The research: You quantitatively examined several scenarios to determine which was the most suitable for a Mars mission. What were the difficulties to overcome during this study?
Aaron Berliner and Anthony Abel: The first was to turn a pretty basic question about – which of solar energy or nuclear power is better for a human exploration mission to Mars? – to a question we could answer quantitatively. We chose transported mass as the target of comparison because we believed that the power-generating system that would require the least amount of mass to transport from Earth to Mars would leave more room for other important materials (habitat, one or two rovers, other survival goods, etc. .). Next, we set out to calculate this mass for different configurations of power generation systems.
Was it easy to do?
No, and it was here, in our opinion, that another obstacle appeared. Nuclear reactors work pretty much the same everywhere, but solar energy production depends on many factors, including where you are on the planet, the atmosphere there – and especially the amount of dust there – and the surface temperature. To take all of these factors into account, we had to model the behavior of light in the atmosphere of Mars. This is a complex task that required a large number of hours of computation on a supercomputer and was probably the most technically difficult aspect of the project.
How did you proceed?
The question we wanted to answer is: for a given landing site on Mars, what option for electricity generation system (nuclear power or solar energy with different energy storage strategies) requires the least mass when traveling from Earth to Mars? Our model should therefore take several factors into account. First, we had to figure out how much energy it takes to keep the Martian habitat running, which includes warming, the production of oxygen that astronauts can breathe, the production of fertilizer for the plants to feed astronauts, and production of “food” for microbes that will make medicine or plastic to be used as spare parts for the Mars habitat, etc. Many of these data are well known or subject to estimates that we were able to trust.
How did you model the amount of light produced depending on the location, a crucial factor as you pointed out?
The Martian atmosphere includes a number of gases (of which carbon dioxide accounts for the vast majority), some ice particles and lots of dust. We calculated a “light budget”, that is, the amount of light hitting the surface of Mars at a given location over the course of a year, and the light spectrum (how much blue, red, infrared light, etc.). Finally, we used this light balance to determine how much energy solar cells could produce by modeling the physics of light-to-energy conversion in solar cells.
What about storage?
With solar cells, it is not enough to produce electricity, it must also be able to store it when the sun is down and the habitat needs energy. We explored various options for storing this energy, including batteries and converting it into hydrogen, which we could then recycle.
By comparison, which system seems most effective?
Overall, our calculations show that a solar energy production system accompanied by hydrogen storage in terms of mass on board outweighs one that produces nuclear energy, but assuming the mission is in the area around the Mars equator (8.3 tons) to take away for solar energy against 9.5 tonnes for nuclear). But the balance tips to the nuclear option if the mission site is close to the South Pole or Mars’ North Pole (22.4 tons for solar energy). Therefore, the solar-powered system outperforms the nuclear capability on only 50% of the planet’s surface due to varying climatic conditions.
Photovoltaic energy would be the best choice if the planned installation site is in the yellow zone of this Mars planisphere. (Photo credit: Anthony Abel and Aaron Berliner, UC Berkeley)
For a long time, it was thought that nuclear energy would in any case be the most efficient for space missions. Does that time pass for you?
Nuclear engineers have done a great job of developing reactor technology that can work in space. Our benchmark for comparison was the Kilopower system, developed by NASA in 2018 and still under development today. It is a very compact and self-regulating mini nuclear fission reactor, designed specifically for long-term missions to the surface of Mars from the Moon. Nuclear has the advantage that they operate continuously and produce approximately the same amount of energy no matter where you are. Two aspects have long burdened solar energy: their large mass and the Martian dust, whose deposits on the panels can reduce energy production [La sonde Insight se trouvant à la surface de Mars depuis novembre 2018 sera bientôt désactivé ; à cause d’une accumulation de poussière sur ses panneaux solaire, la sonde ne produit plus suffisamment d’énergie. NDLR]. However, flexible and lightweight materials were developed to support the solar panels for the satellites and other applications, making it possible to significantly reduce the mass of a solar panel. Researchers have also developed very clever ways to use electrostatic processes to dust solar panels off; during manned missions, it is also possible to clean the panels manually. In addition, dust storms can block light simply by too much presence in the atmosphere, this is where the role of energy storage comes into play. Solar energy for space is on its way!
Interview by Odyssée Piettre
Anthony Abel is a postdoc researcher at Clark Laboratory at the University of California, Berkeley (USA). His research focuses on biological engineering and biotechnology, especially in connection with long-term space missions.
Aaron Berliner is a student and research assistant at Arkin Laboratory at the University of California, Berkeley. He is a specialist in biotechnology and nuclear in space exploration. Image: The artist’s impression of a Mars mission (Photo credit: NASA / JPL).