  The Space Interferometry Mission PlanetQuest Light (or SIM-Lite) is a new
concept for a space borne astrometric instrument, to be located in a solar
Earth-trailing orbit. SIM-Lite utilizes technology developed over the past ten
years for the SIM mission. The instrument consists of two Michelson stellar
interferometers and a precision telescope. The first interferometer chops
between the target star and a set of Reference stars. The second interferometer
monitors the attitude of the instrument in the direction of the target star.
The telescope monitors the attitude of the instrument in the other two
directions.
  SIM-Lite will be capable of one micro-arc-second narrow angle astrometry on
magnitude 6 or brighter stars, relative to magnitude 9 Reference stars in a two
degree field. During the 5 year mission, SIM-Lite would search 65 nearby stars
for planets of masses down to one Earth mass, in the Habitable Zone, which have
orbit periods of less than 3 years. SIM-Lite will also perform global
astrometry on a variety of astrophysics objects, reaching 4.5 micro-arc-seconds
absolute position and parallax measurements. As a pointed instrument, SIM-Lite
will be capable of achieving 8 micro-arc-second astrometric accuracy on 19th
visual magnitude objects and 15 micro-arc-second astrometric accuracy on 20th
visual magnitude objects after 100 hours of integration.
  This paper will describe the instrument, how it will do its astrometric
measurements and the expected performance based on the current technology.
