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Because we’re sending three missions to a single location in order to establish the first permanent human settlement on Mars, it’s especially important that the location choice be a good one. The intention in this section is not to identify one specific optimal location for the IMRS, which would require a more involved analysis, but to highlight the salient characteristics such a location would have, and to outline a general approach to analysis.
There are several drivers in selecting an optimal location, which must be balanced:
- Availability of key resources. This includes sunlight, heat and water and (secondarily) areothermal energy. Atmospheric resources such as carbon, oxygen, nitrogen and atmospheric water are not location-dependent. Other location-dependent resources that will become more important in the future include wind energy, certain minerals and metals, caves and lava tubes, tourist attractions, and infrastructure.
- Terrain characteristics. For safety in landing, and ease and safety of surface mobility both in marssuits and surface vehicles, we require a location that is reasonably flat, level, and not overly dusty. Low dust is indicated by high thermal inertia, which will also be advantageous for reducing energy storage requirements. In addition, we desire loose regolith to pile on the Hab for radiation and thermal protection.
- Scientific interest. Naturally the best location will be close to sites that can help to answer scientific questions about Mars. Most importantly, has Mars ever hosted, or does it currently host, life as we know it? Other questions relate to the presence of liquid water, Mars’ geologic history, etc.
Previous Mars missions and the DRA have primarily favoured scientifically interesting locations, in line with their intent to continue exploiting the scientific goldmine that is Mars. However, the goal for Blue Dragon and the IMRS is to establish a permanent human presence on Mars as a base from where further and extensive scientific exploration of Mars can occur more cheaply and conveniently. With this in mind (and the general principle that all of Mars is scientifically interesting, especially to humans on the ground), selecting a site of maximum scientific value is considered less important than selecting a site with maximum potential for supporting habitation.
Our preference for a power system based on solar energy (discussed later) is our strongest driver for location selection. Generally speaking, as on Earth, the availability of sunlight reduces with increased latitude. However, due to the eccentricity of Mars’ orbit northern latitudes receive more solar energy than southern, which is fortuitous as there are other reasons for favouring the northern hemisphere. It has been shown that the latitude of 31° north has the highest minimum solar incidence for a single sol over a Martian year (Cooper et al., 2010). Because the minimum solar energy a site receives in a single sol determines the maximum required mass of the energy storage subsystem, this is a good reason for locating the base near this latitude. The ideal location might be within approximately 5-10° of this optimal latitude, as long as it also satisfies other conditions.
In addition, we must not select a location deep in a crater or chasm, as this would increase the amount of time the PV cells are in shadow each day. Rather, we must choose a location out in the open that receives as much sunlight as possible each day. This is harmonious with our need to land somewhere flat.
One of the primary energy requirements of a habitat on Mars is thermal control. The surface temperature on Mars varies between 130K and 308K, with an average of about 218K; temperatures that are, on average, comparable with Antarctica.
The desired temperature inside the Hab, however, is a comfortable 295K (22°C) ± ~5K. Therefore the ECLSS must maintain the interior temperature of the Hab around 80K warmer than the external environment, on average. This is a significant temperature gradient, and one that must be maintained all day, every day, throughout the entire 1.5-year surface stay.
The amount of energy necessary for heating is mitigated to some extent due to warming of the Hab’s atmosphere by human metabolism and operation of electrical and electronic equipment. During warmer days the Hab will need cooling. Further analysis is required to determine the actual energy requirements for thermal control, however, the warmer the location, the lower they will be.
The amount of thermal energy available from the natural environment is partly a function of solar incidence. As we’re already selecting for high solar incidence, this automatically selects for warmth as well.
Thermal inertia of the terrain is also important, as a higher thermal inertia will keep the base warmer for longer after nightfall, reducing energy storage needs. Selecting for higher thermal inertia means choosing a less dusty site. This is somewhat harmonious with our terrain requirements, as a less dusty site is preferable for surface mobility.
For a long surface stay mission architecture (~540 days), the crew is be on Mars for approximately three-quarters of a Martian year (687 days), or three out of four seasons.
Mars seasons are aligned with the eccentricity of its orbit. At perihelion it’s winter in the north and summer in the south. At aphelion it’s summer in the north and winter in the south. The southern hemisphere therefore experiences a more extreme climate, with hotter summers and colder winters than the north.
