Geosynchronous Microlens Parallaxes
  I show that for a substantial fraction of planets detected in a space-based survey, it would be possible to measure the planet and host masses and distances, if the survey satellite were placed in geosynchronous orbit. Such an orbit would enable measurement of the microlens parallax, \pi_e, for events with moderately low impact parameters, \beta <~ 0.05, which encompass a disproportionate share of planetary detections. Most planetary events yield a measurement of the angular Einstein radius \theta_e. Since the host mass is given by M=\theta_e/\kappa\pi_e where \kappa\ is a constant, parallax measurements are the crucial missing link. I present simple analytic formulae that enable quick error estimates for observatories in circular orbits of arbitrary period and semi-major axis, and arbitrary orientation relative to the line of sight. The method requires photometric stability of ~1e-4 on ~1 orbit timescales. I show that the satellite data themselves can provide a rigorous test of the accuracy of the parallax measurements in the face of unknown systematics and stellar variability, even at this extreme level. 
Andrew Gould (Ohio State University)
astro-ph.EP