FREQUENCY: 2,000 YEARS
Less damaging but more numerous are the asteroids with Earth-crossing orbits that are roughly 200 meters (660 feet) across, the size of a large sports stadium. Astronomers have discovered only a minority of the estimated tens of thousands of Near-Earth asteroids this size. An organized program to detect 90 percent of them would take at least 10 years and cost several hundred million dollars.
If an asteroid this size did hit Earth, it would be very likely to land in a relatively unpopulated area or in the ocean, so the death toll would be relatively low. If it landed in a large metropolitan area, however, it could kill millions of people.
The impact from an asteroid of this size would release 10 times the energy of the largest nuclear weapon test, with destruction equivalent to history's worst natural disasters. The heat blast and shock wave from an impact on land would devastate an area the size of greater Los Angeles. An ocean impact could generate a tsunami 10 meters (33 feet) high along many coastal areas.
Topic: Earth Science
Keywords: Meteorites, Astrophysics, Asteroids--Collisions with Earth, Astrogeology, Collisions (Astrophysics), Near-Earth asteroids, Natural disasters