ABSTRACT
ASTEROIDS, METEOROIDS, AND COMETS Trillions of particles remain in our solar system. Astronomers group these particles on the basis of diameter and composition (Table 14.1). They range in size from interplanetary dust a fraction of a millimeter in diameter to larger bodies such as asteroids that range in diameter from about 10 m (&30 ft.) to 1000 kilometers (&620 mi.). For the most part, asteroids are found in the asteroid belt, which is a region between Mars and Jupiter (Figure 14.5). The asteroids, which are composed of rock material, metallic material, or rock-metal mixtures, would pose no threat to Earth if they remained in the asteroid belt. Unfortunately, they move around and collide with one another, and a number of them are now in orbits that intersect Earth’s orbit. Asteroids sometimes break into smaller particles known as meteoroids (Table 14.1), which range in size from dust particles to objects a few meters in diameter. When a meteoroid enters Earth’s atmosphere, it is known as a meteor. Frictional heating of meteors as they streak through the atmosphere produces light, so meteors are sometimes called shooting stars. Meteors occurring in large numbers produce the familiar meteor showers.
