Space security relates to guaranteed access to space and the ability to freely exploit space for various purposes. Traditionally, space security was defined in military terms in relation to the strategic balance between the United States and the Soviet Union. Since the end of the Cold War, a two-dimensional model of military and environmental dimensions of space security has developed. This, in turn, is beginning to be superseded by a three-sector understanding which distinguishes between the uses of space for security and defense: the security of assets in space against natural and man-made threats and security from threats originating in space. Expanding the definition of space security has advantages but also carries certain risks.