  I investigate the prospects for using the Space Interferometry Mission (SIM)
to measure the masses of nearby stars from their astrometric deflection of more
distant sources, as originally suggested by Paczynski and by Miralda-Escude. I
derive an analytic expression for the total observing time T_tot required to
measure the masses of a fixed number of stars to a given precision. I find that
T_tot ~ r_max^{-2}, where r_max is the maximum radius to which candidates are
searched, or T_tot ~ \mu_min^2, where \mu_min is the minimum proper motion to
which candidates are searched. I show that T_tot can be reduced by a factor 4
if source availability is extended from V_s=17 to V_s=19. Increasing r_max and
V_s and decreasing \mu_min all require a significantly more agressive approach
to finding candidates. A search for candidates can begin by making use of the
Luyton proper motion catalog together with the USNO-A2.0 all-sky astrometric
catalog. However, a thorough search would require the all-sky USNO-B
proper-motion catalog which is not yet available. The follow-up observations
necessary to prepare for the mission will become more difficult the longer they
are delayed because the candidate pairs are typically already within 1" and are
getting closer.
