The large crater in North-Arizona USA, one and a half kilometer wide and 200 m deep is result of a metallic meteorite with 30 m diameter, crashing about 50.000 years ago. Of the near-earth asteroids, the 1000 to 2000 medium-size objects with nearly one kilometer diameter, of which one in 300.000 years crashed the earth on average, are a much more extensive threat. It can happen to morrow, after tenth of years or after a million years, the past does not say anything about the future! The still larger asteroids, roughly ten kilometer in diameter, of which there are only a few (perhaps 10 pieces), have hit the earth on average once in hundred million years. Something like that should have taken place some 65 million years ago causing three quarter of all living species on earth to die out. The crater formed by that object with a diameter of 170 km has been found near the peninsula of Mexico. The size of the object is estimated to be 10 - 20 km in diameter.
Can we do something against it? In a sense, yes. There exist "space-watch" - observatories, engaged in identifying number, size and orbit of asteroids, some nights observing 600 asteroids, but nearly all of them are smaller than 10 m. Are they much larger then we have to reckon with 10 to 100 years preparation to cause a change in the orbit of a dangerous one. It requires extremely accurate calculations and very complex spacecrafts to reach the asteroid with its enormous speed in an orbit influenced by many other celestial objects. A small chemical explosion will already be sufficient to push them out of their orbit, but you should know beforehand very precisely what the new orbit will be!!
We certainly will be able in the near future to divert such threads, the question is yet: always?



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