Meteor

From Aetilc

Meteor, appearance of a small particle flying through space that interacts with a planet's upper atmosphere. While still outside the atmosphere, the particle is known as a meteoroid. Countless meteoroids of varying sizes are moving about the universe at any time. Perhaps a billion meteoroids a day enter the atmosphere, their speeds ranging from 10 to 45 mi (16–72 km) per sec. They experience friction due to collisions with the atmosphere; by the time they reach 50 to 75 mi (80–120 km) from the surface, they have been heated to incandescence through friction and are visible as "shooting stars," or "falling stars." Most disintegrate completely before they reach the planet; those large enough to reach the ground are called meteorites. A meteor of considerable duration and brightness is known as a fireball; a fireball that breaks apart with an explosion is a bolide. Meteoroids are composed of stone, iron, or a mixture of stone and iron, with other metals present in very small proportions. Other meteoroids, the carbonaceous chondrites, are stony with a large amount of carbon. Although most meteoroids are quite small, and even though only a very small fraction of them reach the surface, their large quantity accounts for several tons of matter falling on the planet each day. A single observer under a dark sky can see an average of 5 to 10 meteors per hour; more during a meteor shower. More meteors are visible after midnight because the earth's rotation has then positioned the observer's part of the planet in the direction of the planet's motion about the star. The frequency of meteors also increases when the planet passes through certain swarms of particles that intersect the orbit.