Scientists estimated in the past that the likelihood of a major asteroid strike with the Earth large enough to destroy a city is once every 3000 years. An organization called the B612 Foundation has a very different estimate for this sort of major impact. B612 believes that an asteroid large enough to decimate a city hits the Earth as often as once per century.
B612 Foundation is a planetary defense group with former space shuttle astronaut and physicist Ed Lu as the CEO. The estimate for impacts offered by the group comes from data that it has collected from a network of sensors that are designed to detect nuclear explosions. In addition to detecting when a nuclear bomb explodes in the air or under the earth, the sensors can also detect when an asteroid explodes in the air.
Those sensors have detected 26 asteroid blasts since 2000 that were each equivalent to at least 1000 tons of TNT. B612 says that four of those incidents released more energy than the atomic bomb that destroyed Hiroshima. The biggest of the explosions that the sensor detected happened in 2009 just off the coast of Indonesia and released energy equivalent to three Hiroshima bombs.
The biggest asteroid in recent years was the one that entered the atmosphere over Chelyabinsk, Russia in early 2013. That asteroid exploded in the air with enough force to break the windows out of buildings and injured hundreds of people. None of the asteroids that were picked up by the sensors would have destroyed a city; they all exploded high in the atmosphere. However, the data is used to extrapolate that a city killing asteroid can strike every 100 years. Lu says that the real number is probably more like every 150 to 200 years, but even that is much more common than the every 3000 years previous estimates indicated.
SOURCE: USA Today