The primary constituents of the Martian atmosphere are carbon dioxide (CO2), nitrogen (N2) and argon (Ar), whereas Earth’s are mainly N2, oxygen (O2) and Ar. Therefore, all the elements required to make breathable, Earth-like air are available in Martian air.
A Mars habitat may include inflatable extensions to significantly increase habitat volume beyond the size of the initial spacecraft. If the Hab is landed along with the MAV during a pre-deployment phase, it will be remotely activated from Earth, initiating the ISAP system, and causing the inflatable extensions to inflate. If the Hab is sent one launch window earlier than the crew, we will have about 20 months to inflate the Hab and test its other systems before the crew leave Earth. In order to inflate the extensions it will be preferable to make the necessary air using ISRU technology rather than bring air from Earth, which would require tanks of compressed O2 and N2.
The mass of the proposed ISAP system is likely to be less than the mass of full O2 and N2 tanks. More importantly, having the capability to manufacture breathable air from local resources will be a useful advantage for the mission, providing increased safety and reducing limitations on the mission. Locally manufactured air can compensate for air losses due to leaks, airlock cycling, atmosphere scrubbing, refilling of O2 and N2 tanks in pressurised vehicles and marssuits, punctures if they occur, and other potential causes.
The DRA proposes separating N2 and potentially also Ar from the Martian atmosphere for use as a buffer gas. However, it is actually much cheaper and easier to use the gas that remains after CO2 and dust is removed from Martian air, as this is mostly N2 and Ar, both of which are perfectly acceptable buffer gases. The Martian atmosphere may also contain undesirable toxic gases such as ozone (O3), carbon monoxide (CO), and nitric oxide (NO), that exceed safe limits (as specified in JSC 20584: Spacecraft Maximum Allowable Concentrations for Airborne Contaminants). However, these can be relatively easily scrubbed out using ordinary COTS catalytic converters, and this process is far simpler and energetically cheaper than trying to separate out pure N2 from the air. (Note that NO is not actually toxic, but when combined with O2 it rapidly oxidises to form NO2, which is.)
Step 1: Mars atmosphere is drawn into the ISAP system through a dust filter.
Step 2: Water is removed from the gas mix via zeolite adsorption. The captured water is stored in the Hab’s water tank and used to replace recycling losses, and for O2 production.
Step 3: Water is separated into H2 and O2 via electrolysis. The O2 is stored for habitat atmosphere.
Step 4: Microchannel adsorption or cryogenic separation (CO2 freezing) is used to separate CO2 (about 96%) from the gas mix.
Step 5: The CO2 is reacted with H2 via the reverse water gas shift (RWGS) reaction:
CO2 + H2 → CO + H2O
The H2O produced is returned to the water tank. The CO may be stored for use in fuel cells, or it may simply be vented.
The gas mix that remains after CO2 is removed is mostly N2 and Ar, with trace amounts of various gases, which may include neon (Ne), krypton (Kr), xenon (Xe), O3, CO, NO, methane (CH4), hydrogen peroxide (H2O2), and sulphur dioxide (SO2).
Step 6: This gas mixture is first passed through an ozone scrubber, which reduces the O3 to O2.
Step 7: The resultant gas mixture is then passed through an ordinary automobile 3-way catalytic converter, which converts any CO and NO into CO2 and N2.
Step 8: The result is a safe buffer gas comprised mostly of N2 and Ar, with small amounts of O2 and CO2, and traces of Ne, Xe and Kr. This mixture can be combined with additional O2 to provide breathing gas for the Hab. The CO2 level in this gas mixture is slightly higher than the proposed upper limit for the habitat atmosphere, but this excess will be removed by the ECLSS.
The DRA describes several methods for making O2 from CO2, including:
SOCE (Solid Oxide CO2 Electrolysis)
SOCE requires considerable energy, whereas the other two options require H2. Fortunately, H2 is available because the Hab will contain H2O for the crew, which makes SOCE’s less appealing than the other two alternatives.
