The United States requires a logical and practical space power theory that relates to national interests. This monograph explains how activities in the space domain influence US national interests. The relationship between space power and national interests is the basis to present a new space power theory. Further, the proposed space power theory establishes relationships between the activities in the space domain. The space activities identified as critical to the United States'; national security are space exploration, commercial space endeavors, and space enablers. A space protection framework is presented to allow freedom of action within the critical space activities. Previous space power theories are analyzed for both shortfalls and useful ideas. The proposed space power theory applies the Mahan's sea power theory as the framework based on generating national economic prosperity through freedom of action in the space domain.