Callisto's craters are mysterious. There is no rain or wind to wear away the craters. You can not really travel that far to find out. However, a space probe is making the trip for you. A probe is like a remote control car or airplane.
Scientists send messages to the computers on the probe telling it what to do.
The probe sends information about what it sees or hears back to the scientists. The probe is named Galileo after the scientist who first saw the four large moons of Jupiter through a telescope. These messages are sent by radio signals. Galileo has two radio antennas - a big one and a small one. After Galileo was up in space, scientists sent the message to open its big antenna.