  The holy grail of exoplanet searches is an exo-Earth, an Earth mass planet in
the habitable zone around a nearby star. Mass is the most important parameter
of a planet and can only be measured by observing the motion of the star around
the planet-star center of mass. A single image of a planet, however, does not
provide evidence that the planet is Earth mass or that it is in a habitable
zone orbit. The planet's orbit, however, can be measured either by imaging the
planet at multiple epochs or by measuring the position of the star at multiple
epochs by space-based astrometry. The measurement of an exo-planet's orbit by
direct imaging is complicated by a number of factors: (1) the inner working
angle (IWA); (2) the apparent brightness of the planet depending on the orbital
phase; (3) confusion arising from the presence of multiple planets; and (4) the
planet-star contrast. In this paper we address the question: "Can a prior
astrometric mission that can identify which stars have Earthlike planets
significantly improve the science yield of a mission to image exo-Earths?" We
find that the Occulting Ozone Observatory (a small external occulter mission
that cannot measure spectra) could confirm the orbits of ~4 to ~5 times as many
exo-Earths if an astrometric mission preceded it to identify which stars had
such planets. We find that in the case of an internal coronagraph, a survey of
the nearest ~60 stars could be done with a telescope of half the size if an
astrometric mission had first identified the presence of Earth-like planets in
the habitable zone and measured their orbital parameters.
