In the early 1970s, there was a small group of advocates for an infrared space telescope; however, the field of infrared astronomy was only a few years old, and no one had ever built a space-based observatory of the required complexity. Considering the technical, political, scientific, and economic uncertainties, it was not obvious that a project like SIRTF could-or should-be dared by NASA. How did SIRTF manage to overcome these uncertainties? This monograph makes visible the invisible forces that influenced the design of SIRTF's innovative technology. The lessons learned by the project team over the course of building SIRTF, now better known as the Spitzer Space Telescope, are about managing innovation over time and in the face of uncertainty.