英语听力：探索发现 2014-06-18 BBC 地平线：陨石的真相-16

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In this simulation I include more of the physics to be more realistic. We can see that the main shockwave doesn’t come out of the point of the explosion, but it comes out of the point where the fireball descends² to. So by the time the shockwave gets to the ground, it’s much stronger than it would otherwise be. And there’s more damage on the ground because the destructive power was carried downward. 

 

Based on Mark’s calculations, the devastation³ at Tunguska could have been caused by an asteroid, perhaps as small as 30 to 50 metres in diameter, and this carries a worrying implication. Smaller asteroids⁴ are more dangerous than we used to think and because there are so many more smaller asteroids than bigger asteroids we need to take that risk more seriously than we used to. 

 

The lesson of Tunguska helps explain why in Chelyabinsk there’s so much damage, but very little meteorite⁵ to be found. 

 

If we go back to the video footage and we see the object coming in when it’s in the high atmosphere, it suffers very little effect, but just here you get this huge flare-up and that’s because the atmosphere has become so dense⁶ that it’s almost impossible for it to push through any more. And basically something’s got to give and the asteroid gives, and it basically just breaks apart in a huge catastrophic fragmentation event. And that is what creates a shockwave which we hear as this sonic boom. 

 

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