BEWARE the blast from above: small asteroids that explode before they hit the ground may be more dangerous than we thought.
Asteroids a few tens of metres in diameter rip through the atmosphere at between 40 and 60 times the speed of sound, and many explode before they hit Earth. Extreme friction and heating can cause these asteroids to flatten into pancakes, which increases drag even more and eventually tears them apart. The resultant "airburst" is thought to be behind the 1908 Tunguska explosion in Siberia, which levelled 2000 square kilometres of forest.
Because airbursts spread material over a wide area and there is no impact crater, researchers rely on computer simulations to calculate the size of the asteroids that caused them. Previous calculations for the Tunguska event suggested an asteroid around 50 metres in diameter exploding with a force of between 10 and 20 million tonnes of TNT.
Now a computer simulation by Mark Boslough of Sandia National Laboratories in New Mexico shows that a 30-metre asteroid could have been behind the Tunguska blast. That suggests smaller asteroids can do more damage than previously thought — a worry when one considers that objects smaller than 140 metres across are not currently detected as they zip round the solar system.
Previous simulations overestimated the size of the bodies responsible for airbursts because they treated them much like a nuclear explosion at a fixed point in the atmosphere, says Boslough. As a result, the damage they caused was thought to be related only to the size and temperature of the blast, and its distance away from Earth's surface. "That neglects something significant, though — momentum," says Boslough. His calculations show that the resulting fireball would continue to rocket towards Earth as it exploded. In the case of Tunguska, this jet didn't quite reach the surface — stalling at an altitude of around 5 kilometres — but a heat and shock wave would have carried on to Earth's surface to do much of the damage.
It's becoming clear that previous models aren't right, says Boslough, who presented his results at the annual meeting of the American Geophysical Union in San Francisco this month. "If one of these events hit an area of high population density, it could kill 1 million people."