The Outer Space Treaty (OST), formally the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, is a multilateral treaty that forms the basis of international space law. It was opened for signature on January 27, 1967, and entered into force on October 10, 1967. Created during the Cold War, the treaty's primary objective was to prevent the militarisation of space and the stationing of weapons of mass destruction, thereby solving the problem of an arms race extending beyond Earth.
The treaty establishes that outer space shall be free for exploration and use by all States and is not subject to national appropriation by claim of sovereignty. Crucially, Article IV prohibits placing nuclear weapons or other weapons of mass destruction (WMD) in Earth orbit, on the Moon, or on any other celestial body. The Moon and other celestial bodies must be used exclusively for peaceful purposes, forbidding the establishment of military bases or the testing of any type of weapons on them. States are also held responsible and liable for national space activities and any damage caused by their space objects. The OST is the foundational document for international space law and connects to four subsequent UN treaties, including the Rescue Agreement (1968) and the Liability Convention (1972). While the original provisions remain in effect, the treaty has not been formally amended, and its framework is increasingly challenged by the rise of commercial space activities and issues like space debris, which were not anticipated in 1967.