The best-known example of ISRU for Mars was described in the mission architecture published in 1991 by Robert Zubrin and David Baker, called Mars Direct.
Mars Direct triggered a revolutionary shift in the Mars community, being radically cheaper and simpler than any previous proposals.
One of the key innovations incorporated into Mars Direct is the idea of manufacturing LOX/CH4 (liquid oxygen/methane) bipropellant from the Martian atmosphere, rather than transporting it from Earth. This fuel can then be used to launch the crew from the surface of Mars in an ERV (Earth Return Vehicle) on completion of the surface stay. By manufacturing fuel from local resources, rather than carrying it from Earth, the mass needed to be launched from Earth is significantly reduced. This in turn greatly reduces the cost and complexity of the mission.
As one of the earliest and best known examples of ISRU on Mars, and one that will almost certainly be used by Mars missions from the outset, it’s worthwhile to review it here.
The methane portion of the bipropellant is produced by reacting carbon dioxide (CO2) obtained from the Martian atmosphere with hydrogen (H2) brought from Earth, via the Sabatier reaction:
(1) CO2(g) + 4 H2(g) → CH4(g) + 2 H2O(v)
Water (H2O) produced by this reaction is then separated into hydrogen and oxygen gas via electrolysis:
(2) 2 H2O(l) → 2 H2(g) + O2(g)
The hydrogen produced by reaction (2) is recycled back into reaction (1), producing more methane. The oxygen produced by reaction (2) is stored cryogenically as LOX, the oxygen portion of the bipropellant.
The original Mars Direct architecture specifies carrying 6 tonnes of H2 from Earth. By combining these processes, this amount of hydrogen can be used to manufacture 48 tonnes of O2 and 24 tonnes of CH4.
However, the stochiometric ratio for LOX/CH4 bipropellant is 3.5:1 (or 7:2). That means, to burn 24 tonnes of methane, we need 84 tonnes of oxygen. That’s an additional 36 tonnes.
Several processes were proposed for produced the additional O2. Perhaps the most elegant is combining the Sabatier reaction with the reverse water gas shift (RWGS) reaction in the same chamber.
The reverse water gas shift reaction reacts carbon dioxide with hydrogen to produce carbon monoxide and water:
(3) CO2(g) + H2(g) → CO(g) + H2O(v)
Combining reactions (1) and (3), we get:
(4) 3 CO2(g) + 6 H2(g) → CH4(g) + 2 CO(g) + 4 H2O(v)
The H2O is electrolysed, storing the O2 and recycling the H2 back through reaction (4), just as we did with reaction 1. This produces 96 tonnes of O2, which is enough oxygen for the bipropellant plus 12 tonnes spare. The surplus oxygen can be used to top up the hab, pressurised rover or surface suit oxygen tanks. In addition, the CO produced in reaction (4) could potentially be used as rover or generator fuel.
The value of this approach should be clear. Bringing only 6 tonnes of H2 from Earth is significantly cheaper and easier than transporting 108 tonnes of bipropellant! Launching the crew from Mars at the end of the surface stay is one of the most challenging aspects of any H2M mission, which is why many people, including the designers of Mars One, have opted for one-way missions. Yet, naturally, most mission planners – especially those from governmental space agencies – are inclined or obliged to plan for the crew to return to Earth. This one idea presented in Mars Direct suddenly made this a whole lot easier to achieve, which is why it had such a big impact on the Mars community. It became a core design feature of several new H2M architectures, including the NASA Design Reference Mission.
Hydrogen on Mars
Of course, this approach begs the question: if it makes sense to manufacture rocket fuel from local Martian resources, why do we need to carry even 6 tonnes of hydrogen from Earth? Hydrogen is notoriously difficult to store in space. Large tanks are required, due to its low density, and because H2 molecules are so small, at least 0.5% per day boils off and leaks away into space. The amount launched from Earth has to be more than 6 tonnes to allow for this boil-off.
But Mars has plenty of hydrogen. Why can’t we use ISRU techniques to obtain and use it?
The problem is that most of the hydrogen on Mars is in the form of water frozen in the regolith, and this simply isn’t as easy to access as the atmosphere. Methods are indeed being researched to obtain water from the regolith; for example, using robots and microwave radiation. But these are possibly too complex for the first H2M mission, which is what Mars Direct is principally designed for.
Hydrogen represents only 1/9 of the mass of water; therefore, to obtain 6 tonnes of hydrogen requires first obtaining 54 tonnes of water.
The Martian atmosphere contains about 0.021% water vapour, which can be accessed. To obtain 54 tonnes of water would require processing almost 260,000 tonnes of Martian atmosphere. If we attempted to achieve this during a 26-month period between launch windows, this means processing about 10,000 tonnes of atmosphere per month, or over 300 tonnes per day. The equipment required to achieve this could weigh more than 6 tonnes, thus offsetting any benefit to this approach.
Nonetheless, it’s inevitable that techniques will be developed for obtaining water from the Martian atmosphere, and from the regolith. This will be a crucial capability for human settlement of Mars.