Outer space is of vital importance for numerous civilian and military functions in the modern world. The idea of a space weapon involves something used, intended or designed for employment in, to or from outer space to cause injury or damage to the enemy during an armed conflict. Non-injurious, non-damaging space activities that adversely affect enemy military operations or capacity, though not involving the use of weapons, will nevertheless be methods of warfare. Article III of the Outer Space Treaty makes it clear that international law, including weapons law, applies in outer space. Accordingly, the superfluous injury/unnecessary suffering and indiscriminate weapons principles, the environmental protection rules and the prohibition of the use of the environment as a weapon contrary to the Environmental Modification Convention (ENMOD) all apply to space weapons. Ad hoc rules of weapons law, such as prohibitions against chemical and biological weapons, and the restrictions on the lawful use of particular technologies, also apply to space weapons.
Article IV of the Outer Space Treaty requires States party not to place nuclear or mass destruction weapons in Earth orbit, not to install such weapons on celestial bodies and not to station them in outer space. Testing any weapons on celestial bodies is also forbidden. This does not prohibit: the use of conventional space weapons that have a nuclear power source; the deployment of conventional weapons in outer space; or nuclear or mass destruction weapons entering outer space as part of the trajectory of an inter-continental ballistic missile.
Any State that is acquiring a space weapon must conduct a legal review of that weapon to determine whether the principles and rules of weapons law prohibit the use of that weapon or restrict its circumstances of lawful use. There is no internationally prescribed procedure for the conduct of such reviews and there is no obligation to make the text of any such review public.
The article then discusses how the principles and rules of weapons law are likely to apply to particular space weapons or to particular methods of warfare involving outer space. Kinetic anti-satellite operations will raise concerns, inter alia, as to the space debris that is likely to be caused and its potential to exacerbate the existing space debris problem. With both kinetic anti-satellite operations and missile defense operations the focus of attention in a weapon review is likely to be on the indiscriminate weapons principle. The principles and rules of weapons law are applied to jamming operations and to the use of satellites to detect, track and monitor objects on the earth’s surface, such as ships at sea. Such detection activities, if properly conducted, are generally unlikely to prove legally controversial.
The final method of space warfare that is addressed is the use of ground-based lasers to cause power loss in satellites that pass over a high power laser facility, a technology which, depending on the manner of its employment, may sometimes be difficult to reconcile with adherence to the principle of distinction.
While Article III of the Outer Space Treaty is of vital importance in applying international law to outer space, arguably the maintenance of international peace and security in outer space is most likely to be achieved if a tacit acceptance can be achieved among all States that hostility in outer space is simply unacceptable.