Messages travel through space as radio waves, just like the radio waves that you receive with a car radio. Each spacecraft has a transmitter and a receiver for radio waves as well as a way of interpreting the information received and acting on it. Radio waves from a spacecraft need to be received on Earth, and are often quite weak when they get there. NASA has huge radio receivers to gather information from space missions. These must be precisely aimed so they can get the waves. Likewise, NASA must precisely aim transmissions to spacecraft so that these ships can hear the messages.