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.

NASA has satellite mission called NEOCam to put a satellite in space in the Earth’s orbit. There is also the Sentinel space telescope being planned by the B612 Foundation. These should give us earlier warning. The aim would be to know about the big ones a decade in advance, then send up a spacecraft to intercept them.

There is a thing called the Torino Scale to categorise the impact hazard associated with near-Earth objects (NEOs) such as meteors, asteroids and comets. Interestingly the Chelyabinsk event rates zero. The Tunguska event of 1908, a 60 to 190m rock, flattened 2,000 km2 of forest and rated 8 but still didn’t strike the ground.

Speaking of big, the Chelyabinsk rock is thought to be a chip off asteroid 86039 (1999 NC43) which comes in at over 2 km. That one is not expected to strike which is just as well as it would result in a major extinction event.

National Geographic has a list of the ten biggest ever. The Chicxulub impact crater in the Yucatán Peninsula, caused by a rock thought to be 10 km across, is possibly the biggest of all. That not only did in the dinosaurs, it created the opportunity for mammals to thrive and multiply, leading eventually to us, so it wasn’t all bad.

This is a reconstruction of the dent Chicxulub made:

There is obviously a range of serious strikes between the tens-of-metres category and the top ten list. Some years ago I recall discussion of the problem of these strikes causing tsunamis. I found an article in the New Scientist. Back in 2007 a research team looked at strikes in the 100 to 500 metre range which happen about every 10,000 years on average.
They found China, Indonesia, India, Japan and the US as most vulnerable. Here’s their vulnerability map:

I recall them saying that satellite imagery shows the oceans pock-marked with these strikes.

I gather that total effective vulnerability showing the impact on existing infrastructure is represented in this map:

I don’t know about you, but these midrange strikes at once every 10,000 years bother me a bit. But then we are getting some pretty nasty weather at times, such as Cyclone Haiyan which has already killed more than 10,000 people and is heading for Vietnem, especially as sea level rises: