Nuclear technology applied to space exploration
Nuclear technology has significant applications in fields such as the arts, industry, agriculture, medicine and also in space. The space exploration missions of NASA (the US National Aeronautics and Space Administration) require safe, reliable and long-lasting systems that provide electricity and heat to spacecraft and their scientific instrumentation. In this respect, nuclear energy meets these requirements.
For example, one of the main applications of nuclear batteries is space navigation. With nuclear power and related technologies, interplanetary missions will be faster, more efficient and more cost-effective. As a result, humanity is moving ever closer to a new era of space travel to Mars, the solar system and even beyond.
Satellites and probes
The aim is to power the instruments on Earth-orbiting satellites and planetary probes using slightly more powerful generators, so that they can reach nearby planets and transmit information back to Earth.
Isotope power generators are devices containing a hermetically sealed radionuclide, the radiation from which is absorbed by the walls of the capsule. This acts as a heat source, as the capsule converts the energy of the radiation. An electrical circuit is connected to this source to generate an electric current that powers the instruments.
The power source will be long-lasting if the half-life of the radionuclide is long, and the radionuclides used are always alpha emitters, because this radiation is stopped within the first few micrometres of the walls of the subsequent stages; for example, for missions to Mars, the assembly is encapsulated (usually in stainless steel). Plutonium-238 and curium-244 are the preferred choices, as they can provide low levels of electrical power for many years.
The European Space Agency is investigating the replacement of plutonium-238 with another isotope that generates electricity to power the electrical and electronic equipment used for measurement and data transmission to Earth. One of the isotopes under consideration is americium-241, commonly used in fire detectors; it is also an alpha emitter with a decay rate similar to that of plutonium-238, but has a half-life of 432.2 years, meaning it can be used for longer missions, although a greater quantity will be required to produce the same amount of energy.
Spacecraft propulsion
For the foreseeable future, spacecraft launched into space will continue to rely on fossil fuels for propulsion. However, once in orbit, nuclear engines could take over and provide the thrust needed to increase speed.
Unmanned journeys to planets beyond Earth’s solar system have been carried out by missions equipped with robotic devices powered by electricity generated by the radioisotope plutonium-238. This isotope has a half-life of 87.74 years and a duration of activity which, for all practical purposes, can meet the needs of space missions for several centuries. Plutonium-238, which is not fissionable like other plutonium isotopes, is derived from irradiated uranium fuel; it would shorten journey times and reduce costs if spacecraft were to use nuclear propulsion systems, which are more efficient than current rockets. They would operate by transferring heat from the nuclear reactor to a liquid propellant, which would turn into gas and expand to provide thrust for the spacecraft.
There are two key nuclear technologies for propulsion: with thermonuclear propulsion, the fuel load is lower and a journey to Mars would be shorter; with electronuclear propulsion, fuel efficiency is much higher and the journey would take even less time.
Thermonuclear propulsion (NTP)
It involves using a nuclear fission reactor to heat a liquid propellant, such as hydrogen. The heat turns the liquid into a gas, which expands through a nozzle to generate thrust and propel the spacecraft.
One of its main advantages is that space flights would require less fuel, and NTP engines would make the journey shorter. For example, a trip to Mars would be reduced by 25 per cent compared with traditional chemical rockets. Furthermore, a shorter time in space also reduces astronauts’ exposure to cosmic radiation.
Nuclear-electric propulsion (NEP)
hrust is generated by converting the thermal energy from a nuclear reactor into electrical energy. With this type of technology, the thrust is lower but continuous, and fuel efficiency is much higher. Speed increases, with a reduction of more than 60 per cent in the journey time to Mars compared with traditional chemical rockets.
The spaceflight company Ad Astra Rocket Company is building a NEP system: the Variable Specific Impulse Magnetoplasma Rocket (VASIMR). It is a plasma-powered spacecraft in which electric fields heat and accelerate a propellant to form a plasma. When the plasma is ejected from the engine, magnetic fields direct it in the correct direction, generating thrust. The VASIMR design would allow large amounts of energy to be processed whilst maintaining the high fuel efficiency characteristic of electric spacecraft.
The VASIMR engine is expected to be used for a wide range of high-energy applications, from solar power in cislunar space to nuclear power in interplanetary space. In the longer term, the VASIMR could be the forerunner of future fusion spacecraft, which are still at the conceptual stage.
Power systems
Radioisotope Power Systems (RPS) are a tried-and-tested technology that meets the safety, reliability and long-life requirements needed to supply electricity and heat for a wide range of space exploration missions.
One example in this category is the Radioisotope Thermoelectric Generator (RTG), a nuclear space system that converts heat into electricity without any moving parts.
RTGs convert heat from the natural decay of radioactive isotopes into electricity. They consist of two main components: the heat source containing the radioisotope (primarily plutonium-238) and the solid-state thermocouples that convert the decay heat into electricity.
