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Thursday, April 18, 2024

Mars Colonies Will Need Solar Power—and Nuclear Too

Science fiction authors like Ray Bradbury, Kim Stanley Robinson, Andy Weir, and the creators of The Expanse have long envisioned how people might one day assemble functioning settlements on Mars. Now that NASA and the European Space Agency aim to send astronauts to the Red Planet within the next 20 years, and SpaceX CEO Elon Musk has talked about sending humans there as well, it’s time to address the practical questions involved in making those visions a reality.

One of the biggest: What’s the most practical way to power future Mars colonies? The seemingly simple question took UC Berkeley engineering students Anthony Abel and Aaron Berliner four years of hard work to figure out.

In findings published last week in Frontiers in Astronomy and Space Sciences, they and their colleagues argue that both solar and nuclear energy sources can provide enough power for long-term crewed missions—but astronauts will face certain limitations, including how much weighty equipment they can bring from faraway Earth, how much energy solar panels can glean once there, and how well they can store energy for when it’s not so sunny. “It depends where you are on Mars,” Abel says of their results. “Near the equator, solar seems to work better. And near the poles, nuclear works better.”

The engineers based their study on the energy options for a Martian habitat built for a six-person crew. For such a remote outpost, the first astronauts would have to bring almost everything they need with them, including the photovoltaic (PV) cells, battery stacks, and nuclear reactors needed to generate enough energy for them to survive. That means these crewed missions would be shaped by how much one can bring aboard a rocket—what Abel and Berliner refer to as “carry-along mass.” “Bringing stuff from Earth to Mars is really tough, and it’s really expensive, so you want to minimize it,” Abel says.

For their study, the engineers calculated how much energy the solar or nuclear options would generate and how much carry-along mass would be needed to produce that energy. In particular, they found that over about 50 percent of the Martian surface—especially near the equator, where many of the Mars rovers and landers have alighted so far—PV solar energy outperforms other solar alternatives and requires only around 8.3 tons of carry-along mass to power a six-person habitat, thanks to advances in lightweight solar panels. (Of the three solar energy options they tried, panels with electrolysis and compressed hydrogen storage were the most efficient.) That satisfies an estimated average power demand of about 40 kilowatts, used for things like heating, lighting and rover travel, and for producing oxygen for breathing, fertilizer for crop growth, and methane for rocket fuel for the return trip.

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But the weight of the needed solar equipment would go up to more than 20 tons for a Mars outpost closer to the poles. Mars is tilted on its axis by about 25 degrees, slightly more than Earth is, and its orbit is less circular, so less sunlight would reach those PV cells during parts of the year. That means nuclear power becomes more viable at the poles. The power generation equipment needed to produce that much nuclear energy would add up to about 9.5 tons of carry-along mass to produce the same 40 kilowatts of energy. That lift is doable for massive next-generation rockets like NASA’s Space Launch System and SpaceX’s Starship and Super Heavy, which can each carry payloads of at least tens of tons into deep space. (The poles also harbor ice that could provide a water source for the astronauts.)

These same kinds of trade-offs have already arisen with energy technologies used by Mars rovers. Engineers need to find the right balance between transportation weight, storage needs, and an energy system that can handle variations in the availability of sunlight. Significant sunlight reaches the surface only during the Martian day and only when dust and cloud particles don’t get in the way, says Guillem Anglada-Escudé, an astronomer at the Institute of Space Sciences in Barcelona who was not involved in the study. He’s also a member of the Sustainable Offworld Network, a collaboration of researchers, engineers, and architects studying how future colonies on Mars and other worlds might work.

Anglada-Escudé agrees with Abel and Berliner’s findings. He also believes that, if possible, one shouldn’t look at solar and nuclear energy as either/or. “Our conclusion is, you want to have both solar and nuclear,” he says. “It’s a matter of resilience. Things can fail in many different ways. The best option is to have redundancy.”

It’s also important to study solar radiance and how dust and ice affect how much light reaches the planet’s surface, and where that light can best be collected, says Daniel Vázquez Pombo, an energy engineer at the Technical University of Denmark who wrote a paper last year about a possible hybrid power system for a permanent Mars colony that includes PV arrays and storage. Maintenance for energy systems can be risky for those conducting repairs, another argument for having options.

“Do you really want to rely on a single technology? What happens if you have a systematic error or a design flaw?” Pombo says. “Diversifying is a smart idea. You don’t put all your eggs in one basket.”

The calculus may also change when it’s not just a handful of astronauts visiting for a couple months or a year but rather a permanent colony with long-term visitors, Anglada-Escudé argues. “Solar panels are a relatively simple technology, and solar gets more attractive for the very long term,” he says. “You may need more mirrors, but it will work. On Mars, finding plutonium to the quality you need for a reactor is not trivial. Solar is there, it’s safe, and we know how to do it.”

In the end, life in Mars’s rugged conditions will be tougher than anywhere on Earth. And the science and technology issues are only half the story. Settlers will have to navigate complex financial and societal issues as well, Abel says. At least when they get there, though, they’ll know how to keep the lights on.

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