  Gaia is the next astrometry mission of the European Space Agency (ESA),
following up on the success of the Hipparcos mission. With a focal plane
containing 106 CCD detectors, Gaia will survey the entire sky and repeatedly
observe the brightest 1,000 million objects, down to 20th magnitude, during its
5-year lifetime. Gaia's science data comprises absolute astrometry, broad-band
photometry, and low-resolution spectro-photometry. Spectroscopic data with a
resolving power of 11,500 will be obtained for the brightest 150 million
sources, down to 17th magnitude. The thermo-mechanical stability of the
spacecraft, combined with the selection of the L2 Lissajous point of the
Sun-Earth/Moon system for operations, allows stellar parallaxes to be measured
with standard errors less than 10 micro-arcsecond (muas) for stars brighter
than 12th magnitude, 25 muas for stars at 15th magnitude, and 300 muas at
magnitude 20. Photometric standard errors are in the milli-magnitude regime.
The spectroscopic data allows the measurement of radial velocities with errors
of 15 km/s at magnitude 17. Gaia's primary science goal is to unravel the
kinematical, dynamical, and chemical structure and evolution of the Milky Way.
In addition, Gaia's data will touch many other areas of science, e.g., stellar
physics, solar-system bodies, fundamental physics, and exo-planets. The Gaia
spacecraft is currently in the qualification and production phase. With a
launch in 2013, the final catalogue is expected in 2021. The science community
in Europe, organised in the Data Processing and Analysis Consortium (DPAC), is
responsible for the processing of the data.
