The Science of the Space Interferometry Mission

NASA is anticipating a 2008-2009 launch for the Space Interferometry Mission, an instrument that will improve on current practice in Optical Astrometry by three orders of magnitude. The scientific returns from such a large gain in capability is hard to imagine. SIM will be able to provide 10% distances to any object in the Galaxy brighter than V=20. Planetary masses will be determined for companions down to a couple Earth masses out to 10pc and to Jupiter masses out to a kiloparsec. A global measurement of the potential of the Galaxy will be made at 20-30 kpc from the center. In short, an astonishing number of fundamental questions will finally be laid to rest. In this talk I will describe the instrument and its main technical constraints. I will then give an overview of the scientific gains we expect to achieve.