RWGS, which produces CO and H2O, is preferred over the Sabatier reaction, which produces CH4 and H2O, because in RWGS all the H2 is converted to H2O, which is a valuable resource in its own right, and from which H2 can be easily recovered via electrolysis. Using the Sabatier reaction would either consume H2, or the H2 would have to be recovered from the CH4. In theory CH4 could be used in a fuel cell to provide additional energy to the Hab. Combustion of the CH4 would produce H2O, which could be captured. However, it remains to be seen if fuel cells will be relevant for the Hab, and this process would add a layer of complexity and inefficiency to H2 recovery. Choosing RWGS eliminates the need for any method to recover H2 from methane. An effective method for separating H2O from the gas stream existing the RWGS chamber is to adsorb it with zeolite 3A, as in step 2.
The DRA specifies that H2 be brought from Earth in order to make water for the crew; however, in the Blue Dragon architecture H2O is obtained from the atmosphere, and potentially also from the ground. Research into extraction of H2O from the Martian atmosphere (Grover et al, 1998; Williams et al, 1995) has shown how 3.3kg of H2O per day (enough to replace losses through life support regenerative processes) could potentially be obtained from the Martian atmosphere using adsorption into zeolite 3A. This idea has been incorporated into the ISAP process above, indicating that H2O necessary for the RWGS reaction need not necessarily come from the Hab’s supplies. The amount of H2O obtainable from the atmosphere may be small, but this doesn’t matter because predeploying the Hab allows plenty of time to collect the necessary H2O and make the air, and the H2 is recycled anyway.
More research needs to be done into this system. We need to investigate efficient gas separation techniques, the rate at which air can be manufactured, the unit’s mass, volume and energy requirements, and how it integrates with the ECLSS.
Growing food on Mars is an exercise in efficiency. The facilities will not (at least initially) be available to grow every kind of fruit, vegetable, grain, nut and herb that we are used to. We may only be able to grow small amounts of a small number of crops. For a while there may be restrictions on the number of available ingredients and therefore the meal choices. The challenge is to find which crops deliver the most nutrition for the amount of volume, mass and energy required to produce them.
Top of this list would surely be spinach. Tomatoes, mushrooms, cabbages, garlic, kale, carrots would perhaps also make the list. What is the best way to grow each of these?
There are 3 primary mechanisms proposed for producing food on Mars:
Experiments are already underway to grow food in Martian soil simulant. Because all the chemical elements necessary for life are available in Martian soil, it should be possible to grow plants in it; however, it will be necessary to analyse the crops thus produced, in order to determine if they have healthful levels of vitamins and minerals. They will not necessarily have the exact same nutrient profile as their counterparts on Earth, because of the availability of those elements in the soil.
Some chemical processing of the soil may be necessary to prepare it for plant growth; for example, it may be too acidic or salty. Therefore the addition of a specially prepared fertiliser may be necessary. It may be beneficial to introduce worms to the soil and feed them with food scraps, so they can process the dirt grains and organic material together to make fertile soil. Of course, for this to happen we would initially need food scraps, which would have to come from somewhere, so this would not be an option for the first crops.
In any case, the first crops are more likely to be grown using hydroponics or aeroponics. These are similar setups in that the plants are grown in a dirt-free environment, fed with nutrient-rich water. In the case of hydroponics, the water flows through pipes in which sit the roots of the plants, so they can access the nutrients in the water. In the case of aeroponics, the plants are suspended, with their roots exposed to the air; nutrient-rich water is provided to the roots as a mist.
The primary advantage of aeroponics over hydroponics is that the water requirement is minimal, which will be important for a Mars base where water may be scarce in the early years. The disadvantage, however, is that the mist greatly increases the relative humidity of the greenhouse atmosphere. Relative humidity of controlled environments like spacecraft atmospheres should not exceed 70% (according to NASA guidelines), as this can interfere with electronics or cause build-up of mould. This could be addressed by separating the greenhouse environment from the main habitat environment by a gate.
A double-gate, whereby a person transitioning from the habitat to the greenhouse would open one gate, step through, close that gate behind them, open a second gate, step through and close that one behind them, may be an effective method of containing humidity in the greenhouse. However, this level of control is probably unnecessary, and a single gate will work fine if people don’t leave it open. The small amount of water vapour that would travel across from the greenhouse to the habitat would be easily soaked up by the habitat’s ECLSS.
One way to mitigate the migration of water vapour from the greenhouse into the habitat would be to place intake fans near the gate, which draw air form the region around the gate into the THC (Thermal and Humidity Control) subsystem of the habitat’s ECLSS, which will remove any surplus water vapour from the atmosphere.
Aeroponics may therefore be the preferred choice. The questions remain:
What crops would be good to commence experimentation with? (e.g. spinach and tomatoes)
What nutrients are added to the water provided to the system, if any?
What mass and volume of equipment is required to produce what mass, volume and nutrient value of food?
Another downside of aeroponics is that in some cases the crops will not grow as large as they would in a hydroponic system or if they were grown in soil. This is usually due to lack of available nutrients in the provided water. When using water enriched with sufficient nutrients, and with the proper equipment, aeroponic crops can grow to full size.
Therefore, if we are to experiment with an aeroponic system on Mars, it will be important to take with us a supply of nutrients optimised for aeroponic crop production.
Aquaponics combines hydroponics with fish-farming, and is another approach to food production that has been suggested for Mars. The fish can be fed food scraps, possibly supplemented with special food; the water in which the fish swim becomes nutrient-rich due to the metabolic outputs of the fish, and is then provided to the plants via the hydroponic system.
The main problem with aquaponics is the amount of water required, which is naturally much higher than for the other proposed methods. It is far more likely that Martians will be vegetarians, which is perfectly safe and healthy; many millions of people on Earth live healthfully on plants only. Producing food from plants is considerably more efficient in terms of energy and water, which will be crucial on Mars. In fact, it’s also crucial on Earth, but humanity is still learning this. Some scientists predict that by 2050 everyone (or perhaps the majority of people) on Earth will be vegetarian due to expansion of the population. In any case, if aquaponics is implemented on Mars it will probably not be for a few years, when water and energy production are much higher.
Green Dragon is a mission concept to send integrated ISRU packages to function as resource factories to support human Mars missions. It builds on the accomplishments of NASA’s Ice Dragon mission, while functioning as an important precursor to the Humans-to-Mars mission architecture, Blue Dragon. The intention is to test and iteratively improve both ISRU and EDL systems.
The mission is named “Green Dragon” as, like Ice Dragon, it is based on the Red Dragon EDL system. It’s “green” because it demonstrates technologies and provides resources crucial to life support, and to match the pattern of red, green, blue commonly associated with Mars.
The initial Green Dragon program will land 1-3 Dragon capsules on Mars containing integrated ISRU hardware capable of manufacturing several products that will be of great value in exploration and settlement, namely LOX/LCH4 bipropellant, water and breathable air, from local Martian resources.
It will also serve as an opportunity to test and refine the Red Dragon EDL system in preparation for using it to land crew and cargo on Mars in the Blue Dragon mission. Our goal for landing humans on Mars is to maintain g-forces below an acceptable threshold for human health, and to land with a reasonably high degree of accuracy. Green Dragon will ideally serve to bringing the ISRU and EDL systems needed by Blue Dragon up to TRL-9.
Apollos 11-17 succeeded largely because the preceding missions in the program tested and proved each element of the target mission, i.e. the human landing. This approach of a program of precursor missions is not normally described in conjunction with H2M architectures, presumably because the large cost of sending stuff to Mars has typically resulted in very conservative programs. However, if the Red Dragon technology can be shown to reliably deliver a 1-2 tonne package to Mars surface for under $200M per launch-plus-capsule, a program of precursor missions that build the necessary confidence in a human landing will be affordable. Green Dragon would represent a key subset of those missions.
Entry, Descent and Landing
The Ice Dragon mission will inevitably stimulate ideas for improving the Red Dragon EDL system, and it will be important for the success of the Blue Dragon mission that we take the opportunity to implement these improvements and refine Red Dragon further. The more capsules we’ve landed on Mars before we attempt the landing of humans, the greater confidence we will have that the landing will succeed.
