Absolute astrometry in the next 50 years
  With Gaia in orbit since December 2013 it is time to look at the future of fundamental astrometry and a time frame of 50 years is needed in this matter. A space mission with Gaia-like astrometric performance is required, but not necessarily a Gaia-like satellite. It should be studied whether this can be obtained within the budget of a medium-size ESA mission. A dozen science issues for a Gaia successor mission in twenty years, with launch about 2033, are presented and in this context also other possibilities for absolute astrometry with milliarcsecond (mas) or sub-mas accuracies are discussed. The three powerful techniques: VLBI, the MICADO camera on the E-ELT, and the LSST are described and documented by literature references and by an extensive correspondence with leading astronomers who readily responded with all the information I needed. In brief, the two Gaia-like missions would provide an astrometric foundation for all branches of astronomy from the solar system and stellar systems, including exo-planet systems, to compact galaxies, quasars and dark matter (DM) substructures by data which cannot be surpassed in the next 50 years. - In April 2017 ESA selected our proposal Hobbs et al. (2016) for study of a detector with NIR sensitivity for a Gaia successor mission, called GaiaNIR. 
Erik H{\o}g (Niels Bohr Institute, Copenhagen)
astro-ph.IM