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Rocket fuel made on Mars could propel astronauts back to Earth

30 November 2021

Researchers have come up with a new way to make rocket fuel on Mars that could launch future astronauts back to Earth.

Image: Georgia Institute of Technology
Image: Georgia Institute of Technology

The bioproduction process would use three resources native to the red planet: carbon dioxide, sunlight, and frozen water.

It would also include transporting two microbes to Mars. The first would be cyanobacteria (algae), which would take CO2 from the Martian atmosphere and use sunlight to create sugars. An engineered E. coli shipped from Earth would convert those sugars into a Mars-specific propellant for rockets and other propulsion devices. The Martian propellant, which is called 2,3-butanediol, is currently in existence, can be created by E. coli, and, on Earth, is used to make polymers for production of rubber.

Rocket engines departing Mars are currently planned to be fuelled by methane and liquid oxygen (LOX). Neither exist on the red planet, which means they would need to be transported from Earth to power a return spacecraft into Martian orbit. That transportation is expensive: ferrying the needed 30 tons of methane and LOX is estimated to cost around $8 billion. 

To reduce this cost, NASA has proposed using chemical catalysis to convert Martian carbon dioxide into LOX, though this still requires methane to be transported from Earth.

As an alternative, researchers propose a biotechnology-based, in-situ resource utilisation (bio-ISRU) strategy that can produce both the propellant and LOX from CO2.

Researchers say making the propellant on Mars using Martian resources could help reduce mission costs. Additionally, the bio-ISRU process generates 44 tonnes of excess clean oxygen that could be set aside to use for other purposes, such as supporting human colonisation.

“Carbon dioxide is one of the only resources available on Mars. Knowing that biology is especially good at converting CO2 into useful products makes it a good fit for creating rocket fuel,” says first author Nick Kruyer, a recent PhD recipient from the Georgia Institute of Technology’s School of Chemical and Biomolecular Engineering (ChBE).

The new paper in Nature Communications outlines the process, which begins by ferrying plastic materials to Mars that would be assembled into photobioreactors occupying the size of four football fields. Cyanobacteria would grow in the reactors via photosynthesis (which requires carbon dioxide). Enzymes in a separate reactor would break down the cyanobacteria into sugars, which could be fed to the E. coli to produce the rocket propellant. The propellant would be separated from the E. coli fermentation broth using advanced separation methods.

The team’s research finds that the bio-ISRU strategy uses 32% less power (but weighs three times more) than the proposed chemically enabled strategy of shipping methane from Earth and producing oxygen via chemical catalysis. Because the gravity on Mars is only one-third of what is felt on Earth, the researchers were able to be creative as they thought of potential Mars rocket fuels.

“You need a lot less energy for lift-off on Mars, which gave us the flexibility to consider different chemicals that aren’t designed for rocket launch on Earth,” says corresponding author, Pamela Peralta-Yahya, an Associate Professor in the School of Chemistry & Biochemistry and ChBE, who engineers microbes for the production of chemicals. 

Image: Georgia Institute of Technology
Image: Georgia Institute of Technology

“We started to consider ways to take advantage of the planet’s lower gravity and lack of oxygen to create solutions that aren’t relevant for Earth launches.”

“2,3-butanediol has been around for a long time, but we never thought about using it as a propellant. After analysis and preliminary experimental study, we realised that it is actually a good candidate,” says Wenting Sun, an Associate Professor in the Daniel Guggenheim School of Aerospace Engineering who works on fuels.

The team is now looking to perform the biological and materials optimisation identified to reduce the weight of the bio-ISRU process and make it lighter than the proposed chemical process. For example, improving the speed at which cyanobacteria grows on Mars will reduce the size of the photobioreactor, significantly lowering the payload required to transport the equipment from Earth.

“We also need to perform experiments to demonstrate that cyanobacteria can be grown in Martian conditions,” says Matthew Realff, Professor and Fellow in ChBE, an expert in process synthesis and design who works on algae-based process analysis. 

“We need to consider the difference in the solar spectrum on Mars both due to the distance from the sun and lack of atmospheric filtering of the sunlight. High ultraviolet levels could damage the cyanobacteria.”

The researchers emphasise that acknowledging the differences between the two planets is pivotal to developing efficient technologies for the ISRU production of fuel, food, and chemicals on Mars. It’s why they’re addressing the biological and materials challenges in the study, to contribute to the goal of future human presence beyond Earth.

“Application of biotechnology on Mars is a perfect way to make use of limited available resources, with minimal starting materials,” Kruyer says.

A NASA Innovative Advanced Concepts (NIAC) Award funded the work.

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