MACHO velocities from satellite-based parallaxes

Observation of microlensing events by two satellite telescopes in conjunction with Earth-based observations can measure three of the four physical parameters of the lenses. By making these measurements, one can (1) determine whether the lenses are in fact massive compact halo objects (MACHOs) in the Galactic halo or are objects in a thin or thick disk; (2) determine whether and how fast the halo is rotating; (3) identify individual lensing events that arise from LMC MACHOs and so determine the density, flatness, and rotation speed of the LMC halo; (4) better constrain the distribution of low mass stars that give rise to lensing events toward the Galactic bulge. Earth-based observations alone yield only one combination of the physical parameters, the timescale, ω^-1^ = 4GMD_OL_(1-D_OL_/D_OS_)^1/2^/|v|c Here M and D_OL_ are the mass and distance of the lens, and v represents the two components of its transverse velocity. The distance to the source D_OS_ is known. Observation by two satellites yields in addition the two components of the "reduced" transverse velocity, v = (1- D_OL_/D_OS_)^- 1^v. The telescopes should have apertures of d >~ 0.15 m and should be placed in solar orbit about 0.6 AU and 0.3 AU from the Earth. One or both should have eccentricity e~0.3 and inclination i~20^deg^. A few hours of observations per month on the proposed Space Infrared Telescope Facility could substitute for one of the telescopes. Observation by a single satellite can measure v up to a fourfold ambiguity, including a twofold ambiguity in the reduced speed, v. However, it is possible to design a single-satellite experiment which could give good statistical information about v.