In one of the largest asteroid explosions ever observed, a comet with the energy of three Hiroshima bombs has exploded in the atmosphere above Indonesia. According to NASA, an asteroid 5 to 10 metres in diameter detonated high in the atmosphere above South Sulawesi of Indonesia earlier this month, releasing about as much energy as 50,000 tons of TNT, about three times more powerful than the atomic bomb that leveled Hiroshima. The space agency said it was one of the largest asteroid explosions ever observed. The blast was so loud that it was also heard by monitoring stations 10,000 miles away.<br/><br/>Though the explosion caused no damage on the ground, occurring at high altitude, 15 to 20 km above Earth's surface, it reignited fears about the planet's defences against space impacts.<br/>