英语听力：探索发现 2014-06-15 BBC 地平线：陨石的真相-15
时间：2014-08-26 09:02:27
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（单词翻译）
On June 30th, 1908, a huge explosion tore through the forest of Tunguska, Siberia. It was 20 years before the Russians mounted an expedition to the site. What they found astonished them. 60 million trees across an area the size of London had been leveled. Scientists thought it had been caused by a meteorite1 strike. But then why was there no sign of any kind of impact crater2? The answer is that the devastation3 had to be caused by a meteor attack of a very particular kind.
Physicist4 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
asteroid5 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.
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