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The collision of rocks and chunks of ice with the Earth occurs every day, however, most of these are so small that they go largely unnoticed. Around 50,000 of the toughest of these objects survive passage through the atmosphere and fall as meteorites. Occasionally these can even damage property.
Larger objects collide with the Earth much less frequently and the larger they are the less likely they are to collide with our planet. Near Earth Objects around 60 m in diameter, for example, are thought to collide with the Earth every 100 to 300 years and can break-up explosively in the atmosphere and cause intense damage over an area of several thousand kilometres square. By comparison Near Earth Objects of one kilometre in diameter, which could devastate a large country, occur once every few hundred thousand years. Very large collisions that could threaten the existence of all large land species on Earth occur even less often at once every one hundred million years.
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