When a comet enters our solar system and passes by the Sun it heats up and loses debris, like small rocks and ice. These materials float around our solar system and can be seen until Earth passes through them during its revolution around the Sun. Then the debris enters the atmosphere with a velocity of 20 to 60 km/s. These small particles heat up due to friction from the atmosphere and look like brief streaks of fire in the sky. If the debris totally burns up before it his the ground it is referred to as a meteor. But, on the rare occasion one is large enough to withstand the friction in the atmosphere and it makes an impact with Earth it is known as a meteorite.
Meteors are more commonly called 'shooting stars', and are seen in great frequency during meteor showers. A meteor show occurs when Earth passes through the debris field of a comet. Many meteor showers occur yearly at fixed intervals. Every year between November 3 and 15 for example a meteor shower may be seen at night, when Earth crosses the Taurids. This swarm of debris is coming from the Encke comet, which return every 3.3 years. Every year between June 24 and July 6 the Earth again crosses Taurids, but during this time, there is almost nothing to see.
Rarely are the fragments large enough to reach the surface of Earth, in that case we call them meteorites. Only the larger and more dense bodies left from the passing comet reach the Earth's surface in pieces that remain intact. Friction greatly slows down the meteorite, however large and more dense objects are not as effected by friction. When the meteorite comes in contact with Earth, all the energy from the motion of the meteorite is instantly turned into heat. This causes the meteorite to explode and evaporate causing a crater most often 20 times the diameter of the object itself.
Most meteorites are small and cause little or no long term impact. But sometimes large pieces of comets break off and the impacts that they leave are gigantic. On June 31, 1908 eyewitnesses saw a giant fireball in the sky about 500 km away that scientists say would have appeared brighter than the Sun. The meteorite slammed into the ground above the Tunguska River in Siberia. Panic broke out up to 1500 km away because the impact caused severe gusts of wind, an unbelievably loud bang and the ground began to shake.
The explosion flattened the forests over an area of about fifteen hundred square kilometers, it also uprooted trees and peeled off their branches and leaves. The explosion was estimated to have had an impact equivalent to twenty to thirty tons of TNT. This amount of energy is more or less about the same as about five hundred atom bombs that were used in World War II. The explosion was caused by a comet fragment with a mass of approximately 200,000 tons. After it entered the atmosphere the object exploded at a height of about 8 kilometers and evaporated, because of the heat of friction. It was the largest major meteorite impact ever recorded in human history.
Although the Tunguska was very large, scientists believe their have been much larger objects to come into contact with Earth. The dinosaurs might have become extinct because the impact of a comet caused a cloud of dust and dirt to block out the light from the Sun for several months. This meteorite was probably several kilometers and must have weighed at least 3 billion tons. The impact must have released millions of mega tons of energy which would be equivalent to setting off a series of atomic bombs all over the world.