Nuclear power in space
Nuclear power in space is the use of nuclear power onboard spacecraft, for electricity, heat, or propulsion. The most common type is a radioisotope thermoelectric generator, which has been used on satellites, space probes and on the crewed Apollo missions to the Moon. Small nuclear fission reactors for Earth satellites have also been flown, by the Soviet US-A program and American SNAP-10A. Radioisotope heater units are also used to prevent components from becoming too cold to function.
Among nuclear power systems launched into space, plutonium-238 is the most common radioisotope fuel. Its half-life of 87.7 years allows RTGs to power spacecraft consistently for decades at electric outputs of hundreds of watts. Nuclear power systems function independently of sunlight, which is advantageous for Mars and outer Solar System exploration. RTGs used on Mars missions include the Curiosity and Perseverance rovers and Viking landers. All spacecraft leaving the Solar System, i.e., Pioneer ''10 and 11, Voyager 1'' and 2, and New Horizons use NASA RTGs, as did the outer planet missions of Galileo, Cassini, and Ulysses. However, due to costs, the global shortage of plutonium-238, and advances in solar-cell efficiency, more recent Jupiter missions have opted for solar arrays. NASA also developed the Advanced Stirling radioisotope generator.
Spacecraft nuclear fission reactors saw limited experimental use during the Cold War. They can achieve kilowatt electric outputs, approximately an order of magnitude more than RTGs, but this comes with more complex engineering, cost, and hazards. The United States tested the first and its only space reactor aboard SNAP-10A for 43 days in 1965. The Soviet Union's US-A program tested the BES-5 and TOPAZ-I reactors in space, as well as developing Romashka and TOPAZ-II reactors. At five kilowatts of electric power, TOPAZ-I was the most powerful known nuclear system in space. Both countries' satellites also used electrically-powered ion thrusters, making them the only experimental uses of nuclear electric propulsion. The Soviet program caused issues with contamination from Kosmos 954's disintegration over Canada, persistent space debris from its sodium-potassium coolant, and radiation interfering with gamma ray telescopes. NASA also developed the Kilopower reactor.
Aside from nuclear electric propulsion, more powerful nuclear space propulsion systems have been developed and undergone ground testing. Nuclear thermal rockets typically used large reactors with liquid hydrogen as both coolant and propellant, and achieved specific impulses twice those of chemical rocket engines. The United States carried out ground test-firings in the 1960s under Project Rover and NERVA, while the Soviet Union's RD-0410 was ground-tested in 1985. Speculative systems include nuclear pulse propulsion, pulsed nuclear thermal rockets, and nuclear fusion propulsion, and were explored by Project Orion, Project Daedalus, and Project Longshot.
Hazards and regulations
Hazards
After the ban of nuclear weapons in space by the Outer Space Treaty in 1967, nuclear power has been discussed at least since 1972 as a sensitive issue by states. Space nuclear power sources may experience accidents during launch, operation, and end-of-service phases, resulting in the exposure of nuclear power sources to extreme physical conditions and the release of radioactive materials into the Earth's atmosphere and surface environment. For example, all Radioisotope Power Systems used in space missions have utilized Pu-238. Plutonium-238 is a radioactive element that emits alpha particles. Although NASA states that it exists in spacecraft in a form that is not readily absorbed and poses little to no chemical or toxicological risk upon entering the human body, it cannot be denied that it may be released and dispersed into the environment, posing hazards to both the environment and human health. Pu-238 primarily accumulates in the lungs, liver, and bones through inhalation of powdered form, thereby posing risks to human health.Accidents within the atmosphere
There have been several environmental accidents related to space nuclear power in history.In 1964, a Thor-Ablestar rocket carrying the Transit 5BN-3 satellite failed to reach orbit, destroying the satellite in re-entry over the southern hemisphere. Its one kilogram of plutonium-238 fuel within the SNAP-9A Radioisotope Thermoelectric Generator was released into the stratosphere. A 1972 Department of Energy soil sample report attributed 13.4 kilocuries of Pu-238 to the accident, from the one kilogram's 17 kilocuries total. This was contrasted to the 11,600 kilocuries of strontium-90 deposited by all nuclear weapons testing.
In May 1968, a Thor-Agena rocket carrying the Nimbus B satellite was destroyed by a guidance error. Its plutonium SNAP-19 RTG was recovered intact, without leakage from the Pacific sea floor, refurbished, and flown on Nimbus 3.
In April 1970, the Apollo 13 lunar mission was aborted due to an oxygen tank explosion in the spacecraft's service module. Upon reentering the atmosphere, the lunar module equipped with the SNAP-27 RTG exploded and crashed into the South Pacific Ocean, with no leakage of nuclear fuel. This is the only intact flown nuclear system that remains on Earth without recovery.
