Current U.S. policy, including the National Security Strategy and the National Space Policy, declares that access to space has become a vital national interest of the United States. This study examines the justification for this assertion. By examining the definition of vital national interest and U.S. national security policy, this study developed three criteria by which to judge the importance of space-based systems to U.S. national security. These criteria were used to evaluate the contribution of space-based systems to U.S. economic vitality, conventional military capabilities, and physical survival. A numerical model related these criteria, addressing their relative importances by applying different numerical weights to each. This research indicates that the assertion that access to space is a vital national interest of the United States is indeed justified. Denying access to the services provided by space-based systems would have a very negative effect on U.S. economic vitality, conventional military capabilities, and physical survival. The United States should commit as a nation to maintaining leadership in the global space industry and insuring continuity of service from space-based systems in spite of environmental catastrophe or deliberate interference.