Uh-oh

Risk of massive asteroid strike underestimated 

The asteroid that exploded on 15 February this year near the city of Chelyabinsk in the Urals region of Russia was the largest to crash to Earth since 1908, when an object hit Tunguska in Siberia. Using video recordings of the event, scientists have now reconstructed the asteroid's properties and its trajectory through Earth’s atmosphere. The risk of similar objects hitting our planet may be ten times larger than previously thought, they now warn....

The rock was an ordinary chondrite from the asteroid belt located between the orbits of Mars and Jupiter, as revealed by its trajectory and by its elemental and mineral composition, mainly silicates that formed the Solar System billions of years ago. At the time it entered the atmosphere, its mass was of the order of 12,000–13,000 metric tonnes, report two studies published online today in Nature¹ and another study published at the same time in Science². This is nearly twice as heavy as initial estimates had suggested and also larger than revised estimates published in June.

The asteroid roared through Earth’s upper atmosphere at an initial speed of around 19 kilometres per second — more than 50 times the speed of sound. At an altitude of between 45 and 30 kilometres, the heavily fractured, and hence rather fragile, body broke into pieces and finally burst into gas and dust at around 27 kilometres' altitude.

“Luckily, most of the kinetic energy was absorbed by the atmosphere,” says Jiří Borovička, an asteroid researcher at the Astronomical Institute, part of the Academy of Sciences of the Czech Republic in Ondřejov, near Prague. ”A more solid rock that might have blasted closer to the ground would have caused considerably more damage.”