Space travel is a dream shared by many around the world. Some are even contemplating the possibility of living on other planets. However, we still have a few minor problems to solve before we can move to neighbouring planets. The Moon has no atmosphere and neither does Mercury, where temperatures can reach 450° C. Venus is also an inferno with temperatures of 380 °C, an atmosphere made up of 97% CO2 and pressure that is 98 times greater than ours. The other planets are all too far away, except for Mars. The problem with Mars is that 95% of its atmosphere is CO2, its average temperature is around -63 °C and the pressure is a tenth of that on Earth. We could survive for a few hours in a dry suit and wrapped up like Siberian Inuits, but the problem of how we could live there for longer still needs to be solved.
If we attempt to colonise Mars, or other more distant planets (which is unlikely for at least the next few centuries), many of the technologies that have made it possible for us to travel and work in space around the Earth, in spacecraft and space stations, would be useless. Specifically, we wouldn’t be able to use the technology that we have already presented here to produce oxygen and eliminate carbon dioxide. All these technologies involve consuming reagents and turning them into waste, so a supply chain connected to Earth would be needed. A trip to Mars takes a minimum of 150 to 300 days (five to ten months) and is only possible every two years when the orbits of both planets bring them closer together.
The Curiosity rover took about 8 months to make the journey from 2011 to 2012, Opportunity was faster and covered the route in just over 7 months from 2003 to 2004, while NASA's newest rover, Perseverance, docked after travelling for 203 days in February 2021. This means that future space colonies would have to fend for themselves for years on end. They would need to adopt strict circular economy rules. All waste materials produced would need to be recovered and recycled on-site into new usable materials using energy from the sun, other stars or artificial fusion.
How to capture CO₂
It wouldn't be possible to use the simple scrubbers (purifiers that remove carbon dioxide) that are used on the International Space Station to capture CO2 and remove it from the colony's habitat. A cyclic mechanism of internal carbon dioxide absorption and subsequent desorption in the space vacuum wouldn’t work in the Martian atmosphere, which is already saturated with CO2. Experimental systems could be trialled, such as MOXIE, which converts carbon dioxide into precious oxygen and carbon monoxide, or a Sabatier cycle, which uses an electrolyser to convert the water vapour produced when breathing into oxygen and hydrogen. The latter then reacts with carbon dioxide to produce two precious molecules, water and methane. Other technologies for CO2 removal are being studied, such as the Bosch reaction, which produces water and solid carbon, the reverse water-gas exchange reaction, producing water and carbon monoxide or, high-temperature electrolysis, which converts it into oxygen and carbon monoxide. A very recent study suggests that plasmas at very low temperatures can be used to transform CO2 into other products.
In all these examples, an energy source is needed to reverse the chemical reactions. This means converting CO2, the most oxidised, stable and energy-free form of carbon into other less oxidised, more reactive forms which contain energy that can be released and used. There are some significant problems to address here. Firstly, a high number of solar panels would be needed because Mars is further away from the Sun than the Earth, with 590 W/m2 of maximum light energy reaching it, compared to the 1000 W/m2 that reaches our planet. These panels would have to withstand frequent dust storms, with gusts of 100 km/h rising up to 1000 km above the surface, coupled with powerful electrostatic discharges caused by the absence of moisture. Under these conditions, any equipment exposed to the elements would wear out rapidly and colonists would need to make immediate repairs using available materials to avoid compromising the entire habitat.
Some help from Nature: photosynthesis
This is why research is moving in a completely different direction. Why not use the oldest technology in the world to remove CO2 and convert it into oxygen? Several experiments are underway that use air recycling systems based on chlorophyll photosynthesis. For two years, the Biosphere-2 project in Arizona saw 8 terranauts living as colonists and farmers in a huge 180,000 m3 sealed artificial ecosystem greenhouse. But is there enough light on Mars? The Mars Society have come up with the answer. At the Mars Arctic Research Station on Devon Island, located at a latitude 75° north between Canada and Greenland where sunlight and temperature levels are comparable to those on Mars, a team of six colonists proved that tomatoes can be grown under these conditions (in a greenhouse, of course!).
The BIOS-3 project in Siberia looked at algae rather than land plants and demonstrated that 315 m3 of bioreactors filled with Chlorella algae would be sufficient to balance the O2/CO2 ratio for one cosmic colonist. Using photosynthesis optimised LED lights, only 1.8 kW of electrical power per person would be needed, more or less that required to power a domestic oven. So, as we design the spaceships that will take us to who knows where, we continue to research the bio-habitats that will be home to the space farmers of the future.
Read more about scientific research
Selected content on this issue.