  With the advent of optical interferometers that will be coming online in the
next decade, radial velocity searches for extra-solar planets will be
complemented by high angular resolution astrometric measurements. In this
paper, we explore the sensitivity of the astrometric technique, and develop an
analytical understanding of the sensitivity in the regime where the orbital
period is longer than the time-baseline of the survey. As in an earlier paper
that dealt with the radial velocity technique, our analysis of the astrometric
technique utilizes both the orbital amplitude and phase.
