Spinning in space
Gaia’s mission relied on the systematic and repeating observation of star positions in two fields of view. As the detectors repeatedly measured the position of each celestial object, they detected any changes in the object’s motion through space.
To achieve its mission the spacecraft spinned slowly, sweeping its two telescopes across the entire celestial sphere to make four complete rotations per day.
Gaia’s telescopes pointed at two different portions of the sky, separated by a constant 106.5°. Therefore, objects arrived in the second field of view 106.5 minutes after they were observed in the first.
Meanwhile its spin axis precessed around the Sun with a period of about 63 days, allowing different parts of the sky to be scanned. This scanning strategy built up an interlocking grid of positions, providing absolute – rather than relative – values of the stellar positions and motions.
The spacecraft spin axis made an angle of 45° with the Sun direction, ensuring that the payload was shaded from the Sun, but that the solar arrays could still produce electricity efficiently.