The Space Interferometry Mission

The Space Interferometry Mission (SIM) will be 10-m Michelson interferometer in Earth-trailing solar orbit. AS a part of NASA's Origins program, it will be the first space-based optical interferometer designed for precision astrometry. As well as enabling forefront astronomical science, SIM will serve as a technology pathfinder for future Origins missions, such as the Terrestrial Planet Finder (TPF). SIM is being designed 4 microarcsecond (pas) precision absolute position measurements of stars down to 20 magnitude. Launch is currently planned for mid-2006, and the mission duration is 5 years. By searching for the astrometric 'wobble' signature caused by a companion to a target star, SIM will be very sensitive to sub-stellar and planetary companions. By observing stars relative to nearby reference stars, an accuracy of 1 microarcsecond (pas) in a l-hour measurement is expected, which is sufficient to detect planets of less than about three Earth masses around the nearest stars. With precision global astrometry, SIM allows parallax distance measurements to 10% accuracy, even on the far side of the Galaxy. During its 5-year mission, SIM will address a important variety of science questions relating to the formation and dynamics of our Galaxy. In addition to astrometry, SIM will demonstrate the technique of imaging using aperture synthesis. While this is routinely used by ground-based radio telescopes, a space-based optical synthesis imaging demonstration is the first step toward a later generation of large imaging telescopes. SIM will have resolution of 10 milliarcsec in the V band. It will also perform a demonstration of interferometric nulling, in which the on-axis starlight is suppressed to a level of a key requirement for TPF. In this paper we outline the design of the SIM instrument, and how it will be used in the search for extrasolar planets. We also briefly describe some selected topics from the SIM astrometric science program.