Satellite Parallaxes of Lensing Events Towards the Galactic Bulge

In order to understand the nature of the lenses which generate microlensing events, one would like to measure their mass, distance, and velocity. Unfortunately, current microlensing experiments measure only one parameter of the events, the characteristic timescale, which is a combination of the underlying physical parameters. Other methods are required to extract additional information. Parallax measurements using a satellite in an Earth-like orbit yield the projected velocity of the lens: ${\bf{\tilde v}} ={\bf{v}}/(1-z)$, where ${\bf{v}}$ is the transverse velocity of the lens relative to the Earth-source line of sight, and $z$ is the ratio of the distances to the lens and the source. A measurement of ${\bf{\tilde v}}$ could distinguish between lenses belonging to the bulge and disk populations. We show that for photometric precisions of 1\% to 2\%, it is possible to measure the projected speed, $\tilde v$, to an accuracy of $\leq 10\%$ for over 70\% of disk lenses and over 60\% of bulge lenses. For measuring the projected velocity ${\bf{\tilde v}}$, the percentages are 40\% and {30\%}, respectively. We find lines of sight $> 2^{\circ}$ away from the ecliptic are preferable, and an Earth-satellite separation in the range $0.7 {\rm{AU}} - 1.9{\rm{AU}}$ is optimal.