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.
This massive explosion went largely unnoticed because it ignited 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.
On December 18th, the asteroid shot 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.
“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 program manager at Nasa.
“That’s another thing we have in our defence, there’s plenty of water on the planet.”
Military satellites picked up the blast last year; Nasa was notified of the event by the US Air Force.
In 2005, Congress asked Nasa to find 90% of near-Earth asteroids of 140 metres in size or larger by 2020. Space rocks of this size are so-called “problems without passports” because they are expected to affect whole regions if they collide with Earth. But scientists estimate it will take them another 30 years to fulfil this congressional directive.
The latest event over the Bering Sea shows that larger objects can collide with us without warning, underlining the need for enhanced monitoring.
Dr Amy Mainzer, chief scientist on NeoCam at JPL, said: “The idea is really to get as close as possible to reaching that 90% goal of finding the 140m and larger near-Earth asteroids given to Nasa by Congress.
She said that if the mission did not launch, projections suggested it would “take us many decades to get there with the existing suite of ground-based surveys”.