Keeping an Eye on Space Rocks
Part II: Size Matters
About once a year, an automobile-sized asteroid hits Earths atmosphere, creates an impressive fireball, and burns up before reaching the surface.
Every two 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.
If a rocky meteoroid larger than 25 meters but smaller than a kilometer were to hit Earth, it would likely cause local or regional damage to the impact area.
We believe anything larger than 1-2 kilometers could have worldwide effects. At 5.4 kilometers in diameter, the largest known potentially hazardous asteroid is Toutatis.
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.