英语听力：探索发现 2014-05-21 BBC 地平线：陨石的真相-5

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So it is kind of like a listening network around theworld. 

 

That’s right. They’re not set up for fireball orasteroid impacts, but set up to listen for nuclear explosions. 

 

What the monitoring stations picked up were some ofthe largest infrasonic waves ever recorded. Here they have been modified tomake them audible.

 

It’s been detected down in Antarctica. We’ve gotrecords of it up there is Alasska. So the pressure wave from the entry of theobject and the explosive fragmentation was found and seen all over the world. 

 

So from the data that is coming in, it’s early days,obviously, but from the data that is coming in, what is your best guess at thesize of that rocky lump?

 

Well, from the infrasound we know the energyreleased was something like 500,000 kilotons of energy, which is huge.

 

I was thinking it sounded a lot.

 

That’s right. And because we know it came in, fromthe video footage, at about 17.5 kilometres per second, we can combine thatenergy with that velocity¹ to get a mass of the object. And from that mass, wecan get a size and it’s probably about 15 metres across or so.

 

And that’s a rarity, isn’t it?

 

We think these things come in maybe once every 50 or100 years. That is all randomly². So this is a really special and really rareevent of course.

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