Galileo travels around Jupiter in elongated ovals --- each orbit lasts about two months. By traveling at different distances from Jupiter, Galileo can sample different parts of the planet's extensive magnetosphere. The orbits are designed for close-up flybys of Jupiter's largest moons.

To keep track of Galileo's journey, each orbit is numbered, and named for the moon that the spacecraft encountered at closest range. During orbit "C-3" for example --- the third orbit around Jupiter --- Galileo flew near the moon Callisto.

The data collected on Jupiter and the moons are stored on the on-board tape recorder. During the rest of the orbit, the data are sent to Earth using the low-gain antenna. At the same time, measurements are made of Jupiter's magnetosphere and transmitted back to Earth.

The intriguing data gathered during the eleven orbits of the prime mission left many questions to be answered. Since Galileo obviously was capable of much more, its mission was extended.