  SIM PlanetQuest (Space Interferometry Mission) is a space-borne Michelson
interferometer for precision stellar astrometry, with a nine meter baseline,
currently slated for launch in 2015. One of the principal science goals is the
astrometric detection and orbit characterization of terrestrial planets in the
habitable zones of nearby stars. Differential astrometry of the target star
against a set of reference stars lying within a degree will allow measurement
of the target star's reflex motion with astrometric accuracy of 1
micro-arcsecond in a single measurement.
  We assess SIM's capability for detection (as opposed to characterization by
orbit determination) of terrestrial planets in the habitable zones of nearby
solar-type stars. We compare SIM's performance on target lists optimized for
the SIM and Terrestrial Planet Finder Coronograph (TPF-C) missions. Performance
is quantified by three metrics: minimum detectable planet mass, number and mass
distribution of detected planets, and completeness of detections in each mass
range. Finally, we discuss the issue of confidence in detections and
non-detections, and show how information from SIM's planet survey can enable
TPF to increase its yield of terrestrial planets.
