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第三篇: 听力点击这里下载 Well, today we're going to talk about near earth objects, or any uses, and impact they have had and probably will have on the earth history. ok, ah, em, all the we tend to think outspaces empty is actually fuller than you might think. With small objects known as astroroets. The main concentration of these is in a bell between Mars and Jupiter, but suprising number have more eccentric orbits. That can bring them dangerously close to earth. If you look at this picture of the moon, I bet there's something strike you straight away. The craters, right? There are so many of them, and quite a few are huge. Tens or even hundreds of miles across. So big, that they can be seen from earth without a telescope. To create these craters is obvious that sometime in the moon's history it's been hit. Is has some pretty major sized astroroets collapsed with it. What I'm saying is so many big astroes have hit the moon, you can be pretty sure that has sometime in earth's history similar sized objects definitely hit earth, too. In fact, it seems that a 200 feet astroet hit Sybiro as recently as 1908. And this small one are thought to strike about once in a century. The impact in Tunguska, that's Tunguska in June 1908. Fortunately, it hit in a nearly unhabitat area, so few people were affected. So what about medium and larger astroroets? Well, medium sized object maybe half a mile across or say up to a mile which have much bigger impact. The force of the exposion would be about the same as 300,000 metric tons. That's 300'000 million tons of TNT or Denamy. To put that in perspective, the Tunguska impact was about by suppose for surround 50 million tons of TNT equivalent. So a medium sized astroroets could do quite a big more damage. Surprisingly the thing is likely to cause the most damage isn't impact himself, but add mis very poisioning. The released energy would split up Nitrogen and Oxygen molicuocento items and some of these were compound to form toxic oxide or nitrogen which would poision anthing or anyone who breed the toxic fumes. And the really big astroroets? Let me see. We think that the really massive astroroets say 5 or 6 miles across, might hit earth once in about 5 million years. A number of people have suggested that the impact of this kind may have been responsible for the demice of the dinosaurs 65 million years ago. It's maybe word saying a bit more about this. As early as 17 century, the famous astronomer Edmund Halley, I'm sure you all heard the comet he discovered, well, Halley realize the comet and other object in space might collide the earth. You can image how the authorities of those days reacted to that kind of idea. In pretty much, every scientist after that who suggest such a thing was know as quank. This was until the famous 1980 paper by the Alvoristing put forward some hard evidence. Analysis of rock from the cotacious period around the time of the dinosaurs extinction showed a higher than expected amount of radium. This haby metal is common in medium rats, but extremely where on earth. So the most likely expernation is that the riginave from medium rat impact at that time. Without wishing to frighten anyone I want to know that although major impact do happen once in a blue moon and the risk from any uses is real. It is also very, very small. However, given the potential benefits of tracing these objects says not really expensive to do. With enough warning, we already have the technology to handle all potencial impact and prevent all limited damage could do.
Just we two. Here it is so desolate, brumal, desert, gaunt, howling, and inhospitable. mp3 has been uploaded. 
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发表于 2006-8-18 10:20:00
发表于 2006-8-19 01:29:00
