  GRAVITY is an adaptive optics assisted Beam Combiner for the second
generation VLTI instrumentation. The instrument will provide high-precision
narrow-angle astrometry and phase-referenced interferometric imaging in the
astronomical K-band for faint objects. We describe the wide range of science
that will be tackled with this instrument, highlighting the unique capabilities
of the VLTI in combination with GRAVITY. The most prominent goal is to observe
highly relativistic motions of matter close to the event horizon of Sgr A*, the
massive black hole at center of the Milky Way. We present the preliminary
design that fulfils the requirements that follow from the key science drivers:
It includes an integrated optics, 4-telescope, dual feed beam combiner operated
in a cryogenic vessel; near-infrared wavefrontsensing adaptive optics;
fringe-tracking on secondary sources within the field of view of the VLTI and a
novel metrology concept. Simulations show that 10 {\mu}as astrometry within few
minutes is feasible for a source with a magnitude of mK = 15 like Sgr A*, given
the availability of suitable phase reference sources (mK = 10). Using the same
setup, imaging of mK = 18 stellar sources in the interferometric field of view
is possible, assuming a full night of observations and the corresponding UV
coverage of the VLTI.
