For some applications, polar orbits, which pass near both of the earth's poles, are preferred. In a polar orbit, a satellite will periodically pass directly over every point on the earth's surface. 4 Telecommunications Control over unmanned space probes and artificial satellites is maintained from the ground at control centers, where huge electronic computers analyze data and determine the exact moment when a change should be made. These instructions are relayed to the spacecraft by signals carried on certain radio frequencies. Instruments inside the craft also use radio signals to send data back to earth.
Tracking is the continuous reporting of a satellite's or space probe's position in space.
Control includes the overall direction of a spacecraft to achieve the intended trajectory. Rocket propulsion systems vary from the tiny Aerobee sounding rocket to the giant Saturn V used in the Apollo project. For interplanetary flights, propulsion by nuclear or solar energy may be possible. Also being considered are ion and photon engines, which very efficiently provide low thrust that can build up very high velocity during a long flight. Landing on the earth or any planet with a significant atmosphere raises the problem of atmospheric friction, which can instantly burn up any spacecraft.