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 (RTG), 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 (RHU) 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.