Nuclear Reactors for Spacecraft Power 

• Radioisotope power sources have been an important source of energy in space since 1961.
• Fission power sources have been used mainly by Russia, but new and more powerful designs are under development in the USA.

After a gap of several years, there is a revival of interest in the use of nuclear fission power for space missions.

While Russia has used over 30 fission reactors in space, the USA has flown only one - the SNAP-10A (System for Nuclear Auxiliary Power) in 1965.

Early on, from 1959-73 there was a US nuclear rocket program – Nuclear Engine for Rocket Vehicle Applications (NERVA) – which was focused on nuclear power replacing chemical rockets for the latter stages of launches. NERVA used graphite-core reactors heating hydrogen and expelling it through a nozzle. Some 20 engines were tested in Nevada and yielded thrust up to more than half that of the space shuttle launchers. Since then, "nuclear rockets" have been about space propulsion, not launches. The successor to NERVA is today's nuclear thermal rocket (NTR).

Another early idea was the US Project Orion, which would launch a substantial spacecraft - about 1000 ton’s - from the earth using a series of small nuclear explosions to propel it. The project was commenced in 1958 by General Atomics and was aborted in 1963 when the Atmospheric Test Ban Treaty made it illegal, but radioactive fallout could have been a major problem. The Orion idea is still alive, as other means of generating the propulsive pulses are considered.

The United Nations has an Office of Outer Space Affairs (UNOOSA)* implements decisions of the Committee on the Peaceful Uses of Outer Space (COPUOS) set up in 1959 and now with 71 member states. UNOOSA recognizes “that for some missions in outer space nuclear power sources are particularly suited or even essential owing to their compactness, long life and other attributes” and “that the use of nuclear power sources in outer space should focus on those applications which take advantage of the particular properties of nuclear power sources.” It has adopted a set of principles applicable “to nuclear power sources in outer space devoted to the generation of electric power on board space objects for non-propulsive purposes,” including both radioisotope systems and fission reactors.

UNOOSA has the dual objective of supporting the intergovernmental discussions in the Committee and its Scientific and Technical Subcommittee (S&T) and Legal Subcommittee, and of assisting developing countries in using space technology for development. In addition, it follows legal, scientific and technical developments relating to space activities, technology and applications in order to provide technical information and advice to Member States, international organizations and other United Nations offices.

Radioisotope systems – RTGs 

So far, radioisotope thermoelectric generators (RTGs) have been the main power source for US space work over nearly 50 years, since 1961. The high decay heat of Plutonium-238 (0.56 W/g) enables its use as an electricity source in the RTGs of spacecraft, satellites, navigation beacons, etc and its alpha decay process calls for minimal shielding. Heat from the oxide fuel is converted to electricity through static thermoelectric elements (solid-state thermocouples), with no moving parts. RTGs are safe, reliable and maintenance-free and can provide heat or electricity for decades under very harsh conditions, particularly where solar power is not feasible.

So far 45 RTGs have powered 25 US space vehicles including Apollo, Pioneer, Viking, Voyager, Galileo, Ulysses and New Horizons space missions as well as many civil and military satellites. The Cassini spacecraft carries three RTGs providing 870 watts of power as it explores Saturn. Voyager spacecraft which have sent back pictures of distant planets have already operated for over 20 years and are expected to send back signals powered by their RTGs for another 15-25 years. Galileo, launched in 1989, carried a 570-watt RTG. The Viking and Rover Landers on Mars in 1975 depended on RTG power sources, as does the Mars Science Laboratory Rover launched in 2011 (the two Mars Rovers operating 2004-09 use solar panels and batteries).

The latest RTG is a 290-watt system known as the GPHS RTG. The thermal power for this system is from 18 General Purpose Heat Source (GPHS) units. Each GPHS contains four iridium-clad ceramic Pu-238 fuel pellets, stands 5 cm tall, 10 cm square and weighs 1.44 kg. The Multi-Mission RTG (MMRTG) will use eight GPHS units with a total of 4.8 kg of plutonium oxide producing 2 kW thermal which can be used to generate some 110 watts of electric power, 2.7 kWh/day. It is being used in the Mars Science Laboratory, a large mobile laboratory – the rover Curiosity, which at 890 kg is about five times the mass of previous Mars rovers. Another one is due to be launched in 2020.

The Stirling Radioisotope Generator (SRG) is based on a 55-watt electric converter powered by one GPHS unit. The hot end of the Stirling converter reaches 650°C and heated helium drives a free piston reciprocating in a linear alternator, heat being rejected at the cold end of the engine. The AC is then converted to 55 watts DC. This Stirling engine produces about four times more electric power from the plutonium fuel than an RTG. Thus each SRG will utilise two Stirling converter units with about 500 watts of thermal power supplied by two GPHS units and will deliver 130-140 watts of electric power from about 1 kg Pu-238. The SRG and Advanced SRG (ASRG) have been extensively tested but have not yet flown. NASA plans to use two ASRGs for its probe to Saturn's moon Titan (Titan Mare Explorer – TiME) or that to the comet Wirtanen, though these missions have been postponed in favour of the Mars InSight mission in 2016. In November 2013 NASA said it was halting development of the ASRG due to budget constraints and the fact that it had enough Pu-238 for MMRTGs, and production of Pu-238 was being ramped up to 1.5 kg/yr.

Russia has developed RTGs using Po-210, two are still in orbit on 1965 Cosmos navigation satellites. But it concentrated on fission reactors for space power systems. China’s Chang 3 lunar Lander apparently uses RTGs with Pu-238.

As well as RTGs, Radioactive Heater Units (RHUs) are used on satellites and spacecraft to keep instruments warm enough to function efficiently. Their output is only about one watt and they mostly use Pu-238 – typically about 2.7g of it. Dimensions are about 3 cm long and 2.5 cm diameter, weighing 40 grams. Some 240 have been used so far by USA and two are in shut-down Russian Lunar Rovers on the moon. Eight were installed on each of the US Mars Rovers Spirit and Opportunity, which landed in 2004, to keep the batteries functional. China’s Chang’e 3 lunar rover Yutu apparently uses several RHUs.

The Idaho National Laboratory's (INL) Centre for Space Nuclear Research (CSNR) in collaboration with NASA is developing an RTG-powered hopper vehicle for Mars exploration. When stationary the vehicle would study the area around it while slowly sucking up carbon dioxide from the atmosphere and freezing it, after compression by a Stirling engine. Meanwhile a beryllium core would store heat energy required for the explosive vaporization needed for the next hop. When ready for the next hop, nuclear heat would rapidly vaporize the carbon dioxide, creating a powerful jet to propel the craft up to 1000 meters into the 'air'. A small hopper could cover 15 km at a time, repeating this every few days over a ten-year period. Hoppers could carry payloads of up to 200 kg and explore areas inaccessible to the Rovers. INL suggests that a few dozen hoppers could map the Martian surface in a few years, and possibly convey rock samples from all over the Martian surface to a craft that would bring them to Earth.

Both RTGs and RHUs are designed to survive major launch and re-entry accidents intact, as is the SRG.