One of the EDL factors necessary to improve is a reduction in the deceleration forces experienced by the capsules. The first mission to land a Dragon capsule on Mars may involve deceleration forces that would be excessive for a human crew. While this is acceptable for a cargo capsule, the Green Dragon series of missions will aim to reduce deceleration forces to below about 10 g’s, and ideally lower, in preparation for safe landing of humans.
Another factor related to EDL that Green Dragon would seek to improve is the accuracy of landing. Green Dragon missions will make use of image recognition software, as well as signals from spacecraft, to land on preselected coordinates as accurately as possible. Having proven that Red Dragon capsules can be landed on a dime will make it possible to design a layout of the Blue Dragon base with much greater precision, thus improving safety and probability of success. For example, the DragonRider that delivers the crew to Mars surface can be landed exactly the desired distance from the Hab, which is to say, not so close that debris thrown up by the landing could damage the Hab’s inflatable modules or instruments (or any other part), yet not so far away that it would be unreasonably difficult or time-consuming for the crew to reach the Hab after landing.
The more Dragon capsules we land on Mars before sending humans, the more confidence we will have in the technology before using a DragonRider to land a crew on the surface of Mars. We need to know exactly how the capsule behaves during EDL to Mars, how it interacts with the Martian atmosphere and surface (especially dust), how to land one with precision, and where the strengths and weaknesses are in this approach. This knowledge can only be obtained definitively with practice.
By the time we land a crew on Mars in a DragonRider, we will ideally have landed a minimum of four uncrewed Red Dragons:
1 for Ice Dragon
1-3 for Green Dragon
2+ pre-deployed cargo capsules for Blue Dragon
In Situ Resource Utilisation
The Green Dragon mission, or series of missions, is designed to test, demonstrate and develop the ISRU technologies necessary for successful execution of the Blue Dragon architecture. Previous architectures have been reasonably conservative in terms of inclusion of ISRU due to the low TRL of the concepts. Blue Dragon pushes the boundaries of what has previously been considered in terms of ISRU to support a humans to Mars mission, and, being mission critical elements, it’s essential to prove that they will work.
Green Dragon will allow us the opportunity to test a variety of ISRU experiments on Mars, including the manufacture of LOX/LCH4 bipropellant, water and breathable air from local Martian resources.
In particular, the demonstrated ability to obtain abundant water on Mars is considered crucial to exploration and settlement. This capability will reduce the cost and increase the likelihood of success of all future H2M missions by a significant margin, as neither water nor hydrogen will need to be brought from Earth.
A design goal of Blue Dragon is to not take any surplus hydrogen, water, oxygen or nitrogen to the surface of Mars. As all these things are available locally and can be obtained directly from Mars, and since utilising local resources significantly reduces mission cost, there’s no sense carrying them all the way from Earth; that is, if the necessary hardware can be developed to successfully and reliably obtain them, which it certainly can.
Proposals for ISRU tech for Mars have traditionally been very simple and conservative. Considering the success of MER’s, Phoenix and MSL – all very complex robots – this cautiousness is probably unfounded. Although the ISRU experiments encapsulated by Green Dragon are more complex than basic ISPP, they are also much less complex than any of these missions that have already been successful. Furthermore, the whole Green Dragon program should, in theory, cost less than MSL.
Green Dragon contains a set of three integrated ISRU experiments:
ISPP – In Situ Propellant Production: Making LOX/LCH4 bipropellant.
ISWP – In Situ Water Production: Extracting water from the ground.
ISAP – In Situ Air Production: Producing breathable air.
This will include the following subsystems:
Storage tanks for water, LOX, LCH4 and buffer gas
Small, self-contained nuclear fission reactor, or solar panels
The following colour codes are used for tanks:
Buffer gas (mainly N2/Ar)
The following schematic illustrates the flow of materials through the system. The masses are calculated based on Mars Direct ERV propellant requirements, but can be scaled to suit the capacity of the Green Dragon (small or large version).
The concept of ISPP (In Situ Propellant Production), whereby LOX/LCH4 (Liquid Oxygen/Liquid Methane) bipropellant is produced by reacting carbon dioxide obtained from the Martian atmosphere with hydrogen brought from Earth, was most famously described by Robert Zubrin and David Baker as part of the Mars Direct mission architecture in 1990 . They also constructed a small plant to prove that the technique was viable.
