Meteorites are lumps of rock and metal, leftovers of the original solar system, orbiting the sun between the orbits of Mars and Jupiter, in Asteroid Belt. They range in size from dust grains to more than a hundred kilometres across. When some of these orbiting chunks stray near the earth, they are dragged down by the earth's gravity. Because of the friction of the atmosphere, they heat up to a few thousand degrees and glow as a streak of light. Only a few survive the hot journey to the surface of the earth. Not surprisingly, meteorites are made of either silicate rock, iron-nickel or a combination of both, similar to the crust of the earth.
n the past four billion years, the earth has often been struck by sizeable asteroids or comets, sometimes with devastating results. Scientists believe that about 65 million years ago, the entire dinosaur community was wiped out following the impact of bits of a comet. Even a bit of a comet or a meteorite as small as 300 metres across striking the earth at 72,000 km per hour would unleash the energy contained in all the nuclear weapons in the world today.
The Jupiter comet Shoemaker-Levy 9 collision in mid 1994 aroused a lot of interest as the earth may encounter a similar happening. In recent years, astronomers have spotted about 100 near-earth asteroids which could pose a danger to the earth. A meteorite that is a few kilometres across would be equivalent to a Hiroshima. In June 1908, an object several metres across fell over Siberia, unleashing the energy of thousand Hiroshima bombs, flattening an area of 2000 square kilometres. There was little damage to human life but vast amounts of dust was kicked up. A few days later people in London could read newspapers at night by the reflected light.
In December 1992, the Comet Swift-Tuttle appeared in the sky. It is estimated that it will reappear in 2124 AD with the likelihood of a collision with the earth. If this happens, human civilisation could be wipe out entirely.