Microarcsecond Astrometry: Science Highlights from Gaia
  Access to microarcsecond astrometry is now routine in the radio, infrared,
and optical domains. In particular the publication of the second data release
from the Gaia mission made it possible for every astronomer to work with easily
accessible, high-precision astrometry for 1.7 billion sources to 21st magnitude
over the full sky.
  * Gaia provides splendid astrometry but at the limits of the data small
systematic errors are present. A good understanding of the Hipparcos/Gaia
astrometry concept, and of the data collection and processing, provides
insights into the origins of the systematic errors and how to mitigate their
effects.
  * A selected set of results from Gaia highlight the breadth of exciting
science and unexpected results, from the solar system to the distant universe,
to creative uses of the data.
  * Gaia DR2 provides for the first time a dense sampling of Galactic phase
space with high precision astrometry, photometry, and radial velocities,
allowing to uncover subtle features in phase space and the observational HR
diagram.
  * In the coming decade, we can look forward to more accurate and richer Gaia
data releases, and new photometric and spectroscopic surveys coming online that
will provide essential complementary data.
  * The longer term promises exciting new opportunities for microarcsecond
astrometry and beyond, including the plans for an infrared version of Gaia
which would offer the dense sampling of phase space deep into the Milky Way's
nuclear regions.
