Measuring Fundamental Galactic Parameters with Stellar Tidal Streams and   SIM PlanetQuest
  Extended halo tidal streams from disrupting Milky Way satellites offer new opportunities for gauging fundamental Galactic parameters without challenging observations of the Galactic center. In the roughly spherical Galactic potential tidal debris from a satellite system is largely confined to a single plane containing the Galactic center, so accurate distances to stars in the tidal stream can be used to gauge the Galactic center distance, R_0, given reasonable projection of the stream orbital pole on the X_GC axis. Alternatively, a tidal stream with orbital pole near the Y_GC axis, like the Sagittarius stream, can be used to derive the speed of the Local Standard of Rest (\Theta_LSR). Modest improvements in current astrometric catalogues might allow this measurement to be made, but NASA's Space Interferometry Mission (SIM PlanetQuest) can definitively obtain both R_0 and \Theta_LSR using tidal streams. 
Steven R. Majewski (1), David R. Law (2), Allyson A. Polak (1),   Richard J. Patterson (1) ((1) University of Virginia, (2) ...