How often does an airburst like Chelyabinsk happen?
Objects about 20 metres across, like the Chelyabinsk meteoroid, are thought to strike Earth roughly once every few decades to a century, mostly over oceans or remote areas.
The Chelyabinsk event of 15 February 2013 was caused by a meteoroid about 18 to 20 metres across, which exploded about 30 kilometres above the ground with energy estimated at roughly 400 to 500 kilotonnes of TNT. It was the largest known object to hit Earth's atmosphere since Tunguska in 1908.
How common? Estimates from telescope surveys, satellite fireball detections and infrasound records suggest that objects of Chelyabinsk's size strike Earth roughly once every few decades to a hundred years on average. The exact rate is uncertain because events of this size are rare and only recently well recorded.
Why so few noticed? About 70 per cent of Earth's surface is ocean, and much of the land is sparsely populated. Many such airbursts would happen far from people and might be detected only by satellites and infrasound stations. For example, an airburst of about 170 kilotonnes over the Bering Sea in December 2018 went largely unnoticed by the public.
Smaller events. Objects a few metres across, releasing energies of about a kilotonne, arrive several times a year. Metre-sized objects arrive roughly every two weeks.
Larger events. Tunguska-sized objects, perhaps 40 to 60 metres across, are thought to arrive roughly once every few centuries to a thousand years.
Risk. At Chelyabinsk, most injuries were from broken glass. Understanding how often such events occur, and finding objects in advance, is part of planetary defence. Surveys aim to find more small asteroids before they arrive.