Science with The Galileo Star Scanner
Galileo at Io. The silvery cone pointing up and right is the star scannerís light shield.
The Star Scanner is the instrument that tells the Galileo spacecraft which way it is pointed. It is something like a combination of a marinerís compass and sextant. Physically, it is a small telescope that is mounted on the spacecraft and looks out at the stars. Some of these stars it recognizes which allows an onboard computer to calculate the direction that the whole spacecraft is pointed. There are sun sensors and gyroscopes that the spacecraft can use to figure out its attitude as well, but the star scanner provides the final and most accurate answer. It was intended to perform this and only this purpose in the mission. However, it has been recognized that the star scanner is capable of producing usable scientific data and so it has taken on additional duties such as studying the trapped radiation belts at Jupiter and measuring the light variations of distant stars.