When Alpha Crew departs for Mars in 2031, it will be close to perihelion, which is the reason we choose that particular year to travel. The distance to Mars will be at its minimum in the approximately 16-year cycle of perihelic oppositions. Therefore, it will be winter in the northern hemisphere at time of launch. The crew will arrive approximately at the end of the northern winter and spend the next three seasons on Mars, and leave at approximately the beginning of the next northern winter. Alpha Crew will therefore miss the northern winter, which is optimal for light and heat. This is fortunate, as there are several other reasons to locate the base in the north.
Bravo and Charlie Crews, however, will be at the IMRS during northern winter. The intention is to increase power production at the base by at least 100kW with each mission, thus compensating for degradation of PV cells, developing increasing energy security at the base, and ensuring ample power for surviving suboptimal conditions such as winter and dust storms. Because Alpha Crew will have the least infrastructure and experience, the intention is to make their mission as safe as possible.
Choosing an especially flat area (low surface roughness) is important for:
- Safe landings.
- Mobility in marssuits and surface vehicles.
The Blue Dragon architecture requires safely landing the MAV, Hab, and at least two cargo capsules in approximately the same location. However, note that they will not be directly adjacent to each other, because landing any item will kick up a lost of dust and debris; therefore, each item must be landed some distance, perhaps about 100-1000 metres, from the others.
The Rover must be able to negotiate the terrain between these elements, and the surrounding area. In addition, the AWESOM (Autonomous Water Extraction from the Surface Of Mars) robot must be able to traverse the ground around the MAV.
As this is our first landing on Mars with a human crew, we must minimise the probability of landing on a large boulder or a slope, so the flatter and smoother the better. Fortunately this corresponds with our desire to find a location near a potential source of areothermal energy, because it’s the locations that have most recently been covered in lava (and therefore have few craters) that are most likely to still be areologically active.
The downside of choosing an especially flat area is that there may not be much of areological interest in the vicinity, which means more exploration will have to be performed using the Rover rather than simply in marssuits. However, safety is a priority. The goal will be to locate the base within, at most, a few hours drive away from areologically interesting regions.
The flattest region of Mars is Vastitas Borealis, the vast low-lying northern region that may have once been the floor of a huge ocean (known as “Oceanus Borealis”).
The following map shows surface roughness, with smooth areas appearing dark and rough areas appearing light. Regions of low surface roughness between 26-36N are highlighted. Note that the region west of Olympus Mons is one of the smoothest on Mars:
The Martian atmosphere is warmest at lower elevations, as the atmosphere is thicker and functions as a thermal blanket.
Vastitas Borealis is at a much lower altitude than the southern highlands, which again leads us to favour this region. The following map shows the topography of Mars, based on data returned by the MOLA (Mars Orbiting Laser Altimeter). Low elevation regions between 26°N and 36°N are highlighted:
Because Martian air is warmer at lower elevations, some researchers have proposed Hellas Basin as a good location for a base. However, this choice would not exploit other advantages of the northern hemisphere, it is not very flat, and it’s a dust trap. Hellas Basin is one of the regions from where dust storms frequently erupt.
Thermal inertia refers to the ability of the terrain to retain heat. Small particles, such as dust, have low thermal inertia, i.e. they lose heat rapidly. Boulders and exposed bedrock have high thermal inertia, i.e. they retain heat longer. (This is why people use polished concrete indoors, as a thermal mass to reduce heating costs.) Thermal inertia at the location is important for several reasons:
- A very low thermal inertia implies thick dust, which could impede mobility in marssuits and surface vehicles. Dust is also less useful for piling around and on the Hab for thermal and radiation protection.
- A high thermal inertia is advantageous as the ground will serve to keep the base warmer after sunset, reducing energy requirements.
- A very high thermal inertia would imply large boulders or large regions of exposed bedrock.
- A medium thermal inertia implies loose regolith, which we desire for piling around and on the Hab, both for insulation to maintain habitat temperature after nightfall, and for radiation protection.
In other words, when it comes to thermal inertia, it’s a case of not too low and not too high.
The following map shows thermal inertia, with areas of intermediate thermal inertia between 26°N and 36°N highlighted:
One of the key goals of Blue Dragon is to make use of locally obtained water, in order to avoid transporting hydrogen from Earth.
The amount of water that could reasonably be obtained from the air would not provide enough hydrogen for ISPP. Therefore, we must look to ground-based sources. Fortunately there’s plenty of water on Mars. The following map shows, circled, the wettest areas between 26°N and 36°N:
The ground on Mars is above 20-30% water beyond ~60° north and south, which is attractive. However, at these latitudes the solar incidence is somewhat low. if we seek to remain close to 31°N, we may select a location around 35-45°N as a compromise. The wettest locations in this zone may contain as much as 10% H2O, which would be a useful quantity.
