The European Space Agency {\Gaia} mission: exploring the Galaxy
  The {\Gaia} astrometric mission was approved by the European Space Agency in
2000 and the construction of the spacecraft and payload is on-going for a
launch in late 2012. {\Gaia} will continuously scan the entire sky for 5 years,
yielding positional and velocity measurements with the accuracies needed to
produce a stereoscopic and kinematic census of about one billion stars
throughout our Galaxy and beyond. The main scientific goal is to quantify early
formation and the subsequent dynamic and chemical evolution of the Milky way.
The stellar survey will have a completeness to $V = 20$ mag, with a precision
of about 25 $\mu$as at 15 mag. The astrometric information will be combined
with astrophysical data acquired through on-board spectrophotometry and
spectroscopy, allowing the chemical composition and age of the stars to be
derived. Data acquired and processed as a result of the {\Gaia} mission are
estimated to amount to about 1 petabyte. One of the challenging problems is the
close relationship between astrometric and astrophysical data, which involves a
global iterative solution that updates instruments parameters, the attitude of
the satellite, and the properties of the observed objects. The European
community is organized to deal with {\Gaia} products: (a) the Data Processing
and Analysis Consortium is a joint European effort in charge of preparation and
execution of data processing, (b) the GREAT network is a platform for
collaboration on the preparation of scientific exploitation.
