The Single Aperture Large Telescope for Universe Studies (SALTUS) is a mission concept for a far-infrared observatory developed under the recent Astrophysics Probe Explorer opportunity from the National Aeronautics and Space Administration. The enabling element of the program is a 14-m diameter inflatable primary mirror, M1. Due to its importance to SALTUS and potentially other space observatories, we focus entirely on M1. We present a historical overview of inflatable systems, illustrating that M1 is the logical next step in the evolution of such systems. The process of design and manufacture is addressed. We examine how M1 performs in its environment in terms of the operating temperature, interaction with the solar wind, and shape change due to non-penetrating particles. We investigate the longevity of the inflatant in detail, show that it meets mission lifetime requirements with ample margin, and discuss the development and testing to realize the flight M1.