Just nine months ago, a massive asteroid blew up above the city of Chelyabinsk in Russia. The explosion created by the Chelyabinsk asteroid on Friday 15 February 2013 was the largest explosion on Earth since the one that occurred over the Tunguska region of Siberia in 1908.
But whereas hardly anyone saw or were able to record information at the time of the Tunguska explosion, the Chelyabinsk asteroid blew up over a relatively densely populated region and more importantly, the journey through the air of the Chelyabinsk asteroid was recorded by numerous CCTV cameras and webcams that many Russian drivers install on their cars. The video footage of the impact event was broadcasted by the media, and thus witnessed by people from all over the world.
Based on data from those videos and visits to some 50 local villages, researchers have published their findings on such a rare event. But is it really such a rare event?
Russian Impact Events in Statistics
The Chelyabinsk asteroid entered our atmosphere with a mass of 13,000 tonnes and a diameter of about 19.8 metres. When the meteor blew up above Russia, it was travelling at almost 70,000 kilometres per hour and it produced an airburst with an energy equivalent to about 500 kilotonnes of TNT.
By comparison, the Tunguska explosion event had an estimated energy of 5-15 megatonnes of TNT.
Beneath the fireball’s path, the shock wave was strong enough to knock people off their feet. First-hand witnesses found it painful to look at the bright fireball. And about 2.2% of those who were polled on the event, reported getting sunburnt. Although most buildings were left structurally undamaged, about 44% of all apartments in Chelyabinsk sustained shattered windows and broken glass. Over 1,000 people were injured as a result.
The asteroid that burned up over Russian skies last February induced a shock wave that formed 90 kilometres above the ground. The shock wave was so intense that it travelled twice around the globe, according to Le Pichon et al. (2013).
The Chelyabinsk asteroid broke into small pieces at altitudes of between 30 to 45 kilometres, creating various smaller meteorites that landed on the ground. As much as 76% of the asteroid evaporated on contact with the Earth’s atmosphere, with most of the remainder turning to dust.
Between 4 and 6 tonnes of the Chelyabinsk asteroid survived the explosion (roughly 0.03-0.05 % of its initial mass), including one piece that made a 7 metre hole in the 70 centimetre-thick ice sheet of Lake Cherbarkul. Studies of its internal composition have revealed that the Chelyabinsk asteroid is an LL chondrite – the same type of rock as the Itokawa asteroid, from which the Japanese Hayabusa probe collected samples a few years ago.
Enhanced Hazard from Small Asteroidal Impactors
Researchers have found that space rocks of a similar size to the one that exploded over Chelyabinsk are hurtling into the Earth’s atmosphere with surprising frequency (Brown et al., 2013). They now think the space rock hit-rate has been underestimated and say early warning systems need to be put in place.
The risk of a strike from asteroids of the same scale appears to have been vastly underestimated. The team from the University of Western Ontario (Canada), estimated that the strike rate of asteroids that are tens of metres in size is actually between 2 and 10 times higher than previously thought. Although an impact event like Chelyabinsk would only be expected every 150 years on the basis of telescopic information, extrapolating the data shows that these impact events seem to be happening every 30 years or so…