Since people first launched   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 spacecraft. 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 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 roofs is not the biggest worry. Scientists are concerned about the “Kessler” problem. Imagine what happens when an empty rocket strikes another while orbiting 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.

Marco Castronuovo, an Italian Space Agency researcher, says launching 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 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 could 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 clean­up have been too great. Mr.Castronuovo says his system could be put in place for a much more reasonable amount of money. So what can be done to clean up the space around our planet?

【小题1】We can learn from the text that much of the space junk ________.

A．was left by the astronauts on purpose

B．is mainly made up of broken spaceship

C．is usually made of small pieces of things

D．burns up before it reaches the atmosphere

【小题2】Now scientists are most worried that space junk will ________.

A．harm the future space vehicles

B．do some harm to the atmosphere

C．fall on people's house roofs

D．destroy the habitat of wildlife

【小题3】It implies in the last but one paragraph that ________.

A．clearing up the space will actually cost little

B．small rockets also do harm to the atmosphere

C．only robots can clear up the space completely

D．it is still difficult for us to clear up the space

【小题4】What is the text mainly about?

A．Space junk threatens earth's orbital environment.

B．Scientists use different methods to clear up space junk.

C．Spacecrafts will meet with a lot of space junk in the future.

D．Marco comes up with many methods to deal with space junk.