Interplanetary travel powered by nuclear systems is moving from science fiction to near-term reality. NASA plans to launch the Space Reactor-1 Freedom mission by December 2028, billed as the first nuclear-powered interplanetary spacecraft, and is also developing a small fission reactor for the lunar surface as part of Artemis. The U.S. government has established a national initiative on space nuclear power, and a growing number of nations, agencies and private firms are now exploring nuclear options for off-Earth missions.
Not all space nuclear systems are the same. Radioisotope power systems turn the heat from the natural decay of isotopes such as plutonium-238 into electricity and have long supported planetary probes and surface instruments. Fission reactors split atoms to produce heat that can be converted to electricity; on the Moon or Mars they could run habitats, life-support systems and surface infrastructure. Fission systems can also generate electrical power for propulsion systems, enabling higher-thrust or higher-efficiency engines that can shorten transit times and reduce astronauts exposure to cosmic radiation.
Practical reasons are driving renewed interest. The lunar day-night cycle lasts about 29.5 Earth days, leaving long, cold nights in which solar panels may be inadequate without massive energy storage. A compact, controllable power source would support sustained science, exploration and human presence. For interplanetary missions, nuclear electric propulsion promises shorter journeys and more flexible mission profiles than chemical propulsion alone.
Nuclear power in space is not new. Apollo missions used radioisotope thermoelectric generators to power experiments. RTGs continue to run Mars rovers and deep-space probes, and the Voyager spacecraft still rely on radioisotopes decades after launch. The U.S. flew a fission reactor, SNAP-10A, during the Cold War, and the Soviet Union operated nuclear-powered radar satellites. Those precedents demonstrate both technical feasibility and the long-term operational value of space nuclear sources.
They also underscore real risks. The 1978 uncontrolled re-entry of the Soviet satellite Kosmos 954 scattered radioactive debris across a large area of Canada, affecting Indigenous lands and prompting an extensive cleanup. Launch accidents or re-entry failures could transfer radiological hazards back to Earth; even localized contamination would have serious consequences for communities and environments. Designers therefore focus on containment, robust safety features, and preventing reactors from becoming critical until they are safely deployed.
Technical challenges remain significant. Reactors and radioisotope systems must survive extreme temperatures, vacuum, vibration at launch, and intense radiation over long durations. Materials science, reactor control, shielding, and reliable thermal-to-electric conversion are active research priorities. End-of-life planning for reactors and radioactive components is also critical, raising questions about decommissioning, disposal and intergenerational stewardship.
Legal and policy frameworks exist but are limited. The Outer Space Treaty forbids placing nuclear weapons in orbit or on celestial bodies, but it does not ban non-weapon nuclear power sources. After Kosmos 954, the United Nations adopted principles governing the use of nuclear power sources in outer space, and the UN Committee on the Peaceful Uses of Outer Space together with the International Atomic Energy Agency developed a safety framework in 2009 covering launch authorization, emergency preparedness and operational phases. Those instruments recommend safety assessments, notifications, international assistance in case of incidents, and state responsibility and liability for launches.
However, the UN principles and the IAEA-related framework are non-binding guidance. Much of the oversight of space nuclear activities falls to national regulators and licensing authorities, which can produce a patchwork of rules, varying tolerances for risk, and differing levels of transparency. Given that accidents can have cross-border effects, inconsistent domestic regulation creates gaps in international protection.
To govern space nuclear power responsibly, states and industry must consistently implement existing international guidance and cooperate to update it where necessary. That means stronger multilateral coordination, routine information sharing on safety assessments, harmonized licensing standards, and clear mechanisms for emergency notification and response. Domestic regulators should adopt the highest practicable safety standards, resist pressure to compress timelines for political or commercial reasons, and ensure accountability for environmental and public health impacts, including the rights and concerns of Indigenous and other affected communities.
Space nuclear systems will likely play an important role in humanity’s expansion beyond Earth. If they do, the priorities must be rigorous engineering, transparent regulation, international cooperation and ethical stewardship. Safety and accountability should guide deployment so that the benefits of nuclear power in space do not come at the expense of people or environments on Earth or other celestial bodies.
