2008PASP..120...38U

Precision astrometry at microarcsecond accuracy has applications for a wide range of astrophysical problems. This paper is a study of the science questions that can be addressed using an instrument with flexible scheduling that delivers parallaxes at about 4 µas on targets as faint as, and differential accuracy of 0.6 µas on bright targets. The science topics are drawn primarily from the team key projects, selected in 2000, for the Space Interferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilities of this mission to illustrate the importance of the next level of astrometric precision in modern astrophysics. SIM PlanetQuest is currently in the detailed design phase, having completed in 2005 all of the enabling technologies needed for the flight instrument. It will be the first space-based long-baseline Michelson interferometer designed for precision astrometry. SIM PlanetQuest will contribute strongly to many astronomical fields, including stellar and galactic astrophysics, planetary systems around nearby stars, and the study of quasar and AGN nuclei. Using differential astrometry SIM PlanetQuest will search for planets with masses as small as Earth orbiting in the ``habitable zone'' around the nearest stars, and could discover many dozen if Earth-like planets are common. It will characterize the multiple-planet systems that are now known to exist, and it will be able to search for terrestrial planets around all of the candidate target stars in the Terrestrial Planet Finder and Darwin mission lists. It will be capable of detecting planets around young stars, thereby providing insights into how planetary systems are born and how they evolve with time. Precision astrometry allows the measurement of accurate dynamical masses for stars in binary systems. SIM PlanetQuest will observe significant numbers of very high- and low-mass stars, providing stellar masses to 1%, the accuracy needed to challenge physical models. Using precision proper-motion measurements, SIM PlanetQuest will probe the Galactic mass distribution, and, through studies of tidal tails, the formation and evolution of the Galactic halo. SIM PlanetQuest will contribute to cosmology through improved accuracy of the Hubble constant. With repeated astrometric measurements of the nuclei of active galaxies, SIM PlanetQuest will probe the dynamics of accretion disks around supermassive black holes, and the relativistic jets that emerge from them.