On December 7, 1995 Galileo began its prime mission: a two-year study of the Jovian
system.
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.
Next: Extended Tours - GEM and the Millennium Mission 
