The article is dedicated to outer space as a space, including the Moon and other celestial bodies, open for exploration and use by all. Celestial bodies are natural resources of the common heritage of humanity. Their exploration and use for the benefit and discovery of all countries is the result of the ownership of all mankind. However, since the end of the 20th century and especially in the 21st century, outer space has gradually become militarized. This is due to changes in the system of warfare, which is likely to be heavily transformed in the coming future. In this transformational system, all domains of warfare will be interlinked and outer space will play a significant role. One example of such processes is the Gulf War, also called the First Space War, in which the US Army successfully used the outer space systems for its Command, Control, Communications, Computers, Intelligence, Surveillance and Reconnaissance (C4ISR) activities. Since then, outer space has become an integral part of US military operations. Consequently, other great powers like Russian Federation and People’s Republic of China are also trying to develop the same capability to counter US dominance in outer space. Simultaneously, the US is continuing its counter-space capabilities to maintain the dominance in outer space. The growing dependence on outer space is not only applicable to the military operation but also to commercial and civilian activities. As a result, great powers are more actively engaging in various space and counter-space activities to pursue their national interests; such activity turns outer space into an arena for inter-state rivalry.