The NASA Carruthers Geocorona Observatory will provide unprecedented constraints on a key geospace state parameter, exospheric hydrogen density, whose specification is needed for a wide range of scientific investigations (atmospheric evolution, polar wind, ring current decay, plasmaspheric refilling) and for the accurate analysis of data acquired by other sensing systems (ground-based visible airglow sensors, ENA cameras, and X-ray imagers). Historical observations of exospheric density are extremely sparse and physics-based modeling is notoriously challenging, so the Carruthers mission fills a longstanding measurement gap.