The Gaia mission

Gaia is a cornerstone mission in the science programme of the EuropeanSpace Agency (ESA). The spacecraft construction was approved in 2006,following a study in which the original interferometric concept waschanged to a direct-imaging approach. Both the spacecraft and thepayload were built by European industry. The involvement of thescientific community focusses on data processing for which theinternational Gaia Data Processing and Analysis Consortium (DPAC) wasselected in 2007. Gaia was launched on 19 December 2013 and arrived atits operating point, the second Lagrange point of the Sun-Earth-Moonsystem, a few weeks later. The commissioning of the spacecraft andpayload was completed on 19 July 2014. The nominal five-year missionstarted with four weeks of special, ecliptic-pole scanning andsubsequently transferred into full-sky scanning mode. We recall thescientific goals of Gaia and give a description of the as-builtspacecraft that is currently (mid-2016) being operated to achieve thesegoals. We pay special attention to the payload module, the performanceof which is closely related to the scientific performance of themission. We provide a summary of the commissioning activities andfindings, followed by a description of the routine operational mode. Wesummarise scientific performance estimates on the basis of in-orbitoperations. Several intermediate Gaia data releases are planned and thedata can be retrieved from the Gaia Archive, which is available throughthe Gaia home page.