  HaloSat is a small satellite (CubeSat) designed to map soft X-ray oxygen line
emission across the sky in order to constrain the mass and spatial distribution
of hot gas in the Milky Way. The goal of HaloSat is to help determine if hot
gas gravitationally bound to individual galaxies makes a significant
contribution to the cosmological baryon budget. HaloSat was deployed from the
International Space Station in July 2018 and began routine science operations
in October 2018. We describe the goals and design of the mission, the on-orbit
performance of the science instrument, and initial observations.
