  In May 2013, I responded with the present paper to ESA's call for White
Papers for the definition of Large missions. This was half a year before the
launch of ESA's large astrometry mission Gaia. The present proposal for a Gaia
successor mission is similar to Gaia, because two Gaia-like missions at an
interval of about twenty years would provide a foundation for studies of
dynamics in all branches of astronomy from the solar system and stellar systems
to compact galaxies, quasars and dark matter by astrometric data which cannot
be surpassed in the next 50 years.
