An asteroid, with the energy of three Hiroshima bombs, has exploded in the atmosphere above Indonesia - reigniting fears about Earth's defences against space impacts.

Witnesses in Indonesia heard the explosion and spotted a huge fireball in the sky. The blast was also heard by monitoring stations 10,000 miles away, according to a report by scientists at the University of Western Ontario.

The explosion, on October 8, caused no damage on the ground because it occurred at high altitude, 15 to 20km above Earth's surface.

Nasa estimates the explosion was the equivalent to 50,000 tons of TNT, making it one of the largest asteroid explosions ever observed.

Scientists are worried that it was not spotted by telescopes ahead of the impact, and that had it been larger and at a lower altitude it could have caused a disaster.

A Nasa statement said: "We estimate size to be about 5-10 meters in diameter. As a rule, the most common types of stony asteroids would not be expected to cause ground damage unless their diameters were about 25 meters in diameter or larger."

Tim Spahr, director of the Minor Planet Center in Cambridge, Massachusetts, says that objects as small as 20 or 30 metres across may be capable of doing damage on Earth.

He warned that it was inevitable that these will go unnoticed:

"If you want to find the smallest objects you have to build more, larger telescopes. A survey that finds all of the 20-metre objects will cost probably multiple billions of dollars."

Youtube: the aftermath

New Scientist video: Asteroid alert