The Stellar Imager (SI): An ultra-high angular resolution ultraviolet/optical observatory

The Stellar Imager (SI) is envisioned as a space-based, ultraviolet-optical interferometer composed of 10 or more one-meter class elements distributed with a maximum baseline of 0.5 km and providing a resolution of 60 micro-arcseconds at 1550 Angstrom. It will image stars and binaries with sufficient resolution to enable long-term studies of stellar magnetic activity patterns and their evolution with time, for comparison with those on 1 he Sun. It will also sound their interiors through astro-seismology to image internal structure, differential rotation, and large-scale circulations. SI will enable us to understand the various effects of magnetic fields of stars, the dynamos that generate these fields, and the internal structure and dynamics of the stars in which these dynamos operate. The ultimate goal of the mission is to achieve the best-possible forecasting of solar activity as a driver of climate and space weather on time scales ranging from months up to decades, and an understanding of the impact of stellar magnetic activity on life in the Universe. The road to that goal will revolutionize our understanding of stars and stellar systems, the building blocks of the Universe. Fitting naturally within the NASA and ESA long-term time lines, SI complements defined missions, and with them will show us entire other solar systems, from the central star to their orbiting planets. In this paper we describe the scientific goals of the mission, the performance requirements needed to address those goals, and the design concepts now under study. We also note other fields of astrophysics that would benefit from a long-baseline interferometer in space and report on the current status of the mission concept development efforts.