Our goal is to use hydrogen manufactured via electrolysis of locally collected water, rather than bringing it from Earth as in the original ISPP concept.
The process is as follows:
Step 1: Carbon dioxide is obtained from the Martian atmosphere by filtering out dust, removing water via zeolite adsorption, and freezing the CO2 out of the remaining gas mix, enabling it to be separated from the remaining gases. The small amount of water captured by zeolite adsorption may be cycled into step 3.
Step 2: The AWESOM rover traverses back and forth across a patch of ground, microwaving the regolith below and collecting released water. When the rover’s tank is full, it returns to the MAV to deliver its payload of water, then returns to work. This step requires some form of LPS (Local Positioning System) so the rover knows which ground it has covered.
Step 3: Water is separated into hydrogen and oxygen gas via electrolysis:
2 H2O(l) → 2 H2(g) + O2(g)
The oxygen is stored cryogenically as LOX.
Step 4: Methane is produced by reacting carbon dioxide (CO2) obtained from the Martian atmosphere with hydrogen (H2) produced in step 3, via the Sabatier reaction:
CO2(g) + 4 H2(g) → CH4(g) + 2 H2O(v)
The Sabatier reaction occurs at high temperatures, optimally around 600K. The methane is liquified to LCH4 and stored in the red tank. The water produced by the Sabatier reaction is stored in the blue tank, where it may be used to make more hydrogen.
Combining the reactions in step 3 and 4, the overall result is:
CO2(g) + 2 H2O(g) → CH4(g) + 2 O2(g)
66 tonnes of carbon dioxide combined with 54 tonnes of water (a total of 120 tonnes) will produce 24 tonnes of methane and 96 tonnes of oxygen. This gives us a surplus of 12 tonnes of oxygen that may be used for air. As the Martian atmosphere is about 95% CO2, we need to process about 70 tonnes of it to obtain 66 tonnes of CO2, which therefore results in about 4 tonnes of buffer gas.
ISWP (In Situ Water Production) via extraction of water from the Martian ground is well understood to be a crucial enabling technology for human settlement of Mars, and would similarly be a boon for preliminary exploration. The robotic technology required to achieve this is likely to be simpler than that we have already deployed to Mars in the form of Spirit, Opportunity and Curiosity, and is therefore well within our capability.
The advantage of not having to bring hydrogen from Earth is of such value that developing a system for obtaining water on Mars is one of the most important outcomes of Green Dragon. Not only will we save the mass of the hydrogen needed for ISPP, but also the water required by the crew during the 1.5-year surface mission. This capability will greatly reduce the launched mass, landed mass, cost and complexity of any H2M mission.
Producing LOX/LCH4 propellant using hydrogen obtained from Martian atmospheric water vapour, instead of hydrogen brought from Earth, has only been discussed theoretically and not demonstrated. One goal of Green Dragon is to demonstrate that.
Water collected by the ISWP experiment will be collected in a tank. This will be connected to the electrolysis unit so that any amount can be converted to H2 and O2 as required. H2 will not be stored, but will be fed directly into the ISPP unit. In this way we avoid the need to store hydrogen gas, and thereby losing any through leakage.
Note also that water is also produced by the Sabatier reaction used in the ISPP experiment. This water will simply be captured and collected in the same tank, thus recycling the hydrogen.
ISAP (In Situ Air Production) will be an important aspect of Blue Dragon. The Martian atmosphere contains plenty of nitrogen and oxygen, which is readily accessible, and considering the benefits of minimising launched mass there is no justification for bringing air from Earth. Of course, the Hab, MAV and rover may be launched full of air, but we need more than that.
Because we intend to utilise inflatable modules to expand the Hab volume, our primary requirement for air is to inflate these modules. Also, if the rover or MAV becomes depressurised for any reason, they will need to be re-pressurised.
Air pressure within the Hab may be lost due to:
Airlock cycling (a daily occurrence during the surface mission).
Punctures or tears.
