First Space-based Microlens Parallax Measurement of an Isolated Star: Spitzer Observations of OGLE-2014-BLG-0939

We present the first space-based microlens parallax measurement of an isolated star. From the striking differences in the lightcurve as seen from Earth and from Spitzer (∼ 1 AU to the west), we infer a projected velocity {{\tilde{v}}_hel}∼ 250 km {{s}^-1}, which strongly favors a lens in the Galactic Disk with mass M=0.23+/- 0.07 {{M}_⊙ } and distance {{D}_L}=3.1+/- 0.4 kpc. An ensemble of such measurements drawn from our ongoing program could be used to measure the single-lens mass function including dark objects, and also is necessary for measuring the Galactic distribution of planets since the ensemble reflects the underlying Galactic distribution of microlenses. We study the application of the many ideas to break the four-fold degeneracy first predicted by Refsdal 50 years ago. We find that this degeneracy is clearly broken, but by two unanticipated mechanisms: a weak constraint on the orbital parallax from the ground-based data and a definitive measurement of the source proper motion.