Anti-satellite weapons that rely on violent impacts and create space debris are regarded as a major threat to the exploration and use of space, including the use of space assets for communications and Earth imaging. This chapter examines two ways in which the testing of such ‘kinetic’ weapons might already have become illegal. First, the accepted interpretation of Article I of the Outer Space Treaty may be evolving as a result of the changing practice of the parties to that treaty. In short, many states are behaving as if tests of anti-satellite weapons that create debris are contrary to the ‘freedom of exploration and use of space’. Second, the same practice and an accompanying opinio juris may be contributing to the development of a parallel rule of customary international law. This chapter also examines the legality of the use of kinetic anti-satellite weapons, as opposed to their testing. Two additional, separate bodies of international law are relevant here: the jus ad bellum governing the recourse to armed force, which includes the right of self-defence, and the jus in bello governing the conduct of armed conflict. A close analysis leads to the conclusion that any use of a kinetic anti-satellite weapon would be illegal today because of the growing crisis of space debris.

The global governance regime for space is grounded in six decades of co-operation between the Soviet Union and then Russia on the one hand, and the United States and its allies on the other.

But continued co-operation is not guaranteed. Following the Russian invasion of Ukraine, some elements of international space co-operation broke down immediately. Other elements, such as the International Space Station, notably continued. More worrying, perhaps, is the heavy reliance of Russian and Ukrainian forces on satellites, including, in the case of Ukraine, satellites owned and operated by private companies. This raises challenging issues of international law, including whether these private satellites are legal targets. It also raises the question of how far Russia might go in this regard. What if it decided to target SpaceX’s Starlink mega-constellation?

Humanity’s ascent into space began in 1929 when the German Army tested its first rocket, the A-1. But while militaries have always accounted for a large portion of human space activity, their use of the space environment has been constrained by a mutual self-interest in preserving access to it for communications, navigation, reconnaissance, weather forecasting, arms control verification and early warning. In 1962, the ‘Starfish Prime’ nuclear test demonstrated that nuclear explosions in space pose a major and indiscriminate threat to satellites. This prompted the United States and the Soviet Union to negotiate the 1963 Limited Test Ban Treaty, which prohibits nuclear tests in space. This chapter addresses such tensions between the expansion of military capabilities in space and the need to keep space free of direct conflict. The chapter highlights the growing need for a treaty to ban the testing of ‘kinetic’ anti-satellite weapons, i.e. weapons that rely on violent impacts to destroy a satellite and thus create space debris. Although Russia tested such a weapon in November 2021, the very next month the United Nations General Assembly created an ‘Open Ended Working Group on Reducing Space Threats through Norms, Rules and Principles of Responsible Behaviours’. The chapter concludes with an examination of the potentially destabilising effects of an imminent extension of military activities to cis-lunar space, the region between Earth and the Moon, including special Moon–Earth orbits.