As you know I’m always up for a thread of doom, so when I heard talk of asteroid strikes happening more frequently than previously thought I decided to investigate.

The story starts with an asteroid that exploded in the air in the Chelyabinsk region in February 2013. There was a collection of videos at Slate. Car alarms were set off by the shock wave, but I gather most of the damage came from broken glass. Over 1000 people were injured. There are some stills of damage here.

The rock was about 19 metres across (equivalent to a six-story building), with a mass of about 12,000 tons. When it hit the atmosphere at a speed of 20 kilometers per second (many times faster than a rifle bullet) the energy released was equivalent to about 500,000 tonnes of TNT, and the brightness around 30 times that of the sun.

This Slate article has a description of what happened physically. Broadly:

It came in over Russia at a low angle, slamming into our atmosphere, violently compressing the air in front of it. That created a vast amount of heat and pressure, which simultaneously melted and broke up the asteroid into smaller fragments. Within seconds, the huge energy of motion of the rock was suddenly and violently dissipated, creating an explosion equal to about 500,000 tons of TNT detonating.

I think 500,000 tonnes of TNT is about the equivalent of 40 Hiroshima bombs.

As that article says (see also the BBC and the ABC) asteroid strikes are now thought to happen more frequently than previously thought (paywalled research here and here), perhaps as much as ten times more. Chelyabinsk-type events were thought to happen every 150 years on the average. Now the estimate has moved to every 25 to 30 years. And then there’s all the others in the range from say 1 to 50 metres. Previously we relied on visual records, but some, over the sea, for example, have escaped notice.

Apparently there are literally millions of objects in the tens-of-metres-of-size range that could come near Earth and we only know about 1000. Continue reading A bolt from the blue →