Precision astrometry with a space-based interferometer

A flexibly-scheduled astrometric interferometer can be used to address a wide range of problems in astrophysics. We use NASA's Space Interferometry Mission (SIM) Lite with microarcsecond accuracy astrometry on targets as faint as V=19 to illustrate the opportunities. SIM Lite can be scheduled to efficiently detect Earth-mass planets around nearby stars, including multiple planet systems, seriously test models of the astrophysics of stars, probe dark matter in our galaxy, and to track changes in the parsec-scale structure of distant active galactic nuclei. A space-based optical interferometer enables microarcsecond precision astrometry of stars, for a wide range of interesting problems in Galactic and stellar astronomy, including planet detection and characterization. The Space Interferometry Mission Lite will be the first space-based Michelson optical interferometer for precision astrometry. In this paper, we briefly summarize the many science applications of this flexibly-scheduled instrument. Details of the design and operation of SIM Lite are covered in other papers in this conference. One of the most important science areas for SIM Lite is the detection and characterization of planets orbiting other stars via the well-known astrometric wobble. With a precision of smaller than one microarcsecond in a single observation, SIM Lite has the capability to detect Earth-like planets around at least 60 nearby stars. This ability to sensitively survey our local stellar neighborhood is a unique opportunity. SIM Lite will be able to characterize multiple-planet systems, which are now known to exist, studying their dynamical properties including long-term stability. Detailed follow-up of the most interesting (perhaps Sun-like) systems is an exciting prospect. Astrometry is complementary to other techniques such as radial velocity, which has already yielded many new planets, because it enables measurement of planetary masses rather than mass lower limits. It will detect small planets around young stars (up to 100 Myr old) to help understand the formation and evolution of planetary systems; these are hard to study other than by astrometry. Thus astrometry permits the study of the nature and evolution of planetary systems in their full diversity, including age, by including young (0.5-100 Myr) solar-type stars. Because it is a pointed instrument, SIM Lite maintains its full astrometric accuracy on targets as faint as V=19, which opens up a range of rare (and therefore distant) stellar types to be observed. Stellar masses and luminosities can be measured to accuracies better than 1%, which is currently hard to do, especially for rare types. Its reach extends to probing dark matter in our Galaxy, and tracking changes in the nuclei of distant active galaxies. SIM Lite will make astrometric measurements by observing a grid of reference stars covering the sky, and make inertial observations of distant quasars; in this frame SIM Lite will deliver positions and parallaxes to better than 4 microarcsecond. SIM Lite uses technologies developed during more than a decade of testbed work and will see application in many future astrophysics missions, so this mission paves the way to the future technically as well as scientifically. The mission is currently in NASA Phase B, and is being considered for full-scale development.