英语听力：探索发现 2014-07-18 BBC 地平线：陨石的真相-29

 Approaching from beneath the planet, asteroid¹ 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 proximity², scientists knew that it posed a threat. So we know we are safe for at least 100 years from most near-Earth asteroids³ 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 previously⁴ detected. That is always the risk we face. Until we can catalogue and identify all the hazardous⁵ 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.

1 asteroid   

n.小行星；海盘车(动物)

参考例句：

• Astronomers have yet to witness an asteroid impact with another planet.天文学家还没有目击过小行星撞击其它行星。
• It's very unlikely that an asteroid will crash into Earth but the danger exists.小行星撞地球的可能性很小，但这样的危险还是存在的。

2 proximity   

n.接近，邻近

参考例句：

• Marriages in proximity of blood are forbidden by the law.法律规定禁止近亲结婚。
• Their house is in close proximity to ours.他们的房子很接近我们的。

3 asteroids   

n.小行星( asteroid的名词复数 )；海盘车，海星

参考例句：

• Asteroids,also known as "minor planets",are numerous in the outer space. 小行星，亦称为“小型行星”，在外太空中不计其数。
• Most stars probably have their quota of planets, meteorids, comets, and asteroids. 多数恒星也许还拥有若干行星、流星、彗星和小行星。

4 previously   

adv.以前，先前（地）

参考例句：

• The bicycle tyre blew out at a previously damaged point.自行车胎在以前损坏过的地方又爆开了。
• Let me digress for a moment and explain what had happened previously.让我岔开一会儿，解释原先发生了什么。

5 hazardous   

adj.(有)危险的，冒险的；碰运气的

参考例句：

• These conditions are very hazardous for shipping.这些情况对航海非常不利。
• Everybody said that it was a hazardous investment.大家都说那是一次危险的投资。