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Precision astrometry at microarcsecond accuracy has application to a widerange of astrophysical problems. This paper is a study of the science questionsthat can be addressed using an instrument that delivers parallaxes at about 4microarcsec on targets as faint as V = 20, differential accuracy of 0.6microarcsec on bright targets, and with flexible scheduling. The science topicsare drawn primarily from the Team Key Projects, selected in 2000, for the SpaceInterferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilitiesof this mission to illustrate the importance of the next level of astrometricprecision in modern astrophysics. SIM PlanetQuest is currently in the detaileddesign phase, having completed all of the enabling technologies needed for theflight instrument in 2005. It will be the first space-based long baselineMichelson interferometer designed for precision astrometry. SIM will contributestrongly to many astronomical fields including stellar and galacticastrophysics, planetary systems around nearby stars, and the study of quasarand AGN nuclei. SIM will search for planets with masses as small as an Earthorbiting in the `habitable zone' around the nearest stars using differentialastrometry, and could discover many dozen if Earth-like planets are common. Itwill be the most capable instrument for detecting planets around young stars,thereby providing insights into how planetary systems are born and how theyevolve with time. SIM will observe significant numbers of very high- andlow-mass stars, providing stellar masses to 1%, the accuracy needed tochallenge physical models. Using precision proper motion measurements, SIM willprobe the galactic mass distribution and the formation and evolution of theGalactic halo. (abridged)

Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest