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 space rock exploded with 10 times the energy released by the Hiroshima atomic bomb.
Lindley Johnson, planetary defence officer at Nasa, told BBC News a fireball this big is only expected about two or three times every 100 years.
What do we know?
At about noon local time on 18 December, the asteroid barrelled through the atmosphere at a speed of 32km/s (20 miles per second) , 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.
"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.
Dr Johnson said the fireball came in over an area not too far from routes used by commercial planes flying between North America and Asia. So researchers have been checking with airlines to see if there were any reported sightings of the event.
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