Our sci-fi dreams of sending humans to Mars could become reality within the next few decades, but how are we going to bring them back?

The return journey to Earth will need rocket fuel. Unfortunately, just the fuel to get to Mars will be heavy enough, and adding several tons more for a round trip will weigh down the spacecraft while making expenses skyrocket. But wait. We could just forget about extra fuel tanks and instead bring along microbes that weigh almost nothing and, when powered by sunlight and CO2, will be able to create rocket fuel.

Researchers from Georgia Tech think we could pull this off with cyanobacteria (blue-green algae also found in many supplements), a bioengineered strain of E. coli, and the right conditions for them to thrive.

“Biological systems can convert CO2 into chemicals, and Mars has 20 times more CO2 than Earth…offering an excellent carbon source for hydrocarbon rocket propellant production,” the research team said in a study recently published in Nature Communications.

Cyanobacteria are photosynthetic microorganisms that can morph CO2 and sunlight into organic matter that can be used as fuel, with a byproduct of molecular oxygen or O2. These autotrophs convert inorganic substances like CO2 into organic substances. The nutrients in their output can sustain other organisms such as E. coli bacteria (not all strains will give you food poisoning), which are already known for their ability to produce clean energy if engineered the right way. They can also produce mass quantities of it.

E. coli are heterotrophs that need to feed on complex organic substances to survive. The cyanobacteria can provide that for them. Using cyanobacteria alone to convert CO2 into rocket fuel would take much too long, since they do not produce much of anything over a short stretch of time, but growing them as food for the E. coli would eventually produce enough rocket fuel to blast a spacecraft back to the home planet. The E. coli can produce a fuel known as 2,3 butanediol (2,3-BDO) if they eat the cyanobacteria.

If you compare this to the main chemical strategy being considered for a return from the Red Planet, the differences are glaring. There is only one downside. The infrastructure for bacteria cultivation will have to be shipped from Earth, meaning a higher overall payload. That still doesn’t outweigh using bacteria, which can produce both fuel that will burn about a third less power and 44 extra tons of molecular oxygen. The O2 can launch more spacecraft or be a fuel source for human exploration and colonization on Mars.

There are still some things that need to be figured out before the researchers are sure the bacteria will keep going in the harsh Martian environment. At least, it seems harsh to us. For cyanobacteria, the extra CO2 over there is an advantage. More of this resource will enable them to create more nutrients for the E. coli, whose biomass can then be reused to start the cycle all over again. However, freezing temperatures on Mars could be an obstacle. There are no microbes known to grow at -67 degrees Fahrenheit.

Because of the intense cold and nearly nonexistent atmosphere, one suggested solution is growing the bacteria in a greenhouse-like structure, where the temperature is always regulated. Something like that would have to be part of the infrastructure that rides with the payload leaving Earth. Bacteria also require water, nitrogen, phosphorus and trace metals. Martian water may have to be pre-treated to get rid of any harmful chemicals. Nitrogen, phosphorus, and trace metals exist on Mars but will also need to be purified.

“The biological production of rocket propellant on Mars is within reach using state-of-the-art technology,” the team said. “Addressing the identified biological and materials challenges should take us closer to enabling interplanetary space travel and Mars colonization.”

For now, NASA does not allow Earth microbes on other bodies in space to prevent  contamination, but further bioengineering could make it possible in the future.