|
|
| Fig. 1: Illustration of a Kilopower reactor on the lunar surface. (Courtesy of NASA) |
The space industry is no stranger to using technologies considered dangerous. One needs to look no further than the foundation of modern rocketry itself: rocket fuel, a substance that can spontaneously ignite when exposed to an oxidizer. While militaries used this resource to bring forth a new age of warfare, space agencies instead used this discovery to put humanity on the moon. As NASA and its partners begin preparation for the Artemis Program - the return of humans to the moon and eventual journey to Mars - sustainability has become a large focus of ongoing research, where another so-called dangerous technology could be a solution: nuclear power.
Nuclear power has already been used to some degree on space exploration missions. For example, the most recent Martian rover - Perseverance - is powered through a radioisotope thermoelectric generator, a power system that collects the heat from the decaying plutonium placed within the rover. [1] Whereas previous rovers had to cautiously consider the Martian dust that could cover their solar panels, cutting off their primary source of power, Perseverance's plutonium will continue to power the rover regardless of dust buildup or time of day.
Independence from the sun is an essential step toward the future of space exploration. Solar panels generate power by catching the photons emitted by the sun, but the concentration of these photons decreases at farther distances from the sun, an implication of the Inverse-Square Law. The result is a limited exploration range, where effective missions can only occur on planets closest to the sun. By switching to nuclear power, power independent of the sun, missions will be able to travel farther from the center of the solar system without losing functionality due to decreasing power. Even on Mars, where solar panels have been able to work with some success, cave exploration remains nearly impossible due to the lack of photons under the surface.
There are drawbacks of using plutonium decay, but these are mostly irrelevant on autonomous missions. The heat generated by the plutonium is radioactive by nature, which poses some health risks when in constant exposure. Not only does the lack of a crew eliminate most safety concerns, but the radiation emits alpha particles, which are weak enough that they can be stopped by ceramic. This allows construction crews to work on the spacecraft without significant hazard. Personnel at the launch site are at little risk even in the event of a flight accident, where the maximum predicted radiation dosage is still below the average radiation dosage received by a person throughout a year. [2] Another drawback is that the radioactive decay destroys the plutonium, lowering power production over time. The most common isotope of plutonium used is Pu-238, which boasts a half-life of about 88 years. [2] The half-life equation
| N | = | N0 2- t/t1/2 |
| = | N0 e- t ln(2)/t1/2 |
then reveals that decaying by ten percent will take over thirteen years. This means that the power system will still be limited by the non-nuclear components. With so few hazards, plutonium decay could easily become the standard for smaller spacecraft. The reliable access to power, even in unfavorable locations, will be a necessity for future missions, and the move away from solar power will extend the operating distances to allow exploration to the farthest reaches of the solar system.
The nuclear power systems that rovers utilize work great considering the rover's low power consumption, but such a system does not scale well with power demand. Perseverance's radioisotope thermoelectric generator delivers 110 Watts of power, providing ample power to operate the rover's systems, but this power production is well below the expected 40,000 Watts required for a Martian outpost. [3] Nuclear power might still be viable, just in a different form: fission reactors. NASA's most recent investigation into nuclear fission is the Kilopower Reactor, as seen in Fig. 1. Like the nuclear power systems used by small spacecraft, nuclear fission operates independent of most external conditions. A crewed mission naturally comes with higher risk, especially in the vacuum of space and the negative temperatures of Mars where power failures can mean human fatalities, so having access to sustainable and dependable power is a necessity. This is where fission reactors like Kilopower could excel, generating constant power whether hidden from sunlight in a lunar crater or withstanding a weeks-long dust storm on Mars, situations where solar panels would be inefficient. [4]
Instead of relying on passive decaying of plutonium, fission reactors like Kilopower use Stirling engines to convert the heat released from fission - the splitting of atoms - into usable power. Current design targets hope for a reactor capable of producing 10 Kilowatts, which would allow just four Kilopower reactors to be capable of powering the 40 Kilowatts needed for a Martian outpost. [3] Like all space missions, weight of the power system is a restricting feature, but if modular reactors like Kilopower could be successfully developed, then new reactors could be added to an existing grid to boost power production or even placed upon orbiting satellites. [5]
Achieving such a lightweight design remains a difficult task. The process through which fission reactors generate heat is radioactive, meaning that mission designs must again mitigate radiation exposure, ever-more important when the mission is crewed. Unlike plutonium decay, nuclear fission produces a significant amount of gamma radiation, which can no longer be stopped with a small layer of ceramic but with higher mass materials. The shielding alone could have a mass upwards of 500 kilograms per reactor, but techniques such as shadow shielding - adding radiation shielding only in the direction of equipment and crew since other directions will be uninhabited - could be used to decrease the necessary weight and improve fission practicality. [6]
Nuclear fission could prove a useful source of power for large-scale missions, giving missions access to plentiful power even in extreme weather conditions, but the weight of radiation shielding could be a limiting factor in the viability of fission reactors in space.
Solar power has been reliable for past missions to space, but as space exploration looks beyond just the moon, the falloff of solar power production is too great and too restrictive. While solar power may be worthwhile as a supplementary source of power on a power grid, nuclear power could be beneficial for future space missions. Plutonium decay has already proven itself on many space missions and could become the standard for all low-power vehicles on other planets, allowing these craft to explore longer and farther. Nuclear fission, as best seen by NASAs Kilopower reactor, could power planetary outposts and even larger spacecraft, allowing for a future of reliable and renewable power systems, but the requirement of heavier shielding could limit their effectiveness. Much like rocket fuel became the catalyst for putting the first human on the moon, nuclear power could very well be the accelerant needed to put the first human on another planet.
© Fletcher Newell. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. O'Callaghan, "The First 100 Days on Mars: How NASAs Perseverance Rover Will Begin Its Mission," Scientific American, 18 Feb 21.
[2] S. Cole, "Mars 2020: What to Do in a Launch Accident," U.S. National Aeronautics and Space Administration, March 2020.
[3] D. Bukszpan, "Why NASA Wants to put a Nuclear Power Plant on the Moon," CNBC, 15 Nov 2020.
[4] G. Bennett, "Space Nuclear Power", in Encyclopedia of Physical Science and Technology, 3rd Ed., ed. by R. A. Meyers (Academic Press, 2001).
[5] R. Boyle, "Will NASA Go Nuclear to Return to the Moon," Scientific American, 15 May 18.
[6] D. Poston, "Space Fission Power: Technology Options - Radiation Shielding", in Encyclopedia of Nuclear Energy, ed. by E. Greenspan (Elsevier, 2021).