Oxygen is easily produced via electrolysis of water, as described above. Every kilogram of hydrogen manufactured from water for ISPP will produce eight kilograms of oxygen that may be used for air.
Earthian air is approximately 78% nitrogen, 21% oxygen, 1% argon, plus traces of other gases. Oxygen is needed by animals; nitrogen is needed by plants and certain microbes, and acts as a buffer gas. Argon is non-reactive and safe to breath as long as there’s sufficient O2 in the mix. Ideally in our habitats we will want a full 1 atmosphere of pressure, as on the ISS, because then we can use COTS equipment which is much cheaper. However, the buffer gas does not necessarily need to be comprised of the same ratio of nitrogen and argon as on Earth.
The ISPP process separates carbon dioxide from the remainder of the Martian air using fractional distillation, which means cooling the gas to below the boiling point of carbon dioxide, thus causing it to become liquid, in which state it can be easily separated from the remaining gases.
There are at least 12 gases present in the Martian atmosphere. As part of the ISPP process, dust and water will be removed from the atmosphere first, then carbon dioxide is separated from the remainder. This produces a residual gas mix with the following approximate composition, which is about 98% safe to breathe.
Preferred buffer gas.
When combined with oxygen this will oxidise to NO2, which is toxic.
Creating an exact match for Earthian air by separating out the nitrogen and argon would be energetically costly due to the low boiling point of these gases (77K and 87K respectively). However, the above gas mix can be made breathable by the removal of toxic elements, highlighted above in green. Fortunately, this is a PSP (Previously Solved Problem).
Scrubbing CO and NO
A standard automobile catalytic converter (~$200 – $500) can be used to scrub CO and NO from the gas mixture.
The reduction catalyst (platinum and rhodium) in the catalytic converter converts NO into N2 and O2:
2NO → N2 + O2
The oxidation catalyst (platinum and palladium) in the catalytic converter converts CO into CO2:
2CO + O2 → 2CO2
It will also oxidise the tiny amount of methane:
CH4 + 2O2 → CO2 + 2H2O
One issue to address is that COTS catalytic converters operate at high temperatures, whereas the ambient temperature on Mars is comparatively very low. The nuclear power plant can provide ample electricity to heat the converter as required. We can simply position the catalytic converter somewhere near the Sabatier reactor, which operates at around 600K.
Ozone is relatively easily eliminated using a COTS ozone scrubber (or “ozone destruct unit”) (~$1200). Ozone scrubbers work best with dry gas, therefore, since the catalytic converter will produce a small amount of water in the process of oxidising the methane, it’s preferable to pass the gas through the ozone scrubber before the catalytic converter.
A breathing gas comprised of 20% oxygen plus 80% of the resultant buffer gas mixture (which has a little O2) will contain about 48.3% nitrogen, 28.6% argon, 21.7% oxygen, 1.4% carbon dioxide, and traces of neon, krypton, xenon and water vapour.
This is a very good breathing gas mixture except that it’s a little high in CO2, and would cause drowsiness. This can be easily removed by the air recycler in the ECLSS (Environment Control and Life Support System). In other words, once the air has been manufactured in this way, it is first treated in the same way as stale air coming from the Hab and pumped through the air recycler to scrub out the excess CO2. In fact, it may even be better to incorporate the ozone scrubber and catalytic converter in the ECLSS rather than the ISAP unit, and simply treat all buffer gas as stale/dirty air.
At one atmosphere of pressure, this breathing gas is sufficiently similar to Earth-normal that no difference would be noticeable except perhaps a reduction in audio frequencies (including astronaut voices) due to the high percentage of argon.
However, it is not a firm requirement that the habitat air pressure be one atmosphere, although that would be ideal in some respects (the internal atmospheric pressure of ISS is one atmosphere, which simplifies design of other components). Green Dragon produces about three times as much surplus O2 as buffer gas, therefore we may opt for a lower concentration of buffer gas (say, 20-50% instead of the usual 80%), which would probably be acceptable.
