6.6.2 The following document was updated on 08-04-88 and contains 4991 characters. A R T I F I C I A L S A T E L L I T E S An artificial satellite is a manmade "moon." It circles the Earth in space along a path called an orbit. An artificial satellite may be designed in almost any shape such as a ball, drum, or box. It does not have to be streamlined because there is little or no air where it travels. Most satellites measure only a few feet across; some, such as Echo balloons, may be 100 feet or more in diameter. A satellite's size and shape depend on its purpose. Artificial satellites stay in space for varying lengths of time. The lifetime of each satellite depends on its size and distance from Earth. When a satellite swings close to Earth, it runs into air which slows it. To stay in orbit, a satellite must maintain a certain speed. If it slows below this speed, it plunges into the atmosphere and burns due to friction. The gradual slowing of a satellite by air is called decay. Large, light satellites in low orbits decay rapidly. Small, heavy ones in high orbits decay slowly. The six inch Vanguard I orbits more than 400 miles above the Earth and will orbit for hundreds of years. Every satellite carries some kind of radio transmitter. One kind is called a radio beacon which sends signals that enable engineers to track the satellite. Another kind of transmitter sends scientific information gathered by the satellite's instruments. This sending of information is called telemetry. Some satellites carry equipment to receive signals. Engineers beam signals to these satellites to turn the instruments on and off. Most satellites stop functioning long before they fall to Earth. Their batteries go dead, or their electronic equipment breaks down. They become "silent" and are of no further use. Many silent satellites are circling the Earth. Artificial satellites may be classified according to their purpose: weather, communication, navigation, earth resource, scientific, or military satellites. Weather, or meteorological, satellites help scientists to forecast and study weather. The first of these was called TIROS. These satellites carry television cameras that take pictures of the Earth's surface showing how the clouds move through the atmosphere. They also show snow and ice. By studying the pictures, weathermen can discover and inform people of approaching weather conditions. Communication satellites make it possible to send radio messages, telephone calls, and television programs between distant parts of Earth. These satellites are classified passive or active depending on the way they operate. Passive communication satellites act as a mirror for radio waves. A sending station on Earth beams waves to the satellite, the waves bounce off the satellite, and return to Earth. They reach a receiving station perhaps thousands of miles from the sending station. Active satellites receive signals from sending stations, amplify them, and send them back to Earth. Navigation satellites help pilots and sailors find their exact positions in all kinds of weather. Navigators can use it to find their position much as they would use a star. But instead of looking at the satellite, they listen to its radio signals. Resource satellites are used to monitor Earth's natural resources. This series include such satellites as Landsat and Seasat. Scientific satellites carry a variety of instruments around the world. Some instruments measure radiation; other instruments called magnetometers measure the Earth's magnetic field. Satellites that explore the upper parts of the atmosphere by beaming radio waves down into the atmosphere and then measure the reflected signals are called topside sounders. Orbiting observatories are the largest and most complicated of the scientific satellites. The U.S. uses three types: geophysical, solar, and astronomical. The orbiting geophysical observatory explores space near Earth, and scientists use it to study how the Earth's magnetic field affects energy coming from the sun. The orbiting solar observatory studies the sun by measuring radiation that cannot get through the Earth's atmosphere. The orbiting astronomical observatory looks deep into space at stars and galaxies measuring rays that never reach Earth. The U.S. builds and launches some scientific satellites in cooperation with other countries. The first international satellite, Ariel, carried instruments built by British scientists. The topsider sounder, Alouette, was designed and built by Canadian engineers and launched by the U.S. Military satellites are used for communication and navigation. Reconnaissance satellites can photograph enemy ground forces. Warning satellites can guard against surprised missile attacks. They can discover a missile launch by measuring the heat of the missile rocket exhaust. Since 1963, the U.S. has kept Vela sauullites in space to detect any "sneak testing" of nuclear bombs in space by other nations. ---- NASA Fact Sheet, ARTIFICIAL SATELLITES, Dec. 1984 (36F585) MSFC