  The Space Interferometry Mission (SIM) is expected to make precise
astrometric measurements that can be used to detect low mass planets around
nearby stars. Since most nearby stars are members of multiple star systems,
many stars will have a measurable acceleration due to their companion, which
must be included when solving for astrometric parameters and searching for
planetary perturbations. Additionally, many of the stars with one radial
velocity planet show indications of additional planets. Therefore, astrometric
surveys like SIM must be capable of detecting planets and measuring orbital
parameters in systems with multiple stellar and/or planetary companions. We
have conducted Monte Carlo simulations to investigate how the presence of
multiple companions affects the sensitivity of an astrometric survey such as
SIM. We find that the detection efficiency for planets in wide binary systems
is relatively unaffected by the presence of a binary companion, if the
planetary orbital period is less than half the duration of the astrometric
survey. For longer orbital periods, there are significant reductions in the
sensitivity of an astrometric survey. Additionally, we find that the signal
required to detect a planet can be increased significantly due to the presence
of an additional planet orbiting the same star. Fortunately, adding a modest
number of precision radial velocity observations significantly improves the
sensitivity for many multiple planet systems. Thus, the combination of radial
velocity observations and astrometric observations by SIM will be a
particularly valuable for studying multiple planet systems.
