  In the context of the ESA M5 (medium mission) call we proposed a new
satellite mission, Theia, based on relative astrometry and extreme precision to
study the motion of very faint objects in the Universe. Theia is primarily
designed to study the local dark matter properties, the existence of Earth-like
exoplanets in our nearest star systems and the physics of compact objects.
Furthermore, about 15 $\%$ of the mission time was dedicated to an open
observatory for the wider community to propose complementary science cases.
With its unique metrology system and "point and stare" strategy, Theia's
precision would have reached the sub micro-arcsecond level. This is about 1000
times better than ESA/Gaia's accuracy for the brightest objects and represents
a factor 10-30 improvement for the faintest stars (depending on the exact
observational program). In the version submitted to ESA, we proposed an optical
(350-1000nm) on-axis TMA telescope. Due to ESA Technology readiness level, the
camera's focal plane would have been made of CCD detectors but we anticipated
an upgrade with CMOS detectors. Photometric measurements would have been
performed during slew time and stabilisation phases needed for reaching the
required astrometric precision.
