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Nuclear power in space is the use of nuclear power in outer space, typically either small fission systems or radioactive decay for electricity or heat. Another use is for scientific observation, as in a Mössbauer spectrometer. One common type is a radioisotope thermoelectric generator, which has been used on many space probes and on manned lunar missions, and another is small fission reactors for Earth observation satellites such as the TOPAZ nuclear reactor. A radioisotope heater unit provides heat from radioactive decay of a material and can potentially produce heat for decades.
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Russia has sent about 40 reactors into space and its TOPAZ-II reactor can produce 10 kilowatts. The Romashka reactor family uses uranium and direct thermoelectric conversion to electricity, rather than using a heated fluid to drive a turbine. The United States tested a nuclear reactor in space for 43 days in 1965. While not yet tested in space, the test of the Demonstration Using Flattop Fission (DUFF) on September 13, 2012 was the first test of a nuclear reactor power system for space since then.
Examples of nuclear power for space propulsion systems include nuclear electric rocket (nuclear electric propulsion), radioisotope rocket, and radioisotope electric propulsion (REP). One of the more explored is the nuclear thermal rocket, which was tested in the NERVA program. See also Category:Nuclear spacecraft propulsion (category link). Nuclear pulse propulsion was the subject of Project Orion (nuclear propulsion)
Benefits
While solar power is much more commonly used, nuclear power offers great advantages in many areas. Solar cells, although efficient, can only supply energy to spacecraft in orbits where the solar flux is sufficiently high, such as low Earth orbit and interplanetary destinations close enough to the Sun. Unlike solar cells, nuclear power systems function independently of sunlight, which is necessary for deep space exploration. Nuclear reactors are especially beneficial in space because of their lower weight-to-capacity ratio than solar cells. Therefore, nuclear power systems take up much less space than solar power systems. Compact spacecraft are easier to orient and direct in space when precision is needed. Estimates of nuclear power, which can power both life support and propulsion systems, suggest that use of these systems can effectively reduce both cost and flight time.
Selected applications and/or technologies for space include
Radioisotope systems
For more than fifty years, radioisotope thermoelectric generators (RTGs) have been the United States’ main power source in space. RTGs offer many benefits; they are relatively safe and maintenance-free, resilient under harsh conditions, and can operate for decades. RTGs are particularly desirable for use in parts of space where solar power is not a viable power source. Dozens of RTGs have been implemented to power 25 different US spacecrafts, some of which have been operating for more than 20 years. The Stirling Radioisotope Generator (SRG) produces roughly four times the electric power of an RTG, but it has yet to be implemented on an actual mission. NASA plans to utilize two advanced SRGs to explore Titan in the distant future. Radioactive heater units (RHUs) are also used on spacecrafts to warm scientific instruments to the proper temperature so they operate efficiently.
Extremely slow-decaying radioisotopes have been proposed for use on realistic interstellar probes with multi-decade lifetimes.
Another direction for development is a RTG assisted by subcritical reactions.
Fission systems
Fission power systems may be utilized to power a spacecraft’s heating or propulsion systems.
In terms of heating requirements, when spacecraft require more than 100 kW for power, fission systems are much more cost effective than RTGs. Over the past few decades, several fission reactors have been proposed, but these fission systems haven’t been utilized in US space projects as prominently as radioisotope systems have. The Soviet Union, however, launched 31 BES-5 low power fission reactors in their RORSAT satellites utilizing thermoelectric converters between 1967 and 1988. Shortly after, the Soviet Union developed TOPAZ reactors, which utilize thermionic converters instead. In 2008, NASA announced plans to utilize a small fission power system to be used on the surface of the moon and Mars, and began testing "key" technologies for it to come to fruition.
Nuclear thermal propulsion systems (NTR) are based on the heating power of a fission reactor, offering a more efficient propulsion system for thrust in launches and landings than one powered by chemical reactions. Current research focuses more on nuclear electric systems as the power source for providing thrust to propel spacecraft that are already in space. Heatpipe Power System (HPS) reactors are very compact, fast fission reactors that can supply power for 10 years to either spacecrafts or vehicles exploring the surface of a planet. Since 1994, HPS reactors have been developed as relatively resilient, safe, and reliable systems.
Other space fission reactors for powering space vehicles include the SAFE-400 reactor and the HOMER-15. In 2020, Roscosmos (the Russian Federal Space Agency) plans to launch a spacecraft utilizing nuclear-powered propulsion systems (developed at the Keldysh Research Center), which includes a small gas-cooled fission reactor with 1 MWe.
As of 2010, more than 30 small fission power system nuclear reactors have been sent into space in the Soviet RORSAT satellites, with only one—SNAP-10A—by the US. In addition, over 40 radioisotope thermoelectric generators have been used globally (principally US and USSR) on space missions.
Proposed fission power system spacecraft and exploration systems have included SP-100, JIMO nuclear electric propulsion, and Fission Surface Power.
A number of Micro nuclear reactor types have been developed or are in development for space applications:
Project Prometheus
In 2002, NASA announced an initiative towards developing nuclear systems, which later came to be known as Project Prometheus. A major part of the Prometheus Project was to develop the Stirling Radioisotope Generator and the Multi-Mission Thermoelectric Generator, both types of RTGs. The project also aimed to produce a safe and long-lasting space fission reactor system for a spacecraft’s power and propulsion, replacing the long-used RTGs. Budget constraints resulted in the effective halting of the project, but Project Prometheus has had success in testing new systems. After its creation, scientists successfully tested a High Power Electric Propulsion (HiPEP) ion engine, which offered substantial advantages in fuel efficiency, thruster lifetime, and thruster efficiency over other power sources.
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Examples of nuclear powered spacecraft:
See also Radioisotope thermoelectric generator#Nuclear power systems in space