  Astrometry from space has unique advantages over ground-based observations:
the all-sky coverage, relatively stable, and temperature and gravity invariant
operating environment delivers precision, accuracy and sample volume several
orders of magnitude greater than ground-based results. Even more importantly,
absolute astrometry is possible. The European Space Agency Cornerstone mission
Gaia is delivering that promise. Gaia provides 5-D phase space measurements - 3
spatial coordinates and two space motions in the plane of the sky - for a
representative sample of the Milky way's stellar populations, including over
two billion stars, being about one percent of the stars over about 50 percent
of the radius. Full 6-D phase space data is delivered from Gaia's line-of-sight
(radial) velocities for the 300 million brightest stars. These data make
substantial contributions to astrophysics and fundamental physics on scales
from the Solar System to cosmology. A knowledge revolution is underway.
