Nuclear technology applied to space exploration

One of the main applications of nuclear batteries is space travel. With nuclear power and its related technologies, interplanetary missions will become faster, more efficient, and more economical. As a result, humanity is ever closer to a new era of space travel to Mars, the solar system,m and beyond.

Satellites and probes

The aim is to power the instrumentation of terrestrial satellites and planetary probes with slightly more powerful generators so that they can reach nearby planets and transmit information back to Earth.

Isotopic power generators are instruments containing a hermetically encapsulated radionuclide whose radiation is absorbed in the walls of the capsule. This is the equivalent of a heat source since the capsule transforms the energy of the radiation. An electrical circuit is coupled to this source to generate an electric current to power the instruments.

The source will be long-lived if the half-life of the radioisotope is long and the radionuclides used are always alpha emitters because this radiation is stopped in the first microns of the walls of the later stages, e.g. for travel to Mars, the process is capsulated (usually stainless steel). Preferably plutonium-238 and curium-244 are used, which can provide small electrical powers for many years.

The European Space Agency is studying the replacement of plutonium-238 with another isotope that generates electricity to meet the needs of electrical and electronic equipment for measuring and transmitting data to Earth. One of the isotopes under consideration is americium-241, commonly used in fire detectors, also an alpha emitter with a similar decay value to plutonium-238, but with a half-life of 432.2 years, which can be used for longer missions, although a larger quantity will be required to achieve the same 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 create propulsion to accelerate speed.

Unmanned travel to planets outside Earth's solar system has been accomplished by missions equipped with robotic equipment powered by electricity produced by the radioisotope plutonium-238. This isotope has a half-life of 87.74 years and an activity lifetime that can practically serve the needs of space missions for several centuries. Plutonium-238, which is not fissile like other plutonium isotopes, has its origin in irradiated uranium fuels. They would work by transferring heat from the nuclear reactor to a liquid propellant, which would be converted to gas and expand to provide thrust for the spacecraft.

There are two key nuclear technologies for propulsion: With thermonuclear propulsion, there is less fuel load and a trip to Mars would be shorter. With electronuclear, the fuel efficiency is much higher and the trip would be even shorter.

Thermonuclear propulsion (NTP)

It involves using a nuclear fission reactor to heat a liquid propellant, such as hydrogen. The heat converts the liquid into a gas that expands through a nozzle to generate thrust and propel the spacecraft.

One of the main advantages is that spaceflight would require less refuelling, and NTP engines would make the trip shorter. For example, a trip to Mars would be reduced by 25% compared to traditional chemical rockets. In addition, reduced time in space also reduces astronauts' exposure to cosmic radiation.

Electronuclear propulsion (NEP)

Thrust is produced by converting the thermal energy of a nuclear reactor into electrical energy. With this type of technology, thrust is lower but continuous, and fuel efficiency is much higher. The speed is increased, with a reduction of more than 60 % in transit time to Mars compared to traditional chemical rockets.

Spacecraft company Ad Astra Rocket Company is building an NEP system: the Variable Specific Impulse Magnetoplasma Variable Impulse Magnetoplasma Rocket (VASIMR). This is a plasma spacecraft in which electric fields heat and accelerate a propellant to form a plasma. When the plasma shoots out of the engine, magnetic fields direct it in the right direction and the thrust is generated. The VASIMR design would allow large amounts of energy to be processed while maintaining the high fuel efficiency that characterises electric spacecraft.

The VASIMR engine is envisaged to be used for a wide range of high-energy applications, from solar electricity in cislunar space to nuclear electricity in interplanetary space. In the longer term, VASIMR could be the precursor to future fusion spacecraft, which are still at the conceptual stage.

Fusion ships

Fusion spacecraft such as the Field Configuration Reactor Reversal (PFRC) being developed at Princeton's Plasma Physics Laboratory could produce a direct fusion pulse (DFD), which directly converts the energy of charged particles produced in fusion reactions into propulsion for the spacecraft.

The possibilities of DFD technology open the door to interstellar space, human missions to Mars, and a stable power supply for a future lunar base, says Princeton Satellite Systems. Other advantages are that they are small in size and require very little fuel. Just a few kilos can power a spacecraft for ten years.

Energy for the extraterrestrial surface

Nuclear reactors could also be used to provide a reliable source of surface power for extended exploration missions, facilitating sustainable human presence on other planetary bodies. Fission surface power reactor designs are microreactors that could provide electrical power in the tens of kW range for decades. The current focus is on using low-enriched uranium fuels or peaceful uranium fuels with high enrichment.

In the words of NASA's Space Nuclear Technology Portfolio representative, 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 surface of the Moon and a future human Mars mission, scalable to power levels above 100 kWe; it must also be able to meet the needs of the NEP system’.

NASA is developing a fission surface energy system for lunar surface applications and a future human Mars mission.

Energy for spacecraft on-board systems

Spacecraft need electrical power for propulsion and maintain their life support systems, communications, and other equipment and systems. Radioisotope thermoelectric generators (RTGs), which have powered the Voyager spacecraft for decades far beyond the Sun thanks to their potential to provide heat and electricity for long periods to spacecraft on-board systems in the cold temperatures of space, were mentioned with particular emphasis at the expert meeting.

Future nuclear solutions such as DFD technology could provide electricity simultaneously. According to NASA studies, a fusion-powered direct-drive engine can produce power and thrust with the best performance, generating electrical power and propulsion with a single engine.

With the support of nuclear power, future space missions will have a much wider range of applications. In the words of Mikhail Chudakov of the IAEA's Department of Nuclear Energy, ‘our way to the stars is through the atom’.

If you want to know more about it, take a look at the infographic below: