Physicist Mark Boslough has been fascinated by how so much destruction can be caused without any apparent direct contact. The explosion at Tunguska was caused by an asteroid that entered the atmosphere, got close to the surface and exploded before it hit the ground. And that explosion created a blast wave with hurricane force winds that knocked trees over for thousands of square miles. Scientists call it an airburst, a massive explosion in the atmosphere rather than on the ground. As it enters the atmosphere at speeds of up to 24 kilometres per second, the air resistance decelerates the asteroid so fast (that) it breaks apart in a huge explosion.