Formation flying for very high precision astrometry: NEAT and micro-NEAT mission concepts

The nearest solar-type stars are of prime interest for the science of exoplanets because they are the objects most suitable for direct detection and future spectroscopic study of telluric planets. In addition, the habitable zone is a region of special interest for astrobiology and for comparison with our own Earth. These two features point to astrometry as a unique tool for a systematic study of the architectures of planetary systems in that domain because the astrometric signal is larger the nearer the stars and the wider the planetary orbits whereas the other techniques favour smaller orbits. The astrometric technique is highly complementary and better suited for planets in the habitable zone (HZ). Recently, we have proposed two astrometric missions to ESA with different scales, but both of them use flying formation capability for the platform: Nearby Earth Astrometric Telescope (NEAT) as an M-class mission with a 1 m telescope and nNEAT as an S-class mission with a 0.3 m telescope. nNEAT can search and characterise giant planets, i.e., Neptune's mass and heavier, in the HZ around these stars, whereas NEAT can detect even smaller planets down to an Earth mass. The scientific impact of NEAT/nNEAT would be tremendous because they would provide the first exhaustive census of terrestrial/giant planet architecture in the HZ and beyond, around the nearest solar-type stars.