Chances of an asteroid hitting are pretty small. NASA has tracked 90 percent of the near-Earth asteroids that are at least a half-mile (0.8 kilometers) wide and believes none of them has a significant chance of hitting Earth.

Still there could be some bigger asteroids out there that NASA doesn’t know about. If a mile-wide asteroid hit Earth, it would strike the planet’s surface at about 30,000 miles per hour (48,280 kilometers per hour). An asteroid that big traveling at that speed has the energy roughly equal to a 1 million-megaton bomb.

It’s difficult to imagine 1 million megatons, so let’s try some smaller sizes. Let’s say that an asteroid the size of a house hit Earth at 30,000 miles per hour (48,280 kilometers per hour). It would have the energy roughly equal to the bomb that fell on Hiroshima — perhaps 20 kilotons. An asteroid like this would flatten reinforced concrete buildings up to half a mile from where it hit, and flatten wooden structures perhaps a mile and a half (0.8 kilometers) from ground zero. It would, in other words, do extensive damage to any city.

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If the asteroid was as big as a 20-story building (200 feet [61 meters] on a side), it could have the amount of energy equal to the largest nuclear bombs made today — about 25 to 50 megatons. This size asteroid would flatten reinforced concrete buildings 5 miles (8 kilometers) from ground zero. It would completely destroy most major cities in the United States.

For an asteroid to wipe out most everything on Earth, it would have to be massive. Scientists estimate it would take an asteroid about 7 to 8 miles (11 to 12 kilometers) wide crashing into the Earth. Once it made impact, it would create a tremendous dust plume that would envelope the entire planet, block out the sun and raise temperatures where the asteroid made impact. Billions would die, and much of life on the planet would be destroyed. But, scientists believe some would survive. NASA scientists say it would take an asteroid 60 miles (96 kilometers) wide to totally wipe out life on Earth.