In early 1978, the Soviet spacecraft Kosmos 954, powered by a 45-kilogram highly enriched uranium reactor, went into an uncontrolled descent. Due to the unpredictable impact point, preparations were made for potential contamination of inhabited areas. This event underscored the potential danger of space objects containing radioactive materials, emphasizing the need for strict international emergency planning and information sharing in the event of space nuclear accidents. It also led to the intergovernmental formulation of emergency protocols, such as Operation Morning Light, where Canada and the United States jointly recovered 80 radioactive fragments within a 600-kilometer range in the Canadian Northwest Territories. COSMOS 954 became the first example for global emergency preparedness and response arrangements for satellites carrying nuclear power sources.
NaK droplet debris
The majority of nuclear power systems launched into space remain in graveyard orbits around Earth. Between 1980 and 1989, the BES-5 and TOPAZ-I fission reactors of the Soviet RORSAT program suffered leakages of their liquid sodium–potassium alloy coolant. Each reactor lost on average 5.3 kilograms of its 13 kilogram total coolant, totaling 85 kilograms across 16 reactors. A 2017 ESA paper calculated that, while smaller droplets quickly decay, 65 kilograms of coolant still remain in centimeter-sized droplets around 800 km altitude orbits, comprising 10% of the space debris in that size range.Trapped-positron problem
Orbital fission reactors are a source of significant interference for orbital gamma ray observatories. Unlike RTGs which largely rely on energy from alpha decay, fission reactors produce significant gamma radiation, with the uranium-235 chain releasing 6.3% of its total energy as prompt and delayed gamma rays:Pair production occurs as these gamma rays interact with reactor or adjacent material, ejecting electrons and positrons into space:
These electrons and positrons then become trapped in the magnetosphere's flux tubes, which carry them through a range of orbital altitudes, where the positrons can annihilate with the structure of other satellites, again producing gamma rays:
These gamma rays can interfere with satellite instruments. This most notably occurred in 1987, when the TOPAZ-I nuclear reactors aboard the twin RORSAT test vehicles Kosmos 1818 and Kosmos 1867 affected the gamma ray telescopes aboard NASA's Solar Maximum Mission and the University of Tokyo/ISAS' Ginga. TOPAZ-I remains the most powerful fission reactor operated in space, with previous Soviet missions using the BES-5 reactor at altitudes well below gamma ray observatories.
Regulations
National regulations
The presence of space nuclear sources and the potential consequences of nuclear accidents on humans and the environment cannot be ignored. Therefore, there have been strict regulations for the application of nuclear power in outer space to mitigate the risks associated with the use of space nuclear power sources among governments.For instance, in the United States, safety considerations are integrated into every stage of the design, testing, manufacturing, and operation of space nuclear systems. The NRC oversee the ownership, use, and production of nuclear materials and facilities. The Department of Energy is bound by the National Environmental Policy Act to consider the environmental impact of nuclear material handling, transportation, and storage. NASA, the Department of Energy, and other federal and local authorities develop comprehensive emergency plans for each launch, including timely public communication. In the event of an accident, monitoring teams equipped with highly specialized support equipment and automated stations are deployed around the launch site to identify potential radioactive material releases, quantify and describe the release scope, predict the quantity and distribution of dispersed material, and develop and recommend protective actions.
International regulations
At the global level, following the 1978 COSMOS 954 incident, the international community recognized the need to establish a set of principles and guidelines to ensure the safe use of nuclear power sources in outer space. Consequently, in 1992, the General Assembly adopted resolution 47/68, titled "Principles Relevant to the Use of Nuclear Power Sources in Outer Space." These principles primarily address safety assessment, international information exchange and dialogue, responsibility, and compensation. It stipulates that the principles should be revisited by the Committee on the Peaceful Uses of Outer Space no later than two years after adoption. After years of consultation and deliberation, in 2009, the International Safety Framework for Nuclear Power Source Applications in Outer Space was adopted to enhance safety for space missions involving nuclear power sources. It offers guidance for engineers and mission designers, although its effective implementation necessitates integration into existing processes.The "Safety Framework" asserts that each nation bears responsibility for the safety of its space nuclear power. Governments and international organizations must justify the necessity of space nuclear power applications compared to potential alternatives and demonstrate their usage based on comprehensive safety assessments, including probabilistic risk analysis, with particular attention to the risk of public exposure to harmful radiation or radioactive materials. Nations also need to establish and maintain robust safety oversight bodies, systems, and emergency preparedness to minimize the likelihood and mitigate the consequences of potential accidents. Unlike the 1992 "Principles," the "Safety Framework" applies to all types of space nuclear power source development and applications, not just the technologies existing at the time.
In the draft report on the implementation of the Safety Framework for Nuclear Power Source Applications in Outer Space published in 2023, the working group considers that the safety framework has been widely accepted and demonstrated to be helpful for member states in developing and/or implementing national systems and policies to ensure the safe use of nuclear power sources in outer space. Other member states and intergovernmental organizations not currently involved in the utilization of space nuclear power sources also acknowledge and accept the value of this framework, taking into account safety issues associated with such applications.