Gaia scanning the sky
This animation illustrates how Gaia will scan the sky during its all-sky survey.
Gaia is ESA's second space mission dedicated to astrometry. It builds on the legacy of the successful Hipparcos mission (1989-1993). Like Hipparcos, Gaia's observation strategy is based on detecting stellar positions in two fields of view separated by a 'basic angle', which for Gaia is 106.5 degrees. This strategy allows astronomers to establish a coherent reference frame over the entire sky, yielding highly accurate measurements of stellar positions.
The first part of the animation shows Gaia's payload module, where the telescopes, the focal plane and the related electronics are housed. Gaia comprises two telescopes, each consisting of five mirrors (some shared) that focus and repeatedly fold the light over a total distance of 35 m before it reaches the detectors.
The two primary mirrors, which are rectangular, curved and measure 1.45 m × 0.5 m, are mounted on top of the optical bench, or torus. Opposite each primary mirror are the two apertures – windows on the Universe. The subsequent mirrors and other elements in the optical bench are also visible in this animation.
From its orbit around the second Lagrange point (L2) of the Sun-Earth system, Gaia performs an uninterrupted scan of the sky. The spacecraft slowly spins with a period of six hours. The angle between the rotation axis of the spacecraft and the direction to the Sun – also known as the solar aspect angle – measures 45 degrees. The spacecraft spinning axis undergoes a slow precession motion, describing circles around the Sun direction, with an average period of about 63 days.
The combination of spin and precession allows Gaia to scan the entire sky repeatedly over the five years of its nominal mission. The second part of the animation shows Gaia performing a complete all-sky survey.