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. At about noon...
