In planning the mission, engineers and scientists decide what kinds of instruments ride on board the spacecraft and what kind of information is gathered. They plan the spacecraft's journey by making precise calculations of its path through space.

The spacecraft itself is a complex machine that must operate perfectly in alien environments -- under conditions of intense radiation, and extreme cold and heat. The spacecraft receives commands from mission controllers and sends scientific data back to Earth. A computer on board the spacecraft manages the two-way communications equipment and controls the scientific instruments and the other activities of the spacecraft.

DSN 70 meter antenna at Goldstone, California

NASA tracks missions using a world-wide communications system called the Deep Space Network. Huge antennas --- some nearly as big across as a football field --- capture the faint signals from spacecraft. The signals carry the science data, which must be decoded into information or images. The Deep Space Network also has powerful transmitters to send commands to distant spacecraft.