The Space Interferometry Mission (SIM) willbe the fastspace-based long baselineMichelsoninterferometer designed forprecision astrometry.It will addressa wide range ofproblemsinstellar astrophysics and Galactic structure, delivering precision astrometryof starsdown to 20 magnitude throughout the entire Galaxy.SIM usesa 10-m Michelsoninterferometer in Earth-trailing solarorbitto provide 4 microarcsecond(ttas) precision astrometry. With a 5-yearmissionlifetime, SIM will be a

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... ing planetsaround nearbystars, through detection of thestellar reflex motion.The astrometric method complements theradial velocity techniquewhich as alreadyyieldedmany new planets, with an important benefit ofdirectly measuringplanetarymasses.SIM willhave a single-measurement precision of 1 ttas in a frame defined by nearby reference stars, enabling searches for planets with masses as small as a few earth masses around the nearest stars. More massive planets will be detectable to much larger distances. In addition to precision astrometry SIM will also serve an important role as a technology precursor for future astrophysics missions using interferometers. Two technologies demonstrated. will be high dynamic-range aperture synthesis imaging at lO-milliarcsec resolution in the optical, and fringe nulling to 10 -4.