It is estimated that the earth’s surface is struck by about 500 meteorites a year, but only about 5 or 6 are large enough to be detected by weather radar instruments or their fragments recovered. Large collisions that leave discernable impact craters are thankfully, extremely rare events, that occur in intervals of thousands of years on average. For instance, stony asteroids of size 100 meter in diameter strike earth every 5,200 years on average. Such a collision would create a crater 1.2 km across and release energy equivalent to 3.8 mega-ton of TNT, or about a thousand times more powerful than the combined energy of both the Hiroshima and Nagasaki explosions. Larger collisions involving asteroids 1km in diameter are even rarer (every 500,000 years), and collisions with 5km objects are rarer still (once every twenty million years). The last known impact of an object of 10 km in size was the dinosaur killer that happened 66 million years ago.
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