  Of all planet-finding techniques, microlensing is potentially the most
sensitive to Earth-mass planets. However, microlensing lightcurves generically
yield only the planet-star mass ratio: the mass itself is uncertain to a factor
of a few. To determine the planet mass, one must measure both the ``microlens
parallax'' and source-lens relative proper motion. Here we present a new method
to measure microlens masses for terrestrial planets. We show that, with only a
modest adjustment to the proposed orbit of the dedicated satellite that finds
the events, and combined with observations from a ground-based observing
program, the planet mass can be measured routinely. The dedicated satellite
that finds the events will automatically measure the proper motion and one
projection of the ``vector microlens parallax.'' If the satellite is placed in
an L2 orbit, or a highly elliptical orbit around the Earth, the Earth-satellite
baseline is sufficient to measure a second projection of the vector microlens
parallax from the difference in the lightcurves as seen from the Earth and the
satellite as the source passes over the caustic structure induced by the
planet. This completes the mass measurement.
