The Interstellar Boundary Explorer (IBEX) is a remarkable mission of exploration and discovery that provides the first global images of the interstellar boundaries and unveils the physics of the heliosphere's interstellar interaction. This investigation fills in the critical missing piece for understanding the connection of our Sun and solar system to the Galaxy, fulfills requirements from recent NASA and NRC plans, makes fascinating connections to astrophysical phenomena, and addresses a serious challenge facing manned exploration by studying the region that shields out the vast majority of galactic cosmic ray radiation. This breakthrough mission is achieved with an innovative mission design using a simple, high heritage science payload and spacecraft. A world-class science, hardware, and management team guarantees that IBEX's low risk approach, coupled with our very large margins, can be comfortably developed within the resources of a Small Explorer.