Gravitational Microlensing with the [ITAL]Space Interferometry Mission[/ITAL]

The Space Interferometry Mission ( SIM ), with its launch date planned for 2005, has as its goal astrometry with ~1 μas accuracy for stars as faint as 20 mag. If the SIM lives up to expectations, it can be used to measure astrometric displacements in the light centroid caused by gravitational microlensing in the events detected photometrically from the ground. The effect is typically ~0.1 mas, i.e., 2 orders of magnitude larger than planned for SIM 's accuracy. Therefore, it will be possible to determine the mass, the distance, and the proper motion of almost any MACHO capable of inducing a photometric microlensing event toward the Galactic bulge or the Magellanic Clouds, even though no light from the MACHO has to be detected. For strong microlensing events in which the source is photometrically resolved, like the recent MACHO 95-30 event, SIM 's astrometry combined with accurate ground-based photometry will allow the determination of the angular stellar radii, and therefore the effective temperature of the source. The effective astrometric cross sections for gravitational lensing by nearby high proper-motion stars and brown dwarfs are ~(1'') 2 , and the effective timescales are ~1 yr. SIM will provide the only practical way to measure masses of single nearby objects with ~1% accuracy. The times of lensing events can be predicted years in advance.