  We show that space-based microlensing experiments can recover lens masses and
distances for a large fraction of all events (those with individual photometric
errors <~ 0.01 mag) using a combination of one-dimensional microlens parallaxes
and astrometric microlensing. This will provide a powerful probe of the mass
distributions of planets, black holes, and neutron stars, the distribution of
planets as a function of Galactic environment, and the velocity distributions
of black holes and neutron stars. While systematics are in principle a
significant concern, we show that it is possible to vet against all systematics
(known and unknown) using single-epoch precursor observations with the Hubble
Space Telescope roughly 10 years before the space mission.
