The Letter of Intent that has been submitted to ESA for its call for a M4 mission.
Summary: The Theia mission will explore the Universe at unprecedented astrometric precision of 0.3μas over a field of about 1 degree. Theia is the divinity of sight and daughter of Gaia. Similarly, the instrument concept carries on the heritage of HIPPARCOS and Gaia missions combined to latest developments in precision metrology control. While giant telescopes and other observatories will do wonders in spectroscopy, imag- ing, photometry, etc Theia will enable science cases unique to μas astrometry, a precision that will reveal the Universe in motion like Earth-like planets orbiting around our immediate stellar neighbors, the activity of the most extreme objects known (black holes and neutron stars) and unveil the local sub-structure of the dark matter halo in which the Milky Way resides. Conceived as an open observatory class mission, Theia will bring ultra-precise astrometry to the broader community, including target-of-opportunity science in the era of Extremely Large Telescopes.