If a meteor explodes over the ocean and nobody is around to hear it, does it make a sound? James Thew / Alamy Stock Photo

A meteor caused a massive explosion over Earth last year, but nobody noticed until now. It is the second-largest recorded impact in the past century, after the meteor that exploded over the Russian region of Chelyabinsk in 2013.

The giant fireball hit at 2350 GMT on 18 December over the Bering Sea, a part of the Pacific Ocean between Russia and Alaska.

Peter Brown at the University of Western Ontario, Canada, spotted the meteor in measurements picked up by at least 16 monitoring stations globally.


An image of the meteor explosion was captured by the Japanese Himawari-8 weather satellite Simon Proud, University of Oxford/Japan Meteorological Agency

The meteor was 10 metres in diameter, had a mass of 1400 tonnes and impacted with an energy of 173 kilotons of TNT, he wrote on Twitter. The impact energy was about 10 times that of the atomic bomb dropped on Hiroshima in 1945.

The meteor exploded at altitude above Earth’s surface, says Alan Fitzsimmons of Queen’s University Belfast, UK. “It would have been quite spectacular,” he says.

The explosion was detected by infrasound stations around the world, which pick up low-frequency acoustic waves inaudible to humans. These stations were initially set up during the cold war to detect nuclear explosions.

Fireballs spotted larger than 3 kilotons

It is the third-largest impact in modern times, after Chelyabinsk and a massive explosion that occurred in Siberia, Russia, in 1908. Known as the Tunguska event, the air burst flattened an estimated 80 million trees over an area of more than 2000 square kilometres.

“When you see these infrasound waves, you know immediately that there has been an impact or a large release of energy,” says Fitzsimmons. Triangulating the location and source of an explosion requires combining pressure wave data from multiple monitoring stations, which may explain the delay in the data being made public.

The Bering Sea explosion was also picked up by US government monitors that detect fireballs: their sensors pick up electromagnetic radiation in the form of infrared and visible light.

Near-Earth asteroids

Various monitoring groups regularly survey the sky for near-Earth asteroids, says Chris Mattmann at NASA’s Jet Propulsion Laboratory. NASA uses a monitoring system that scans a catalogue of known asteroids for possibilities of future impacts over the next century.

Small objects frequently hit Earth, says Brandon Johnson at Brown University in Rhode Island. “If you go out on a clear night, you’ll see little meteoroids burning up in the atmosphere,” he says. And because 75 per cent of Earth is covered by oceans, many go unreported.

Larger impacts are rarer, because bigger asteroids are less common. But data from sky surveys of asteroids of between 5 and 50 metres in diameter suggest that these objects should hit Earth less frequently than they actually do.

“It implies that there may be more small asteroids than we’re actually seeing in those telescopes,” says Fitzsimmons. Current telescopes are more accurately able to detect objects of several hundred metres or larger in diameter, and less sensitive to smaller objects – but this will change in the future as technology improves, he says.

Asteroid monitoring may also be affected by space junk, particularly the increase in satellite numbers as miniature satellites become commercially available, says Mattmann.

“We really should try to track more bodies to smaller sizes so that we have a better understanding of the threat from these kinds of air blasts,” says Johnson.