  I compare an aggressive ground-based gravitational microlensing survey for
terrestrial planets to a space-based survey. The Ground-based survey assumes a
global network of very wide field-of-view ~2m telescopes that monitor fields in
the central Galactic bulge. I find that such a space-based survey is ~100 times
more effective at detecting terrestrial planets in Earth-like orbits. The poor
sensitivity of the ground-based surveys to low-mass planets is primarily due to
the fact that the main sequence source stars are unresolved in ground-based
images of the Galactic bulge. This gives rise to systematic photometry errors
that preclude the detection of most of the planetary light curve deviations for
low mass planets.
