Solar preferable to nuclear for settlement sites on Mars, study suggests

Many scientists and engineers who’ve thought about the logistics of living on the surface of the Red Planet have assumed that nuclear power is the best alternative, in large part because of its reliability and 24/7 operation.

In the past decade, miniaturised kilopower nuclear fission reactors have advanced to the point where Nasa considers them to be a safe, efficient and plentiful source of energy and key to future robotic and human exploration.

Solar power, on the other hand, must be stored for use at night, which on Mars lasts about the same length of time as on Earth. Solar panels' power production can also be reduced on Mars by the omnipresent red dust that covers everything. Nasa's nearly 15-year-old Opportunity rover, powered by solar panels, stopped working after a massive dust storm in 2019.

However, thanks to today's light, flexible solar panels, photovoltaics may be more practical for long stays, according to the University of California study – 'Photovoltaics-driven power production can support human exploration on Mars' – published this week in the journal Frontiers in Astronomy and Space Sciences.

The study uses a systems approach to compare the two energy technologies head-to-head for a six-person extended mission to Mars involving a 480-day stay on the planet’s surface before returning to Earth. This is the most likely scenario for a mission that reduces the transit time between the two planets and extends time on the surface beyond a 30-day window.

The researchers' analysis found that for settlement sites over nearly half the Martian surface, solar is comparable or better than nuclear, taking into account the weight of the solar panels and their efficiency, provided that some daytime energy is used to produce hydrogen gas for use in fuel cells to power the colony at night or during sandstorms.

“Photovoltaic energy generation coupled to certain energy storage configurations in molecular hydrogen outperforms nuclear fusion reactors over 50 per cent of the planet's surface, mainly within those regions around the equatorial band, which is in fairly sharp contrast to what has been proposed over and over again in the literature, which is that it will be nuclear power,” said UC Berkeley bioengineering doctoral student Aaron Berliner, one of two first authors of the paper.

The study gives a new perspective on Mars colonisation and provides a road map for deciding which other technologies to deploy when planning manned missions to other planets or moons.

"This paper takes a global view of what power technologies are available and how we might deploy them, what are the best-use cases for them and where do they come up short," said co-first author Anthony Abel, a graduate student in the Department of Chemical and Biomolecular Engineering.

"If humanity collectively decides that we want to go to Mars, this kind of systems-level approach is necessary to accomplish it safely and minimise cost in a way that's ethical. We want to have a clear-eyed comparison between options, whether we're deciding which technologies to use, which locations to go to on Mars, how to go and whom to bring."

Previously, Nasa’s estimates of the power needs of astronauts on Mars have generally focused on short stays which don't require power-hungry processes for growing food, manufacturing construction materials or producing chemicals.

As Nasa and the leaders of private companies already building rockets that could go to Mars – such as Elon Musk, CEO of SpaceX, and Jeff Bezos, founder of Blue Origin – talk up the idea of long-term, off-planet settlements, larger and more reliable sources of power need to be considered.

The complication is that all of these materials must be carried from Earth to Mars at a cost of hundreds of thousands of dollars per pound, making low weight essential.

One key need is power for biomanufacturing facilities that use genetically engineered microbes to produce food, rocket fuel, plastic materials and chemicals, including drugs. Abel, Berliner and their co-authors are members of the Center for the Utilisation of Biological Engineering in Space (CUBES), a multi-university effort to tweak microbes using the gene-insertion techniques of synthetic biology to produce necessary supplies for a colony.

The two researchers discovered that without knowing how much power will be available for an extended mission, it was impossible to assess the practicality of many biomanufacturing processes. They set out to create a computerised model of various power supply scenarios and likely power demands, such as habitat maintenance (including temperature and pressure control), fertiliser production for agriculture, methane production for rocket propellant to return to Earth, and bioplastics production for manufacturing spare parts.

Pitted against a kilopower nuclear system were photovoltaics with three power storage options: batteries and two different techniques for producing hydrogen gas from solar energy – by electrolysis and directly by photoelectrochemical cells. In the latter cases, the hydrogen is pressurised and stored for later use in a fuel cell to produce power when the solar panels are not.

Only photovoltaic power with electrolysis – using electricity to split water into hydrogen and oxygen – was competitive with nuclear power. It proved more cost-effective per kilogram than nuclear over nearly half the planet’s surface.

Their model also specified how to tweak photovoltaic panels to maximise efficiency for the different conditions at sites on Mars. Latitude affects the intensity of sunlight, for example, while dust and ice in the atmosphere can scatter longer wavelengths of light.

Abel suggests that photovoltaics are now highly efficient at converting sunlight into electricity, though the best performers are still expensive. The most crucial new innovation is a lightweight and flexible solar panel, which makes storage on the outbound rocket easier and the cost of transport less.

"The silicon panels that you have on your roof, with steel construction, glass backing etc just won't compete with the new and improved nuclear, but newer lightweight, flexible panels all of a sudden really, really change that conversation," he said.

He noted, too, that lighter weight means more panels can be transported to Mars, providing backup for any panels that fail. While kilowatt nuclear power plants provide more power, fewer are needed, so if one went offline the colony could lose a significant proportion of its power.

Much research is going into advancing the reality of human missions to Mars. In order for humans to survive on the surface of such an inhospitable planet, there are such basic barriers to overcome as providing sufficient oxygen to breathe, exploring ways to convert carbon dioxide from the Martian atmosphere into pure, breathable oxygen, and sourcing sufficient fuel locally to enable long-term visitors to Mars to be able to leave the planet and return to Earth.

In November 2021, an isolation experiment designed to test the human psychological response in a simulated trip to Mars revealed significant changes to the way participants communicated with the Earth the longer the experiment ran. Earth-bound communications, in fact, declined over time.