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| Map shows some impact craters on Earth |
Every day, Earth is bombarded with about 25 tons of dust and sand-sized particles.
About once a year, an automobile-sized asteroid hits Earths atmosphere, creates an impressive fireball, and burns up before reaching the surface.
Every thousand years or so, a meteoroid the size of a football field hits Earth and causes significant damage to the area.
Finally, only once every few million years, an object large enough to threaten Earths civilization comes along. Impact craters on Earth, the Moon and other planetary bodies are evidence of these occurrences.
How Big?
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| An object the size of a very large bridge could have worldwide effects. |
Space rocks the size of a car and a little larger up to 50 meters (about 164 feet) in length -- will most likely burn up as they enter Earths atmosphere.
If anything smaller than one kilometer (a little more than one-half mile) were to hit Earth, we think it would cause local damage to the impact area.
We believe anything larger than one kilometer could have worldwide effects.
By comparison, asteroids that populate the main asteroid belt between Mars and Jupiter, and pose no threat to Earth, can be as big as 940 kilometers (about 583 miles) across.