A meteor is a bright streak of light that appears briefly in the sky. Observers often call meteors shooting stars or falling stars because they look like stars falling from the sky. People sometimes call the brightest meteors fireballs. A meteor appears when a particle or chunk of metallic or stony matter called a meteoroid enters the earth's atmosphere from outer space. Air friction heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second.

Most meteoroids disintegrate before reaching the earth. But some leave a trail that lasts several minutes. Meteoroids that reach the earth are called meteorites.

Millions of meteors occur in the earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between about 40 and 75 miles (65 and 120 kilometers) above the earth. They disintegrate at altitudes of 30 to 60 miles (50 to 95 kilometers).

Meteoroids travel around the sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second). The earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the earth's atmosphere head-on, the combined speed may reach about 44 miles per second (71 kilometers per second).

Meteor showers

The earth meets a number of streams (trails) or swarms (clusters) of tiny meteoroids at certain times every year. At such times, the sky seems filled with a shower of sparks. Streams and swarms have orbits like those of comets and are believed to be fragments of comets.

The most brilliant meteor shower known took place on Nov. 12-13, 1833. It was one of the Leonid showers, which occur every November and seem to come from the direction of the constellation Leo.

Meteorites

There are three kinds of meteorites, stony, iron, and stony-iron. Stony meteorites consist of minerals rich in silicon and oxygen, with smaller amounts of iron, magnesium, and other elements. One group of stony meteorites, called chondrites, are pieces of the same material from which the planets formed. Another group of stony meteorites, the achondrites, were once part of a parent body, such as an asteroid, that was large enough to have melted and separated into an iron-rich core and a stony crust. Achondrites come from the outer crust; stony-iron meteorites, from the inner crust; and iron meteorites, from the metallic core. Iron meteorites consist mostly of iron and nickel. Stony-iron meteorites have nearly equal amounts of silicon-based stone and iron-nickel metal.

The size of meteorites varies greatly. Most of them are relatively small. The largest meteorite ever found weighs about 66 short tons (60 metric tons). It fell at Hoba West, a farm near Grootfontein, Namibia. However, much larger bodies, such as asteroids and comets, can also strike the earth and become meteorites.

Meteorites reach the earth's surface because they are the right size to travel through the atmosphere. If they are too small, they will disintegrate in the atmosphere. If they are too large, they may explode before reaching the earth's surface. One such object exploded about 6 miles (10 kilometers) above the Tunguska River in Siberia in 1908, leaving a 20-mile (32-kilometer) area of felled and scorched trees.

Thousands of small meteorites have been found in Antarctica, providing a rich supply of specimens for scientists to study. Scientists study meteorites for clues to the types of material that formed the planets.

Impact craters and basins

When large bodies such as asteroids and comets strike a planet, they produce an impact crater or impact basin. Impact craters are bowl-shaped depressions that measure up to about 10 miles (25 kilometers) in diameter. They have shallow, flat floors and uplifted centers. Impact basins are larger, and inside their rims there are one or more rings on the planet's surface.

Scientists have found more than 120 impact craters and basins on the earth. One of the most famous, the Meteor Crater in Arizona, is about 4,180 feet (1,275 meters) across and 570 feet (175 meters) deep. It formed nearly 50,000 years ago when an iron meteorite weighing 330,000 short tons (300,000 metric tons) struck the earth.

Most impact craters and basins larger than the Meteor Crater are heavily worn away or have been buried by rocks and dirt as the earth's surface changed. The largest known of these is the Chicxulub (CHEEK shoo loob) Basin centered in Mexico's Yucatan Peninsula. The diameter of the basin is about 190 miles (300 kilometers). Rock samples obtained by drilling into the basin indicate that an asteroid struck the earth there about 65 million years ago. This was about the time the last dinosaurs became extinct. The impact hurled much debris into the sky. Many scientists believe this debris caused climate changes that the dinosaurs could not survive.