GRAVITY: Astrometry on the galactic center and beyond

International audience; We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase. GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at the center of the Milky Way. We have identified the key design features needed to achieve this goal and present the resulting instrument concept. It includes an integrated optics, four-telescope, dual feed beam combiner operated in a cryogenic vessel, near-infrared wavefront sensing adaptive optics, fringe tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches the scientific needs; in particular that 10 muas astrometry is feasible for a source with a magnitude of mK = 15 like Sgr A*, given the availability of suitable phase reference sources.