Stellar Imager (SI) is a space-based, UV/Optical Interferometer (UVOI) with over 200x the resolution of HST. It will
enable 0.1 milli-arcsec spectral imaging of stellar surfaces and the Universe in general and open an enormous new
"discovery space" for astrophysics with its combination of high angular resolution, dynamic imaging, and spectral
energy resolution. SI's goal is to study the role of magnetism in the Universe and revolutionize our understanding of: 1)
Solar/Stellar Magnetic Activity and their impact on Space Weather, Planetary Climates, and Life, 2) Magnetic and
Accretion Processes and their roles in the Origin & Evolution of Structure and in the Transport of Matter throughout the
Universe, 3) the close-in structure of Active Galactic Nuclei and their winds, and 4) Exo-Solar Planet Transits and Disks.
SI is a "Landmark/Discovery Mission" in 2005 Heliophysics Roadmap and a candidate UVOI in the 2006 Astrophysics
Strategic Plan and is targeted for launch in the mid-2020's. It is a NASA Vision Mission and has been recommended for
further study in a 2008 NRC report on missions potentially enabled/enhanced by an Ares V launch. In this paper, we
discuss the science goals and required capabilities of SI, the baseline architecture of the mission assuming launch on one
or more Delta rockets, and then the potential significant enhancements to the SI science and mission architecture that
would be made possible by a launch in the larger volume Ares V payload fairing, and by servicing options under
consideration in the Constellation program.