There is proof that, in the past, large comets(彗星)or asteroids(小行星)have struck the Earth. Thirty-five million years ago, a three-kilometer-wide rock hit the ocean floor, 160 kilometers from what is now Washington, D. C., leaving an 85-kilometer-wide crater(坑)buried beneath Chesapeake Bay. Another giant rock, called Titan, ten kilometers in diameter smashed into the Gulf of Mexico around 65 million years ago, giving off thousands of times more energy than all the nuclear weapons on the planet combined. "The whole Earth burned that day," says Ed Lu, a physicist and former astronaut. "The chaos and destruction were unimaginable. Three-quarters of all life forms, including the dinosaurs, died out."
Astronomers have found numerous asteroids big enough to cause a disaster for the whole planet. None is on course to do so in our lifetimes, but there are many smaller asteroids that could strike in the near-future, with destructive effects. On June 30, 1908, an object as big as a 15-story building fell in Tunguska, a remote part of Siberia. The object, an asteroid or a small comet, exploded several kilometers before impact, burning and blowing down trees across 2, 000 square kilometers. Clouds of dust and ice filled the sky. The particles reflected the surfs light onto the Earth, and for days people in Europe could read newspapers outdoors at night. More recently, in 2013, a 20-meter meteor(流星)exploded over Chelyabinsk Oblast, Russia, injuring dozens of people on the ground. It was the largest object to enter the Earth's atmosphere since Tunguska.
The next time a large object falls out of the sky, we may be taken by surprise. Currently, there is no early-warning system for near-Earth objects. However, over the next decade, sky surveys, like the one being done by Tholen should begin filling that gap. Astronomers are making a list of thousands of asteroids to help us predict the next strike. "Every couple of weeks," says Lu, "we're going to be finding another asteroid with, like, a one-in-a-thousand chance of hitting the Earth."
12. When did the largest object mentioned in the text enter the Earth's atmosphere?
A.65 million years ago. | B.35 million years ago. |
C.In 1908. | D.In 2013. |
13. What does the author want to tell us by the examples in Paragraph 2?
A.The recent findings of rare stars. | B.The significance of disaster prevention. |
C.The new knowledge of space travel. | D.The threat of comets and asteroids. |
14. What can we infer from Ed Lu's words?
A.The chance of the earth being hit is decreasing. |
B.Titan destroyed most life forms on earth. |
C.The dinosaurs died out due to a nuclear explosion. |
D.We have already had mature warning systems. |
15. What is the author's attitude towards future predictions?
A.Pessimistic. | B.Indifferent. |
C.Optimistic. | D.Neutral. |