Over the past few years, there have been rumblings out of SpaceX about the usefulness of nuclear-powered spacecraft. Given that SpaceX wants to send its rockets to Mars, they believe a nuclear-powered upper stage is an ideal way to accomplish this, and many manned Mars mission proposals have included the use of NERVA, a nuclear thermal engine developed by NASA in the 1960s, it’s plausible that a nuclear-powered Falcon XX rocket might wing its way to the Red Planet one day. I know this statement breaks my engineer friends’ hearts but, assuming that the engineering problems with a nuclear-powered heavy lift vehicle could be overcome, there are still sizable obstacles in the path of our hypothetical rocket, specifically legal obstacles. The 1972 Liability Convention affects the planning and execution of virtually all space launches but has additional requirements for spacecraft carrying nuclear material. The 1992 UN Resolution on Nuclear Power Sources (“Nuclear Power Principles”) lays out guidelines and criteria for nuclear safety aboard spacecraft using nuclear material for “non-propulsive purposes.” What are these additional roadblocks in the path of our hypothetical nuclear rocket?
The 1972 Liability Convention
The Convention on International Liability for Damage Caused by Space Objects, also known as the Liability Convention, is one of the Big Four space law treaties. 90 countries, including the United States, China, and Russia have ratified or acceded to this treaty and an additional 23 countries have signed it. The Liability Convention holds launching states strictly liable for damage to other nations caused by the launching state’s space object falling to Earth. The launching state is held liable regardless of what type of entity actually launched the spacecraft. For example, if ULA launched a private commercial satellite on a Delta IV and that satellite malfunctioned, crashing in Nepal, the United States would be responsible for paying for the damage to Nepal, including loss of life, personal injury, and damage to private and public property. In most cases the US taxpayer might avoid being on the hook for the entire cost of the damage. US law requires launch services providers to carry insurance covering precisely these kinds of accidents. Without the insurance, a launch services provider is not licensed to operate.
Under the Liability Convention, outer space collisions are handled like automobile collisions; responsibility for damage is assigned based on the fault of the parties involved.
In the case of a potential nuclear-powered spacecraft, the launching state and the actual launch services provider (e.g. SpaceX) face unique challenges in determining the scope of potential damage, due to the lasting effects of radioactive contamination. They would likely attempt to determine the safest flight profile in order to minimize potential damage to nations and people alike and implement safety measures like those found on Cassini. Cassini carried 33 kilograms of plutonium spread among 18 modular units, each with its own heat shield and impact shell. These containers were tested to destruction found to release plutonium only in much harsher conditions than were likely to be present during launch.
The 1992 UN Nuclear Power Principles resolution
UN Resolutions are not explicitly intended to be binding on member nations however many resolutions are arguably part of “customary” international law. Customary international law can be thought of as gentleman’s agreements between nations; countries follow these rules without explicitly drafting up a document saying they are bound to follow them. Customary international law usually has some weight in courts around the world, even in countries which were not part in the original decision making process (e.g. a country that wasn’t a member of the UN at the time it adopts a particular resolution). In the case of the Nuclear Power Principles, countries launching nuclear-powered vehicles have generally complied with the Resolution’s directives. For example, the Nuclear Power Principles requires a launching state to notify the UN of an impending launch of a nuclear-powered spacecraft. In accordance with the Resolution, the United States notified the UN of Cassini’s launch and provided a safety assessment of the vehicle.
The Nuclear Power Principles resolution lays out specific guidance for the use of nuclear power sources in space. Notably, this resolution only lays down operating principles for non-propulsive uses of nuclear power sources therefore it would arguably apply to spacecraft like Cassini or the New Horizons probe, but not to NERVA-propelled rockets! The Nuclear Power Principles directs that “nuclear power sources in outer space shall be restricted to those space missions which cannot be operated by non-nuclear energy sources in a reasonable way.” Of those missions that require a nuclear power source, the nuclear power source must be designed to avoid going critical and/or releasing radioactive material in the event of malfunction, explosion, re-entry and water or land impact. Additionally, all satellites with nuclear reactors must carry systems which can place the satellite in a graveyard orbit which will not decay before radioactivity of the nuclear material has been reduced to that of actinides. SNAP-10A, the only US satellite launched equipped with a nuclear reactor, placed its nuclear reactor in an orbit with an expected duration of 4000 years at the end of its operational life.