  We analyze the lightcurve of the microlensing event
OGLE-2003-BLG-175/MOA-2003-BLG-45 and show that it has two properties that,
when combined with future high resolution astrometry, could lead to a direct,
accurate measurement of the lens mass. First, the lightcurve shows clear signs
of distortion due to the Earth's accelerated motion, which yields a measurement
of the projected Einstein radius \tilde r_E. Second, from precise astrometric
measurements, we show that the blended light in the event is coincident with
the microlensed source to within about 15 mas. This argues strongly that this
blended light is the lens and hence opens the possibility of directly measuring
the lens-source relative proper motion \vec\mu_\rel and so the mass
M=(c^2/4G)\mu_\rel t_E \tilde r_E, where t_E is the measured Einstein
timescale. While the lightcurve-based measurement of \tilde r_E is, by itself,
severely degenerate, we show that this degeneracy can be completely resolved by
measuring the direction of proper motion \vec\mu_\rel.
