Satellites are an important part of our ordinary lives. For example, the information for weather forecasts is sent by satellite. Some satellites have cameras which take photographs of the Earth to show how clouds are moving. Satellites are also used to connect our international phone calls. Computer connections of the World Wide Web and Internet also use satellites. Many of our TV programs come to us through satellites. Airplane pilots also sometimes use a satellite to help them find their exact location. We use satellites to send television pictures from one part of the world to another. They are usually 35,880 kilometers above the equator. Sometimes we can see a satellite in the sky and it seems to stay in the same place. This is because it is moving around the world at 11,000 kilometers an hour—exactly the same speed that the earth rotates. A satellite must orbit the Earth with its antennae (天线) facing the Earth. Sometimes, it moves away from its orbit, so there are little rockets on it which are used to put the satellite back in the right position. This usually happens about every five or six days. Space is not empty! Every week, more and more satellites are sent into space to orbit the Earth. A satellite usually works for about 10-12 years. Satellites which are broken are sometimes repaired by astronauts or sometimes brought back to Earth to be repaired. Often, very old or broken satellites are left in space to orbit the Earth for a very long time. This is very serious because some satellites use nuclear power (核动力) and they can crash into each other.