Once the scientists and engineers agree just on what the spacecraft will do, the mission engineers program the operating instructions into a series of commands that the spacecraft can understand.

Months before launch, hundreds of things are decided -- how much fuel to carry, how the spacecraft can be protected during its journey, and what the scientific instruments will do.

Mission scientists study different things about a planet or moon -- geology, weather, chemistry, and magnetic fields, to name a few. They identify which instruments can best collect the information they need. The scientists and engineers determine how to balance the mission goals against the limited resources on the spacecraft.

When an instrument makes a measurement or observation, it uses power. If the spacecraft needs to maneuver to get a better look at something -- this consumes fuel.

On-board power and fuel are limited. And for Galileo, space on the tape recorder to store scientific data is the most precious resource of all. Everything a spacecraft does is planned in advance for the best use of limited resources.