This paper describes the Polarization Spectroscopic Telescope Array
(PolSTAR), a mission proposed to NASA's 2014 Small Explorer (SMEX) announcement
of opportunity. PolSTAR measures the linear polarization of 3-50 keV
(requirement; goal: 2.5-70 keV) X-rays probing the behavior of matter,
radiation and the very fabric of spacetime under the extreme conditions close
to the event horizons of black holes, as well as in and around magnetars and
neutron stars. The PolSTAR design is based on the technology developed for the
Nuclear Spectroscopic Telescope Array (NuSTAR) mission launched in June 2012.
In particular, it uses the same X-ray optics, extendable telescope boom,
optical bench, and CdZnTe detectors as NuSTAR. The mission has the sensitivity
to measure ~1% linear polarization fractions for X-ray sources with fluxes down
to ~5 mCrab. This paper describes the PolSTAR design as well as the science
drivers and the potential science return.