Buy $50 million worth of new telescopes right now. Then spend $10 million a year for the next 25 years to locate most of the space rocks. "By the time we spot a fatal one," the scientists say, "we'll have a way to change its course."

　　Some scientists favor pushing asteroids off course with nuclear weapons. But the cost wouldn't be cheap.

　　Is it worth it? Two things experts consider when judging any risk are: 1) How likely the event is; 2) How bad the consequences if the event occurs. Experts think an asteroid big enough to destroy lots of life might strike Earth once every 500,000 years. Sounds pretty rare--but if one did fall, it would be the end of the world. "If we don't take care of these big asteroids, they'll take care of us," says one scientist. "It's that simple."

　　The cure, though, might be worse than the disease. Do we really want fleets of nuclear weapons sitting around on Earth? "The world has less to fear from doomsday(毁灭性的) rocks than from a great nuclear fleet set against them," said a New York Times article.

　　21. What does the passage say about asteroids and meteoroids?

　　A. They are heavenly bodies different in composition.

　　B. They are heavenly bodies similar in nature.

　　C. These are more asteroids than meteoroids.

　　D. Asteroids are more mysterious than meteoroids.

　　22. What do scientists say about the collision of an asteroid with Earth?

　　A. It is unlikely but the danger exists.

　　B. Such a collision might occur once every 225 years.

　　C. Collision of smaller asteroids with Earth occur more often than expected.

　　D. It's still too early to say whether such a collision might occur.

　　23. What do people think of the suggestion of using nuclear weapons to alter the course of asteroids?

　　A. It sounds practical but it may not solve the problem.

　　B. It may create more problems than it might solve.

　　C. It is a waste of money because a collision of asteroids with Earth is very unlikely.

　　D. Further research should be done before it is proved applicable.

　　24. We can conclude from the passage that ________.

　　A. while pushing asteroids off course nuclear weapons would destroy the world

　　B. asteroids racing across the night sky are likely to hit Earth in the near future

　　C. the worry about asteroids can be left to future generations since it is unlikely to happen in our lifetime

　　D. workable solutions still have to be found to prevent a collision of asteroids with Earth

　　The journey to the moon had been the first step toward future exploration(探险) in space. The distance between the Moon and Earth is very short indeed when compared with the distances between Earth and the other planets. Mars, the nearest planet to Earth is millions of miles away. Traveling to the planets will be man's next aim. Such travels will be more challenging than the trip to the moon and certainly more adventurous.

　　Recently, two American spaceships, Vikings 1 and 2, landed on Mars trying to discover whether that planet has any life on it. So far the signs of life on Mars has neither confirmed(确认) nor ruled out. Russians have discovered that the surface of Venus(金星) is so hot that it is almost certain that there is no life there. Also the atmosphere of Venus is extremely dense(稠) and the pressure is nearly a hundred times greater than that of the Earth's atmosphere.

　　Scientists believe man may find planets which have the same conditions as those we have on Earth, but man should realize Earth will be his only home for a long time and begin to love and care for it.

25. According to the passage, traveling to the planets will be ________.