An ESA astrometry satellite launched in 2013 December to measure the distances and motions of over a billion stars down to 20th magnitude, building up a three-dimensional picture of our Galaxy. Gaia contains two identical telescopes with rectangular mirrors 1.45 by 0.5 m, pointing in two directions 106°.5 apart. They focus light to a common focal plane where arrays of CCDs will measure positions, proper motions, and parallaxes. Other detectors perform multicolour photometry of the same stars and measure radial velocities down to 17th magnitude. Distances will be accurate to 10% as far away as the galactic centre. In addition, Gaia is expected to discover large numbers of asteroids, extrasolar planets, brown dwarfs, variable stars, and supernovae. Gaia is a successor to ESA’s Hipparcos mission. It is stationed at the L₂ Lagrangian point of the Sun–Earth system, 1.5 million km from the Earth in the direction away from the Sun. The name originated as an acronym of Global Astrometric Interferometer for Astrophysics. Although Gaia is no longer an interferometer, the name has been retained.

http://sci.esa.int/gaia/ ESA mission website.

http://sci.esa.int/gaia/60169-gaia-s-sky-in-colour/ MILKY WAY SEEN BY GAIA Gaia’s view of the plane of our Galaxy, created from its observations of the positions, brightnesses, and colours of nearly 1.7 billion stars. In places, veins of dust and gas obscure light from the stars behind, creating darker channels along the band of the Milky Way. The two bright patches at the bottom right are the Magellanic Clouds