英语听力：探索发现 2014-07-18 BBC 地平线：陨石的真相-29（打印版）

Approaching from beneath the planet, asteroid 2012 DA14 passed inside the orbit of our geostationary satellites before heading off to the north. This asteroid had been successfully tracked for a year. And despite its proximity, scientists knew that it posed a threat. So we know we are safe for at least 100 years from most near-Earth asteroids over a kilometer in size. We are better at detecting objects down to 50 metres across like DA14. But for asteroids smaller than that, like the one which exploded over Chelyabinsk, we still have little or no warning.

There are still some we haven’t found. So there’s this unknown bit of the equation where we are still looking for some. We know they are there but we don’t know where they are. So this is a threat, but hopefully as technology moves on, we’ll always have a much better idea where one’s going to pose a risk to the Earth. We could see an event tomorrow; we could see an event in 10 or 20 years time that we hadn’t previously detected. That is always the risk we face. Until we can catalogue and identify all the hazardous objects in the solar system, that risk will always remain.

And there’s one other factor that can make it particularly hard to spot an incoming object. It’s the reason why no one saw that asteroid that was hurtling towards Chelyabinsk.

英语听力：探索发现 2014-07-18 BBC 地平线：陨石的真相-29（在线收听）