Gaia: the Galaxy in six (and more) dimensions
  The ESA cornerstone mission Gaia was successfully launched in 2013, and is
now scanning the sky to accurately measure the positions and motions of about
two billion point-like sources of 3<V<20.5 mag, with the main goal of
reconstructing the 6D phase space structure of the Milky Way. The typical
uncertainties in the astrometry will be in the range 30-500 muas. The sky will
be repeatedly scanned (70 times on average) for five years or more, adding the
time dimension, and the Gaia data are complemented by mmag photometry in three
broad bands, plus line-of-sight velocities from medium resolution spectroscopy
for brighter stars. This impressive dataset is having a large impact on various
areas of astrophysics, from solar system objects to distant quasars, from
nearby stars to unresolved galaxies, from binaries and extrasolar planets to
light bending experiments. This invited review paper presents an overview of
the Gaia mission and describes why, to reach the goal performances in
astrometry and to adequately map the Milky Way kinematics, Gaia was also
equipped with state-of-the-art photometers and spectrographs, enabling us to
explore much more than the 6D phase-space of positions and velocities.
Scientific highlights of the first two Gaia data releases are briefly
presented.
