Since people first sent rockets into space in the 1950s, we have been leaving behind all sorts of things. Some of them, like the camera, were lost by astronauts while they did work outside their spaceship. But much of the space junk is made up of little pieces of things that were once bigger objects, until they struck each other and broke apart. Some things we send into space fall back toward the earth and burn up in the atmosphere. But larger pieces sometimes survive the extreme heat and hit the ground or the ocean at great speeds. So there is always concern that something may fall from the sky and do some harm. But space junk falling on housetops (屋顶) is not the biggest worry. Scientists are concerned about what happens when an empty rocket strikes another while orbiting (绕轨道飞行) the earth. Two big things become many smaller things. They then hit other things. The pieces get smaller and smaller until they form a cloud of junk that blocks the path of future space vehicles. So what can be done to clean up the space around our planet? Marco Castronuovo, an Italian researcher, talks about sending a satellite into space that would get very close to some of the larger pieces of space junk. The satellite would connect a small rocket to the useless object. When the rocket explodes, it pushes the junk into a lower and slower orbit, nearer the earth. After a time, the junk burns up in the atmosphere. Mr. Castronuovo has proposed (提议) using a number of small satellites with robotic arms. One arm would catch the space junk, and another arm would connect the rocket. He imagines that each satellite would jump from one large piece of junk to another. He thinks this method could destroy about ten large objects each year. Scientists have been concerned about space junk for many years. Right now, the costs of the clean-up have been too great. Mr. Castronuovo says his system could be put in place for a much more reasonable amount of money. 小题1:Much of the space junk _____.
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