The Cassini-Huygens spacecraft, which just had its mission to explore Saturn extended to 2017, is one of the many robotic space probes NASA has used to explore the solar system since 1958. These missions of discovery would not have been possible without the Deep Space Network that provides radio communication and data for all of NASA's interplanetary spacecraft.

The Deep Space Network is operated by the Jet Propulsion Laboratory in Pasadena, Calif. Because spacecraft such as Cassini require continuous communication, there are three communications complexes strategically positioned around Earth: one at Goldstone, Calif.; another near Madrid; and a third near Canberra, Australia.

Each complex consists of several large antenna dishes and ultra-sensitive receiving systems, ranging in size from 70 meters (320 feet) to 11 meters (36 feet). High-power transmitters of nearly a half-million watts tell the spacecraft to turn on computers, activate instruments and make course corrections.

Deep space communications are challenging because of the great distance a signal must travel between the spacecraft and Earth. The spacecraft's small, light communications equipment transmits at power typically limited to 20 watts, about the same as a refrigerator light bulb. Signal power arrives at the antenna at extremely low wattage, about 20 billion times weaker than the power required for a digital wristwatch.

While on its mission, the spacecraft sends images, data about its health and science information from its instruments in a stream of digitally coded numbers called a bitstream to the huge antenna receivers at any one of the three Deep Space Network sites around the globe. The data streams are relayed using microwave links, communications satellites, land lines and submarine cables to the Jet Propulsion Laboratory. High-speed computers at JPL then transform these streams of numbers into meaningful information.

The most exciting and best-known results from the missions are the spectacular pictures the spacecraft sends back to Earth.

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