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1、-Its clear, therefore, that the lack of craters doesnt mean were safe! Scientists predict that a large asteroid, as big as the one that probably killed the dinosaurs, should hit Earth, on average, once every 10 million years. But its 65 million years since the last one! Smaller asteroids are passing
2、 close to Earth all the time. Only in 1989 , an asteroid around 1000 feet in diameterthats about 300 meterscalled 4581 Asclepius, passed through the point where Earth was just six hours before. A hit from it would have caused the largest explosion in history. On average, researchers say, an object b
3、ig enough to cause catastrophic results will hit Earth every 5000 years.Astronomers are already working on this. Since the late 1990 s, NASA has been conducting several surveys of space, the biggest of which is LINEAR. NASA aims to find 90 percent of all the near-Earth objects, in other words, NEOs,
4、 that are more than two thirds of a mile 1 kilometerin diameter. So far, tens of thousands of them have been foundmost of them, fortunately, too far away to be dangerous for now. Also, a couple of near-misses have been identified, although usually rather lateafter the object has passed Earth. In 200
5、2 , an object 250 feetabout 80 metersin diameter passed only 75000 milesthats about 120000 kilometersfrom Earth. So, why isnt Earth covered in craters, just like the Moon and some of the planets ? Well, there are two reasons . For one thing, Earth has an atmosphere but the Moon doesnt. Earths atmosp
6、here slows asteroids down, causing heat which burns off their outer layers . Again, you can see this process yourself. Have you ever seen shooting stars ? They are actually the remains of broken up comets It is the burning that we can see. Most are very small, so they burn up completely and disappea
7、r before they get close to the ground .Another reason comes from geology . Earths surface is constantly movingas you probably know, this is called the plate tectonic theory , and is the reason for earthquakes When plates collide with each other, Earths surface gets pushed up. In this way, mountains or islands are formed. When the continents move apart, huge valleys and seas are created. Thus, a map of Earth from the dinosaurs time wouldve looked very, very different from now . You can easily see how craters disappear as the land changes shape.-第 1 页-