sometimes of lunar or Martian heritage. These fragments orbit the Sun just as the planets do, until something intercepts them and sends them hurtling toward another celestial body.

When these fragments fall to Earth, they are called meteorites. According to some estimates, approximately 19,000 meteorites weighing about 3.5 ounces each shower the Earth every day. However, only about 10 of these are recovered each year. In fact, over 99 percent of meteorite falls are not even witnessed.

Meteorites are classified according to three broad categories: stony, iron and stony-iron. Stony meteorites are the most common, comprising 92 percent of all meteorite falls. Iron meteorites account for 7 percent, and stony-iron for just 1 percent.

When a meteorite falls to Earth, it is sometimes accompanied by stunning visual displays and terrifying sounds. As a meteorite plummets toward the Earth, its outer surface is stripped away in a process called ablation. This produces a fireball made of dust and gas, which can be several times larger than the meteorite itself. Some witnesses have described fireballs as being about the size of the Sun or the Moon. The fireballs may also leave behind a trail of smoke that can linger long after the meteorite has landed.

The sounds accompanying meteorites vary greatly. As meteorites enter the lower atmosphere, they are slowed by the sound barrier, and sometimes result in “sonic booms.” Several of these booms may be heard, and are followed by a rumbling sound like that of distant thunder. Other sounds are described as hissing, sizzling or buzzing noises. Called electrophonic sounds, they are of low intensity, and are sometimes felt or sensed rather than heard directly. There are documented cases in which animals have exhibited panic shortly before the fall of a meteorite, and it is believed they were able to hear or sense these electrophonic sounds. It is thought these sounds may be low frequency radio transmissions, caused by electromagnetic radiation produced by the fireball.

But the most noteworthy thing about meteorites may be their ability to act as witnesses to the birth of the solar system. Most meteorites have been determined to be about 4.5 billion years old--the same age as our solar system. Unlike other celestial bodies, meteorites are untouched by the passage of time. While geologic processes have completely changed the landscape of the Earth, meteorites remain relatively preserved. Because their composition is virtually the same as when the solar system was young, meteorites give scientists a glimpse into how and of what materials the planets were made.

	Barbara Melville BellaOnline's Astronomy Editor