Map shows some impact craters on Earth

About once a year, an automobile-sized asteroid hits Earth’s 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 Earth’s civilization comes along. Impact craters on Earth, the Moon and other planetary bodies are evidence of these occurrences.

How Big?

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 Earth’s 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.