Areothermal energy may be available in some areas on Mars, most likely in regions that have been the most recently areologically active (i.e. the Upper Amazonian epoch). Although not a requirement for the first three missions, it is scientifically interesting and will be important to future settlements. If the IMRS is expanded into a larger-scale settlement, being close to areothermal energy will be a huge advantage.
A reliable source of areothermal energy on Mars will be of exceptional value, potentially superior to all other energy options currently under consideration. Areothermal energy could provide a continuous abundant supply of energy, without the environmental issues of fission or the need for energy storage subsystems as for solar or wind, and can be used directly for settlement heating, water supply, and electricity generation. If it is shown that areothermal energy is only available in a few places on Mars, these places may well be settled first, or may be more successful.
Areothermal energy combined with large ice deposits, such as we see in Arcadia ad Amazonis Planitias, could be indicative of liquid aquifers. Not only would this be a tremendously valuable resource for a settlement, but also a potential home for extant Martian life.
Regions on Mars that have been recently areologically active (Fogg, 1996) include:
- Cerberus Plains
- Hecates Tholus
- Medusae Fossae Formation
- Northwestern Tharsis
- Valles Marineris
The northern hemisphere of Mars is strongly preferred for a variety of reasons:
- More water, in the form of both ground ice and atmospheric water vapour.
- A higher minimum solar incidence.
- Less extreme climate.
- Lower elevation.
- Flatter, smoother terrain.
- Higher probability of areothermal energy sources and underground aquifers.
- More mineral ores, which are often formed by liquid water.
- More thorium, which may be important for LFTR’s (Liquid Fluoride Thorium Reactor).
Because solar energy is preferred (see Power Systems), the first base should be located close to the optimal latitude of 31°N in order to minimise the mass of the power system. However, by going a little further north more water can be accessed. Somewhere in Vastitas Borealis will be suitable, as this region is low, flat and reasonably smooth. The goal will be to find a location in this region that is not too dusty, and has loose regolith.
A higher-capacity energy storage subsystem would effectively eliminate the motivation to remain close to 31°N, which would therefore open up a wider range of options. However, there are a couple of good candidate locations near this latitude that should perhaps be investigated more closely:
Northern Amazonis Planitia, around 35°N 145°W
This may be the sweet spot in this region just northwest of Tharsis. There’s abundant water at relatively low latitudes, with good solar incidence, and it’s low and flat. This is one of the smoothest regions of Mars, and one of the most recently areologically active at only 100 million years old, making it likely to have areothermal energy. Arcadia Planitia is just to the north, which shows signs of near-surface ground water. From Wikipedia:
In a lot of the low areas of Arcadia, one finds grooves and sub-parallel ridges. These indicate movement of near surface materials and are similar to features on earth where near surface materials flow together very slowly as helped by the freezing and thawing of water located between ground layers. This supports the proposition of ground ice in the near surface of Mars in this area. This area represents an area of interest for scientists to investigate further.
Areothermal energy in combination with underground ice could be indicative of aquifers, which would be tremendously valuable resource both materially and scientifically, as they may potentially harbour extant life. This location is close to Olympus Mons, one of the most famous and impressive features in the Solar System, which is both scientifically and aesthetically interesting. The thermal inertia is a bit low, which may make this region overly dusty; however, less dusty sites may be discovered on closer inspection.
Utopia Planitia, just north of Hecates Tholus, around ~40-45°N 150°E
This area may in fact be slightly superior due to a moderate level of thermal inertia. It’s flat and smooth and has about 7-10% H2O, good solar incidence, and low elevation. The area is areologically very young and considered a candidate for areothermal energy. It’s also within striking distance of Phlegra Montes, where radar probing has indicated large volumes of water ice below the surface.
The current version of the SpaceX Dragon capsules, including the one that historically became the first commercial spacecraft to dock with the ISS, are designed to splash down in water, like those used in NASA’s Mercury, Gemini and Apollo programs and like NASA’s new Orion capsule.
