NASA’s Deep Space Network is responsible for tracking, commanding and receiving data from space probes throughout the solar system. Some of the spacecraft are so far away and they use such small transmitters that radio signals received from them are about 20 million times weaker than a watch battery. To receive such weak signals, the Deep Space Network uses huge dish-shaped antennae up to 70 meters across. To dispatch commands to the most remote spacecraft, the same giant dishes are used to send radio signals from immensely powerful, 400-kilowatt transmitters. The Deep Space Network’s antennae are located at three sites – Madrid (Spain), Canberra (Australia), and California (the US). The sites were chosen because they are roughly 120 degrees and a third of the way round the world from each other. As the Earth turns, at least one of the stations is always in contact with a space probe. This ensures a 24-hour observation, with overlapping time to transfer the radio link to the next station. The sites are also surrounded by mountains, which protect them from radio interference.