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. 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.