A meteor was the cause of an enormous explosion in the atmosphere last year, NASA has announced.
A huge fireball exploded in the Earth's atmosphere in December, according to NASA.
The blast was the second largest of its kind in 30 years, and the biggest since the fireball over Chelyabinsk in Russia six years ago.
But it went largely unnoticed until now because it blew up over the Bering Sea, off Russia's Kamchatka Peninsula.
The meteor was 10 metres in diameter, had a mass of 1400 tonnes and impacted with an energy of 173 kilotons of TNT. The space rock exploded with 10 times the energy released by the Hiroshima atomic bomb.
According to NASA's planetary defence officer Lindley Johnson, the fireball exploded near to a common flight route, and so researchers are asking airlines if they saw any signs of it.
At about noon local time on 18 December, the asteroid barrelled through the atmosphere at a speed of 32km/s, on a steep trajectory of seven degrees.
Measuring several metres in size, the space rock exploded 25.6km above the Earth's surface, with an impact energy of 173 kilotons.
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. It was also picked up by US government monitors that detect fireballs, flashes of light in the visible and infrared spectrum.
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.
"That was 40% the energy release of Chelyabinsk, but it was over the Bering Sea so it didn't have the same type of effect or show up in the news," said Kelly Fast, near-Earth objects observations programme manager at NASA.
"That's another thing we have in our defence, there's plenty of water on the planet."
Dr Fast was discussing the event here at the 50th Lunar and Planetary Science Conference in The Woodlands, near Houston, Texas.
Military satellites picked up the blast last year; NASA was notified of the event by the US Air Force.
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, larger impacts are rarer, because bigger asteroids are less common.
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