  ESA recently called for new "Science Ideas" to be investigated in terms of
feasibility and technological developments -- for technologies not yet
sufficiently mature. These ideas may in the future become candidates for M or L
class missions within the ESA Science Program.
  With the launch of Gaia in December 2013, Europe entered a new era of space
astrometry following in the footsteps of the very successful Hipparcos mission
from the early 1990s. Gaia is the successor to Hipparcos, both of which
operated in optical wavelengths, and Gaia is two orders of magnitude more
accurate in the five astrometric parameters and is surveying four orders of
magnitude more stars in a vast volume of the Milky Way. The combination of the
Hipparcos/Tycho-2 catalogues with the first early Gaia data release will give
improved proper motions over a long ~25 year baseline. The final Gaia solution
will also establish a new optical reference frame by means of quasars, by
linking the optical counterparts of radio (VLBI) sources defining the
orientation of the reference frame, and by using the zero proper motion of
quasars to determine a non-rotating frame.
  A weakness of Gaia is that it only operates at optical wavelengths. However,
much of the Galactic centre and the spiral arm regions, important for certain
studies, are obscured by interstellar extinction and this makes it difficult
for Gaia to deeply probe. Traditionally, this problem is overcome by switching
to the infra-red but this was not possible with Gaia's CCDs. Additionally, to
scan the entire sky and make global absolute parallax measurements the
spacecraft must have a constant rotation and this requires that the CCDs
operate in TDI mode, increasing their complexity.
