Unprecedented social media documentation of a small asteroid exploding over a Russian city earlier this year has taught scientists much more about how — and how often — such spectacular events happen.
In one of three scientific papers on the Chelyabinsk meteor published Wednesday, a University of Western Ontario scientist concludes that the heavenly bodies are hitting the Earth two or three times more often than we thought.
"The consensus seems to be that we're seeing more things in the tens-of-metre size hitting us than we previously thought," said Peter Brown, lead author of a paper published in Nature. "Our knowledge of the risk is getting better."
The Chelyabinsk meteor sailed into the Earth's atmosphere on Feb. 15 and exploded in the sky so brightly that it cast daytime shadows in the nearby city of that name. The long, fiery trail gave observers plenty of time to whip out smartphones and videocams to record it — and many did.
More than 400 recordings of it exist.
"It's almost certainly the best-recorded event in any of our lifetimes," said Brown. "Even if it happens again, the odds are it will happen over the oceans and be virtually unrecorded."
The blast knocked people off their feet and smashed thousands of windows. Many residents of Chelyabinsk were injured from flying glass.
Olga Popova, author of another paper published in Science, used the data from security and dashboard cameras to calculate that the asteroid was nearly 20 metres in diameter and hit a speed of 18.6 kilometres per second. The blast was 30 times brighter than the sun and released energy equivalent to 530 kilotonnes of TNT.
Brown and his colleagues were able to use the wealth of data to construct a clearer picture of what happens to asteroids when they hit the atmosphere.
Previously, scientists assumed the resulting explosions were similar in character to nuclear blasts — spherical shock waves expanding from a central point. But Chelyabinsk's explosion looked more like a cylinder, which meant less damage on the ground, but spread over a larger area.
The blast also got Brown and his colleagues wondering about how often such events actually occur.
Brown said there are probably about a million asteroids between 10 and 20 metres across floating in near space. Telescopic surveys have only found about 1,000 of them.
With that in mind, he looked through 20 years worth of data from U.S. government and other sensors that recorded air bursts releasing at least one kilotonne of energy.
"These tens-of-metres-sized objects seem to be hitting the Earth more frequently that just what the telescopic survey numbers would suggest," said Brown.
But the sky, he hastens to add, is not falling.
"These are small objects. They're not, in general, going to cause a lot of damage at the ground."
Telescope surveys have already plotted the orbits of almost all near-space asteroids big enough to pose a real threat, he said, and it wouldn't be worth doing the same for Chelyabinsk-sized objects.
"It would be extremely costly to go out and try to find and catalogue and get the orbits for all of these objects. But what might make sense is to have resources put into ... systems that might detect an object a few days to a week in advance of when it hits."
A brief warning might save people from the type of injuries experienced in Chelyabinsk, he suggested.
"It might have been possible for civil authorities to say, 'When you see this event — it's going to occur at this time — don't go close to windows.'"
The timing of the asteroid could be predicted within minutes, even seconds, said Brown.
Just such an asteroid impact was predicted in 2008. The object exploded as expected over northern Sudan and the fragments recovered.
"The concept could be applied to Chelyabinsk-sized impactors," said Brown.
"You'd get a few days warning and you'd know something was going to hit. You could take appropriate measures."
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