With a source of oxygen plus a suitable buffer gas mix available, we can plug these into the Hab to maintain an optimised internal atmosphere. Whenever the partial pressure of O2 fraction drops below a certain threshold, additional O2 is pumped in; similarly, if the buffer gas partial pressure drops too low, more buffer gas will be automatically pumped in. This is how the ECLSS functions on the ISS.
Larger Green Dragons
The Mars One mission plans to land both cargo and crew on Mars using an upgraded Dragon capsule with a 5 metre diameter. At the time of writing this capsule has not yet been developed, although SpaceX has indicated that it will be developed within the next few years in order to be ready for the first Mars One cargo mission in January 2016. This larger Dragon will be capable of delivering a payload upwards of 2.5 tonnes to Mars surface.
For at least the first Green Dragon mission, the smaller capsule with the 3.66m diameter and the approximately 1-2 tonne payload will be adequate. However, when these larger capsules become available, larger Green Dragons based on these could be sent as resource factories to support Blue Dragon, Mars One, or other missions or bases. The primary advantage of the larger capsules will be the capacity to store greater volumes of their products.
The suggested approach therefore is to use the smaller capsules to develop and refine the design of a self-contained resource factory, then use the larger capsules to support missions.
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:
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.
Apart from ECLSS (Environment Control and Life Support Systems) and space vehicles, ISRU is one of the most important capabilities we need to develop in order to settle Mars.
“In Situ Resource Utilisation”, or “ISRU”, simply means using local resources. To illustrate: when European settlers sailed the ocean blue to new lands past the edge of the world, they did not take everything with them that they’d need for their new life. To do so would have been entirely impractical. On arrival at new lands, explorers drank water from local streams, plucked ripe fruits from local trees and hunted local wildlife. They used wood and stone from near the settlement sites to build homes and other structures. In other words, they utilised resources from their current location, hence, In Situ Resource Utilisation (“in situ” is Latin for “on site” or “at location”).
This is one of the main categories here is because ISRU is a crucial capability for Mars exploration and settlement. (Also because it’s a large and interesting topic.) Launching anything into space from the surface of Earth is extremely expensive, costing between $2,000 and $15,000 per kilogram, depending on the vehicle. Considering that everything needed to send even a small crew of humans to Mars – spacecraft, life support systems, food, fuel, and other supplies and equipment – could weigh 50-100 tonnes or more, that’s a significant cost.
Furthermore, the more mass you need to deliver to the surface of Mars, the more fuel is needed to launch that mass to Earth orbit and then onwards to Mars – and the fuel itself has mass. Rockets also have limits in terms of mass and volume; therefore, more mass can mean a greater number of launches. Thus, as mass increases, costs tend to compound in an exponential way, rather than linear.
Anything we can obtain from the Martian environment therefore translates to a significant reduction in mission cost and therefore an increased likelihood that the mission will be flown; or, a greater number of missions that can be flown. For those of us who would prefer to see H2M happen sooner rather than later, developing ISRU capability on Mars is therefore considered crucial.
Mars has plenty of oxygen, nitrogen, carbon, water, metals and energy that can potentially be used by human explorers and settlers. The challenge is accessing it, which is dependent on technology and scientific knowledge.
For the past half-century we’ve been accumulating the necessary scientific knowledge – characterising the atmosphere, climate, the crust and every other aspect of the planet that we can perceive – in order to determine what resources are available. We now have reasonably detailed understanding of the Martian atmosphere and surface, including an awareness of the large quantities of water available across the surface of Mars.
The atmosphere is by far the most accessible resource on Mars, since air can easily be drawn into a system designed to extract substances from it. The next most easily accessible resource is the regolith – the loose top layer of dust and dirt on the Martian surface.
The topic of Martian ISRU can be organised into two sections:
ISRU Level 1
This refers to the most basic resources that we need to obtain from the local environment for the purposes of survival, and simply in order to make H2M feasible. These ISRU processes are likely to be developed at bases during early exploration of Mars (Stage 1 of Mars settlement).
ISRU Level 2
This refers to advanced ISRU processes to produce materials and other substances necessary for manufacturing and industrial processes. While the full list of potential resources that will eventually be produced on Mars would be far too long to list exhaustively, some of the most obvious include: