ASTEROID An asteroid is any of numerous small planetary bodies that revolve around the sun. Asteroids are also called minor planets or planetoids. Most of them are located in the asteroid belt between the orbits of Mars and Jupiter. The belt contains more than 1,150 asteroids with diameters greater than 30 kilometers. The average temperature of the surface of a typical asteroid is -73 degrees C. One theory is that a planet located between the orbits of Mars and Jupiter broke up, forming the asteroids. According to the other theory, there was enough material for a planet between the two orbits, but no planet formed. Instead, the material simply became a belt of rocky objects. One group of asteroids dominates the outer part of the belt. These asteroids are rich in carbon. Asteroids in the second group, which are located in the inner part of the belt, are rich in minerals. These asteroids formed from melted materials.

METEORITE A meteorite is a chunk of metallic or stony material from space that strikes the surface of Earth. 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, 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.

METEOR A meteor is a bright streak of light that appears briefly in the sky. Shooting stars or falling stars. A meteor appears when a particle or chunk of metallic or stony matter called a meteoroid enters 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. Millions of meteors occur in Earth's atmosphere every day.

FIREBALL A fireball is a meteor that burns brightly as it plunges through Earth's atmosphere. If the fireball explodes at the end of its path, it is generally called a bolide. Some pieces may survive the explosion and fall to the earth. Only brightness makes a fireball different from an ordinary meteor. A fireball is as bright as Jupiter or Venus. In rare cases, it may be as bright as a full moon. A sound like thunder occasionally accompanies the passage of a fireball.

METEOR SHOWER

COMET A comet is an icy body that normally travels around the sun in a long, oval orbit. A comet consists of a solid nucleus (center), a cloudy atmosphere called a coma, and one or two tails. The nucleus resembles a dirty snowball. It is made of ices of various kinds and of rocky dust particles that are stuck in the ices. When the comet approaches the sun, some of the surface ices vaporize. The resulting gases and the particles that were stuck in the ices fly away from the sun, forming the coma and the tails. As a comet gets closer to the sun, increased heat vaporizes ices on the surface of the nucleus, and the solar wind causes the resulting gases and dust particles that were stuck in the ice to fly away from the sun in tails.

COMET

Impact Crater and Basin 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 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 Earth. One of the most famous, the Meteor Crater in Arizona, is about 1,275 meters across and 175 meters deep. It formed nearly 50,000 years ago when an iron meteorite weighing 300,000 metric tons struck the earth.