Watney says that this means that space is subject to maritime law, which is not entirely correct, but it is an easy way to translate to an unlawful public, which means that no State can claim sovereignty over any natural object in space. Many people have the basic idea that the high seas don't belong to anyone. Everything in space essentially counts as international waters. Starting in 1957, with the space race, nations began to discuss systems to ensure the peaceful use of outer space.
The State Party to the Treaty in whose registry an object launched into outer space is recorded shall retain jurisdiction and control over that object and over all its personnel while it is in outer space or on a celestial body. The Outer Space Treaty is one of the most relevant when it comes to dealing with alleged crimes in space. The Outer Space Treaty also incorporates the Charter of the United Nations as a reference and requires parties to ensure that activities are carried out in accordance with other forms of international law, such as customary international law (the custom and practice of states). In addition to the amount of free time (I listen to the Broncos game in the background), it's not stupid to take the time to analyze this scene and identify, discuss and correct the space law that the film tried to use.
Space, like the high seas, is considered res communis: it belongs to everyone and to no one, nor any country can claim it. It is also unlikely that the Subcommittee will be able to agree to modify the Outer Space Treaty in the near future. In recent years, the Legal Subcommittee has not been able to reach a consensus on the debate on a new global space agreement (the idea of which, however, was only proposed by a few member States). The launch by the Soviet Union in 1957 of the world's first artificial satellite, Sputnik 1, directly prompted the United States Congress to pass the Space Act, thus creating the National Aeronautics and Space Administration (NASA).
First of all, there is no “other treaty that expresses the principle” if it is not in the territory of any country, maritime law applies. The Rescue Agreement, the Liability Convention and the Registration Convention detail the provisions of the Outer Space Treaty. In addition, Watney remains under the jurisdiction of the United States through NASA under Article VIII of the Outer Space Treaty and implicitly has the right to enter the Ares IV return vehicle, which means that he cannot requisition or acquire the Ares IV return vehicle by other means of deception. The Outer Space Treaty gives responsibility for regulating space activities, including the governmental and private sectors, to the individual countries in which the activity takes place.
If commercial space transport becomes widely available, with substantially lower launch costs, all countries will be able to directly benefit from the benefits of space resources. More difficulties arise when trying to define the upper limits of space, since it would require further research on the nature of the universe and the role of the Earth as a planet.