NASA has a need for a simple, low-cost, compact, lightweight and rugged instrument to monitor materials degradation in the atomic oxygen (AO) environment of low Earth orbit (LEO). The purpose of the reported SBIR contract work was to build a space-fly able ellipsometer to meet these needs. The small modulation ellipsometer (SME) built uses polarization modulation and an arrangement of optical elements permitting simultaneous and continuous measurement of the ellipsometric parameters without reference to external calibration or to elaborate alignment procedures. This ellipsometer directly measures the most significant errors in system alignment that perhaps might occur at initial construction or during space launch. This ellipsometer contains no moving parts, requires only low electrical power, can work in vacuum conditions, and could be flight-qualified to be operable in space.