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Asteroid impacts occur at different frequencies and sizes. Small impacts occur twice a year and can cause an airburst twice as large as the Hiroshima bomb. Larger impacts occur less frequently and can cause more damage, with asteroids larger than 1 km posing the greatest threat to human life. However, the probability of such an impact occurring in the next 100 years is low.
The smallest asteroid impacts occur about twice a year. Only about 3m (10ft) in diameter, these rocks, moving at an average of 17km/sec, have enough energy to cause an airburst twice as large as the Hiroshima bomb at an altitude of 43km. Originally, it was feared that atmospheric asteroid impacts would be mistaken for nuclear explosions by sky satellites and start a nuclear war, but modern satellites are able to distinguish the characteristic double flash of nuclear bombs. The Royal Astronomical Society considers anything smaller than 50 meters in diameter to be a “meteoroid”. Meteoroids are commonly known as “shooting stars”.
Impacts from larger asteroids, more than 50 m (164 ft) in diameter, occur about every 500 years. Similar to smaller asteroids, asteroids of this size usually lack the kinetic energy to get to the surface and explode in an airburst at an altitude of about 7 km. The energy of the explosion is about 6 megatons of TNT, equivalent to a small hydrogen bomb. An asteroid of this size is believed to have exploded over an area near the Tunguska River in 1908, creating a circle of scorched trees 50 kilometers (30 miles) in diameter. This is called the Tunguska event and it helped encourage governments around the world to take the risk of an asteroid impact more seriously.
Asteroid impacts by bolides about 250 m (820 ft) in diameter occur only once every 2,000 years or so. These asteroids actually tend to hit the surface, although they may break up slightly before doing so. The resulting energy is about a gigaton, about 20 times greater than the most powerful nuclear weapon ever tested, the Tsar Bomba. Such an asteroid impact is thought to have occurred on the Moon in the year 1178, where it was recorded by a monk in Canterbury, England. This impact class leaves a crater miles wide.
Asteroids larger than 1 km (0.62 mi) in diameter are quite rare, occurring less than once every 50,000 years. However, they are the most destructive and most likely to threaten the human race, despite their rarity. Such an asteroid impact releases 50 gigatons of TNT of energy at its source, igniting everything for more than two hundred miles in each direction. If such an asteroid hit a populated area, it could kill millions. However, the probability of one hitting in the next 100 years is less than 1/500, and the probability of randomly hitting a populated area is less than 1/1,000.
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