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  3. Our Future in Space: It Has Already Begun! We are all space travelers. But we’ve stayed close to home until now. One day, we

Our Future in Space: It Has Already Begun! We are all space travelers. But we’ve stayed close to home until now. One day, we

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问题                 Our Future in Space: It Has Already Begun!
    We are all space travelers. But we’ve stayed close to home until now. One day, we may leave our "mother-ship" Earth to make our home among the stars.
    A giant, spherical "spaceship",about 8,000 miles in diameter, is speeding through the solar system right now. It is cruising at an incredible 66,600 miles per hour.
    It’s not a giant, Star Wars mother-ship. It’s spaceship Earth, the home of over four billion people. This water coated spaceship has been traveling through the universe for about five billion years. Only within the past 25 years, however, have some of its passengers broken free of Earth’s gravity.
    But 25 years from now, many people, including you, might live in an orbiting space station 200 miles above the Earth.
    Space Cities
    Scientists have already designed special space factories. These factories will take advantage of the absence of gravity (zero gravity) to produce everything from life-saving drugs to perfect ball bearings.
    Other scientists have designed space colonies, complete with farms, schools, and artificial day and night. Hundreds, or even thousands, of people will live, work, play — even go to school, far above the Earth.
    Our conquest of space, of course, has already begun. We have explored part of the Moon,sent robot spaceships onto the surface of Venus and Mars, and aimed space probes past the planets of Jupiter and Saturn.
    Last June, one robot ship, Pioneer 10, left our solar system forever. And astronauts from the United States have lived in space stations.
    The conquest of space, without question, is one of the greatest adventures human beings have ever set out on. But it may be more than a great adventure. Some scientists think the conquest of space may be a necessity for survival of the human species.
    We are tearing up more and more of the Earth to get raw materials for industry. And we are polluting the air and water as we manufacture products that we need or want. Almost everything that seems to make our lives more comfortable, and from electricity to pesticides, uses up or alters a piece of our planet’s natural environment.
    Why Go into Space?
    Yet our solar system is full of resources. The moon is chock-full of valuable metals. So are the asteroids, the small, rocky, planet-like bodies orbiting the sun most of them between Mars and Jupiter. These metals, if we can get them, could be used to build factories and space stations. Also, in space, there is no atmosphere to filter out the sun’s energy. There is plenty of solar energy to be turned into electricity for manufacturing, for creating comfortable living conditions.
    Getting away from Earth has other advantages, too. Modern industry uses many kinds of metal alloys (mixtures of metal that are better for certain purposes than pure metals). Yet some metal alloys either can’t be made or are very expensive to make on Earth because of gravity. For instance, certain metals don’t mix well on Earth. But in zero gravity, molten (hot, liquid) metals mix more evenly. This is because there is no gravity to pull the heavier metals down, while the lighter ones float on top.
    From space, too, we can look down on the Earth and study the atmosphere, its weather, and the effects of air pollution.
    And because there is no strong gravity to break free from, our future homes away from Earth will be convenient starting points for travel to distant planets.
    But, while going into space might solve some problems, outer space can also be a dangerous place. For example, in outer space, we have to protect ourselves from the dangers of ultraviolet light and cosmic rays. Ultraviolet light from the sun can give us bad sunburns right here on Earth. Yet, Earth’s atmosphere screens out most of that harmful radiation. Cosmic rays are tiny high energy particles from outer space. Again, the Earth shields us from most of them.
    At Home in Space?
    But in space, without special protection, we would be exposed to much stronger radiation from ultraviolet light and cosmic rays. Also, in the zero gravity of outer space, our bones will lose calcium and become weaker. This will be more of a problem the longer people stay out in space. Doctors are looking for a way to keep our bones from losing calcium in outer space. And a small spaceship just might "drive you batty" after a while. But even on a short trip in outer space, you might not feel as well as you’d like to. Space travel could make you seasick!
    Yet, these risks won’t keep people from going into space. Eventually, an Earth-like environment will be built in space. And they will be populated by people with many different interests: medicine, construction, farming, teaching, mining, and so on.
    The next hundred years will be filled with other worldly adventures, exciting scientific discoveries, and danger, as humans leave Earth — perhaps forever.
    Aging in Space
    Suppose a space traveler is moving at a velocity of 186,200 miles per second. For every hour that passes for him, 30 hours pass on Earth. If he travels for a year in this fashion (having accelerated instantaneously) and then turns around and comes back at this speed (having turned around instantaneously), he will find that while he has seemed to himself to have traveled two years, the men on Earth would claim he had been absent for 30 years.
    Suppose the space traveler had left at the age of 30, leaving behind a twin brother also aged 30. When he returned he would be 32, but his stay-at-home twin brother would be 60. That is why the "clock paradox", is sometimes called the "twin paradox".
    Of course it takes quite a long while to accelerate to a high speed, and a long while to make a turn and head back again, so conditions aren’t quite as clear-cut as just described.
The reason some metal alloys can’t be made on Earth is that the heavier metals______together with the lighter ones.
选项
答案do not mix well
解析 综合题目的意思：在地球上无法做出金属合金是因为重金属和轻金属不能很好地相互熔合，所以答案是do not mix well。
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