The conversion of heat into electricity is the operating principle of thermocouples and was discovered by the scientist Thomas Johann Seebeck when he observed that when two different conductive materials are joined in a closed circuit and their junctions are maintained at different temperatures, a potential difference is produced. Unlike solar panels, these systems do not rely on solar radiation, making them ideal for missions to deep space, atmospheres with insufficient solar radiation, or environments where solar panels might be damaged.
The output power of thermocouples depends on the temperature of each junction and the properties of the materials from which they are made. Thermocouples used in RTGs utilise heat from the natural decay of radioactive isotopes such as plutonium-238 to heat the hot side of the thermocouple junction, and the cold of space or the atmosphere of other planets for the cold side.
The US Department of Energy (DOE) has developed several generations of these systems for NASA, which supply electricity and heat for a large number of space exploration missions. The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) was designed with the flexibility to operate both on planets with atmospheres, such as Mars, and in the vacuum of outer space. It is capable of providing an electrical output of 125 watts (W) at launch, sufficient for most requirements whilst meeting the high safety standards required. The main components of the MMRTG can be seen in varying degrees of detail in the image.
It derives its power from eight General Purpose Heat Sources (GPHS) fuelled by plutonium-238 dioxide. Initially, these eight modules produce 2 kW of thermal power, equivalent to 125 W of electrical power.
The MMRTG’s thermocouples are designed using compounds known as PbTe/TAGS, where PbTe refers to a lead-tellurium compound obtained from rocks known as halites, and TAGS is the abbreviation for a material containing tellurium (Te), silver (Ag), germanium (Ge) and antimony (Sb).
The MMRTG is designed to produce 125 W of electrical power at the start of the mission, falling to 100 W after 14 years of service. In relative terms, the MMRTG’s weight of 45 kg translates to a favourable power-to-weight ratio of 2.8 W/kg at the start of its operational life.
It is designed with multiple safety levels, with the aim of preventing or minimising the risk of loss or dispersal of its nuclear material across a wide range of hypothetical accident conditions.
Notable features include the plutonium dioxide ceramic structure, which makes it extremely robust; an iridium cladding; graphite sleeves that further enhance fuel protection; and the rugged carbon-fibre material forming the outer containment casing for the GPHS heat sources located inside the MMRTG, as shown in the image.
Inside, the MMRTG contains 4.8 kg of plutonium-238 dioxide, which initially provides around 2 kW of thermal power, equivalent to 125 W of electrical power under deep space environmental conditions.
The approximate dimensions of the MMRTG are 64 cm in diameter, measured between the heat-sink fins, by 66 cm in height. It weighs approximately 45 kg.
The GPHS Heat Source Module, identical to the one powering the Perseverance rover, protects in the event of an impact with the ground and during re-entry into Earth’s atmosphere that could result in an accident.
In all three cases, the RPS units performed as intended by their design.
Fusion spacecraft
Fusion spacecraft, such as the Reversed Field Configuration (PFRC) reactor being developed at the Princeton Plasma Physics Laboratory, could produce direct fusion drive (DFD), which directly converts the energy of the charged particles produced in fusion reactions into propulsion for the spacecraft.
The potential of DFD technology opens the door to interstellar space, human missions to Mars, and a stable energy supply for a future lunar base, according to Princeton Satellite Systems. Other advantages include their compact size and very low fuel requirements. Just a few kilograms of fuel can power a spacecraft for ten years.
Energy for extraterrestrial surfaces
Nuclear reactors could also be used to provide a reliable source of surface power for long-term exploration missions, thereby facilitating a sustainable human presence on other planetary bodies. Fission-based surface power reactor designs are microreactors that could supply electrical power in the range of tens of kW for decades. The current focus is on using low-enriched uranium fuel or high-enriched uranium fuel intended for peaceful purposes.
According to a representative of NASA’s Space Nuclear Technology Portfolio, the Agency’s priority is to “design, build and demonstrate a low-enriched uranium fission surface power system with a wide range of applications for the Moon’s surface and for a future crewed mission to Mars, scalable to power levels above 100 kWe; it must also be capable of meeting the requirements of the NEP system”.
NASA is working on the design of a fission-based surface power system for applications on the Moon’s surface and for a future crewed mission to Mars.
Power for on-board spacecraft systems
Spacecraft require electrical power not only for propulsion, but also to maintain their life-support systems, communications and other equipment and systems. At the expert meeting, particular emphasis was placed on radioisotope thermoelectric generators (RTGs), which have powered the Voyager spacecraft for decades far beyond the Sun thanks to their ability to provide heat and electricity over long periods to on-board systems in the cold temperatures of space.
Future nuclear solutions such as DFD technology could supply electricity simultaneously. According to NASA studies, a direct-drive engine powered by fusion energy can produce energy and thrust with the highest efficiency, generating both electrical power and propulsion with a single engine.
With the support of nuclear energy, future space missions will be able to utilise a much wider range of applications. In the words of Mikhail Chudakov, from the IAEA’s Department of Nuclear Energy, “our path to the stars lies through the atom”.
