60 asteroids have hit since 1990… And we didn’t notice.

In February, an asteroid screamed into the atmosphere above Russia so fast that it exploded in midair. It produced the explosive force of half a million tons of TNT.

It turns out asteroid strikes like the one that injured almost 1500 people this year are much more common than anyone thought. They just usually hit in remote areas — and even with all our technology and 7 billion pairs of eyes we haven’t noticed. An international team of scientists recently found out that “the strike rate of asteroids that are tens of metres in size is between two and 10 times higher than was previously thought.”

Why do meteors sometimes explode in midair? Coming from space, they’re very cold. They’re moving so fast that the air in front of them is compressed so much it produces incredible heat. The difference between extreme heat and extreme cold makes the space rock shatter into thousands of pieces. Those pieces are also moving at incredible speeds and compressing the air, producing even more heat, and you basically have an explosion. It doesn’t work the same way as a chemical explosion or nuclear explosion, but has similar force.

People often think that meteorites are radioactive because media reports often describe the power of meteor explosions in terms of the strength of a nuclear bomb. (February’s midair explosion had the force of twenty-five Hiroshima bombs). Meteor explosions can have similar strengths to nuclear bombs, but meteors are basically never radioactive.

There’s a global network of sensors set up by the US military and other groups to listen for nuclear explosions to enforce a nuclear test ban treaty. Some of those sensors can hear the really loud low-frequency atmospheric rumblings of a nuclear explosion from very far away.

An international team of scientists examined records from those sensors for readings that match the signatures of exploding meteors. They found 60 of them since 1990 that were up to 20 meters across.

Smaller-scale meteors fall on Earth all the time. I just saw a shooting star last week.

Last month, a meteor blew up in midair over Columbus, Ohio.

No damage. We’re lucky these things usually blow up in the sky instead of on the ground.

UPDATE: There was another meteor fireball over California a few hours ago.