The smallest units in the solar system which can be observed with the naked eye are meteoroids, and they range in size from large fragments of asteroids and comets to small micrometeoroids (Bedient www.corvis.com; NASA www.nasa.gov). Most meteoroid streams come from comets, and comets originate in interstellar space, where they are assembled in the dense molecular clouds of gas and dust between the stars (NASA www.nasa.gov). The nucleus of a comet consists of ice and dust particles. Comets were formed in the outer solar system beyond Saturn's orbit at the same time as the Earth and the solar system were formed. As comets approach the sun, the ice evaporates and the dust particles are ejected into orbit and become meteoroids. When meteoroids enter the Earth's atmosphere, they are heated by friction and are observed as a bright flash of light with a glowing tail streaking across the sky, known as a meteor (Bedient www.corvis.com). Meteors may weigh less than a gram, or more than a kilogram, and if they survive their passage through the Earth's atmosphere to impact the ground, they are known as meteorites. Most meteors burn up in the Earth's atmosphere before ever reaching the ground. Brilliant meteors are sometimes followed by trails of light, which can last for up to 30 minutes. Others explode with a loud sound resembling thunder: these are called bolides.
Meteors occur in the thermosphere layer of the atmosphere (Bedient www.corvis.com). The region where the meteors occur is between 80 km and 120 km in altitude, but very fast meteors may become visible above this height and slow, bright meteors may be seen below this band. Most visible meteors range in size between that of a grain of sand and that of a small pebble, weighing less than one or two grams. The majority of meteors originate from comets, and the remainder have their origins in asteroids. Meteors of asteroidal origin are often composed of dense stony or...