The next generation of Dragon capsules are designed to land on solid ground. Currently in development at SpaceX, they are fitted with eight “SuperDraco” engines, which are a powerful new variation of the Draco engines used by the Dragon RCS (Reaction Control System). Like the Dracos, they use non-cryogenic propellant: monomethyl hydrazine fuel and nitrogen tetroxide oxidiser. However, they’re much more powerful, each capable of delivering about 67 kilonewtons of axial thrust, for a total of about 534kN. These engines will enable the Dragon to land propulsively on solid ground, usually back at the original launchpad, thereby saving the time and expense of water recovery and opening up the possibility for Dragon capsules to land on the Moon, Mars and other worlds with solid surfaces. This is in alignment with SpaceX CEO Elon Musk’s stated purpose of establishing settlements on Mars.
SpaceX offer two basic configurations for the Dragon capsules: cargo and crew. The crewed version is known as a “DragonRider”, and can accommodate up to seven astronauts. These may be used for transporting crew between Earth and the ISS in the near future.
In the Blue Dragon architecture, which is designed for a crew of six, the seventh seat is removed and the volume that it (and a seventh person) would normally occupy is reserved for cargo. This may be last minute personal items from Earth, or samples from Mars. All three DragonRiders in the architecture will be modified to accommodate six people plus storage.
“Red Dragon” is a proposed variant of the SpaceX Dragon capsule currently being investigated by NASA as a low cost alternative for delivering payloads to Mars (Karcz et al., 2012).
Red Dragon will presumably be similarly configured with SuperDraco engines. Alternatively, they may utilise new methane-fuelled “Raptor” engines being developed at SpaceX, which would have the advantage that they could be refuelled on Mars. In addition, Red Dragon will incorporate several modifications necessary for EDL on Mars, including:
- Removal of systems unique to LEO missions, such as berthing hardware.
- Addition of deep space communications.
- Modifications to SuperDraco (or Raptor) engines to suit the Martian atmosphere.
- Reduction of heat shield thickness, since the atmosphere is far less dense.
- Algorithms and avionics for pinpoint landing on Mars.
The gravity on Mars is lower, which reduces the acceleration of the capsule towards Mars; however, in the case of direct entry the capsule will be approaching from interplanetary space at a much higher velocity than if it were descending from orbit. Also, Mars’ atmosphere is much thinner (less than 1%) than Earth’s, so it will play less of a role in reducing spacecraft velocity during EDL. However, for the same reason, there is less heating due to atmospheric friction, and therefore less or, perhaps, different heat shield material may be used. The different conditions will affect the forces experienced by the spacecraft, which may require changes to thrusters, heat shield, avionics and other aspects.
NASA have calculated that a Red Dragon capsule will be capable of delivering payloads of up to 1.9 metric tonnes to the surface of Mars. This delivery mechanism has been receiving increasing attention from NASA, being considerably simpler and cheaper than, for example, the sky crane method used to deliver Curiosity. Not only will it be cheaper per kilogram of payload mass, but much cheaper overall.
Another clear advantage is that a landed capsule can be repurposed as a storage unit, shelter or habitat.
Once the Red Dragon technology has been proven as a reliable mechanism for delivery of cargo, this approach may be used to deliver up to seven crew members to Mars surface, simply by using a DragonRider modified in the same way.
Red Dragon represents a near term technology that can enable comparatively inexpensive and functional Mars missions. It’s a fundamental element of the Blue Dragon architecture (hence the name), being utilised for delivery of both crew and cargo to Mars surface.
The Dragon capsules are being designed to land with a high degree of accuracy. From the SpaceX website:
“SuperDraco engines will power a revolutionary launch escape system that will make Dragon the safest spacecraft in history and enable it to land propulsively on Earth or another planet with pinpoint accuracy.”
This ability to land “with pinpoint accuracy” is supported by the Dragon’s GNC (Guidance, Navigation and Control) system. Due to the lack of GPS (Global Positioning System) on Mars, high-accuracy landings must be achieved using alternate methods. However, this problem has effectively been solved. For example, ESA (European Space Agency) have been developing a system known as “LION” (Landing with Inertial and Optical Navigation) that will enable pinpoint landing on the Moon, Mars and asteroids using image recognition of major landmarks (Delaune et al., 2012). Another important development is the Fuel Optimal Large Divert Guidance (G-FOLD) algorithm (Acikmese et al., 2012), able to autonomously calculate landing trajectories in real-time. This was recently tested successfully with Masten Space System’s Xombie VTOL experimental rocket, with the vehicle making a 750 metre course correction in real time. Considering these developments it’s reasonably safe to assume that the Red Dragon will be capable of pinpoint landings on Mars by the time we begin sending them. Because the position of landed base components can be known with precision, a neat, safe and optimised layout of the base can be designed beforehand.
Red Dragon potentially represents a mechanism for delivering cargo or crew to the surface of Mars that is not only repeatable, but affordable. SpaceX currently charge $135M for a Falcon Heavy launch including the Dragon capsule. Making use of COTS and other pre-developed hardware it may be possible to develop and deliver a payload to Mars for under $250M. This is a mere one tenth of the $2.5B Curiosity rover.
Once SpaceX have developed their RLS (Reusable Launch System) for the Falcon Heavy – a goal likely to be achieved within a few years, considering the recent Grasshopper tests, and thus well before the first H2M mission – this price will come down even further.
Dragon capsules have a diameter of 3.7 metres. However, the architecture for the Mars One mission, which proposes to send 24-40 astronauts on a one-way mission to Mars, proposes to rely on a larger, 5-metre-diameter Dragon capsule for habitat modules. Although these are yet be to be built or demonstrated, their plan is to land the first two of these on Mars in 2020, which is only seven years from the time of writing.
It could perhaps be inferred that plans exist at SpaceX to have these larger Dragon capsules operational and available within seven years. This is well within the timeline of Blue Dragon. However, SpaceX and Mars One do not have a formal association so there is no real evidence of this yet, and as no information about these larger capsules is currently available, the Blue Dragon architecture does not presently include them. This may change if more information becomes available.
NASA have commenced studies of a mission to Mars based on the Red Dragon landing system, which may be flown as early as 2018. Known as “Ice Dragon” (Stoker et al., 2012), it’s being developed in collaboration with SpaceX, and will deliver a science package to Mars including a drill that will penetrate up to two metres into the permafrost to investigate environmental conditions suitable for past or extant life.
There are six objectives currently envisaged for Ice Dragon:
- Determine if life ever arose on Mars.
- Assess subsurface habitability.
- Establish the origin, vertical distribution and composition of ground ice.
- Assess potential human hazards in dust, regolith and ground ice, and cosmic radiation.
- Demonstrate ISRU for propellant production on Mars.
- Conduct human relevant EDL demonstration.
Besides the scientific outcomes of the mission, which will certainly be of tremendous value to human missions, one of the most important contributions of Ice Dragon will be demonstration of the EDL capabilities of the Red Dragon capsule.
B. Acikmese, J. Casoliva, and J. M. Carson III, “G-FOLD: A Real-Time Implementable Fuel Optimal Large Divert Guidance Algorithm for Planetary Pinpoint Landing,” Concepts and Approaches for Mars Exploration, 2012.
J. Delaune, G. Le Besnerais, M. Sanfourche, T. Voirin, C. Bourdarias, and J. Farges, “Optical Terrain Navigation for Pinpoint Landing: Image Scale and Position-Guided Landmark Matching,” Proceedings of the 35th Annual Guidance and Control Conference, 2012.
J. S. Karcz, S. M. Davis, M. J. Aftosmis, G. A. Allen, N. M. Bakhtian, A. A. Dyakonov, K. T. Edquist, B. J. Glass, A. A. Gonzales, J. L. Heldmann, L. G. Lemke, M. M. Marinova, C. P. Mckay, C. R. Stoker, P. D. Wooster, and K. A. Zarchi, “Red Dragon: Low-Cost Access to the Surface of Mars Using Commercial Capabilities,” Concepts and Approaches for Mars Exploration, 2012.
C. R. Stoker, A. Davila, S. Davis, B. Glass, A. Gonzales, J. Heldmann, J. Karcz, L. Lemke, and G. Sanders, “Ice Dragon: A Mission to Address Science and Human Exploration Objectives on Mars,” Concepts and Approaches for Mars Exploration, 2012.
New logo for the International Mars Research Station. It shows the flags for the top 10 space agencies in the world, who I hope will participate in building the IMRS. It also shows the flag of Mars at the top, and the flag of Earth at the bottom representing all of Earth. From the top and going clockwise around, the flags are: Mars, Russia, China, Europe, Brazil, Iran, Earth, South Korea, India, Japan, Canada and USA.
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)
- Sabatier reaction
- RWGS reaction
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.
The advantages of a normal Earthian atmosphere apply equally well to the Hab as they do to the ISS and other spacecraft. However, there are several compelling reasons for a reduced atmospheric pressure for the Hab:
- Lower Hab mass.
- Less air to be manufactured, which therefore reduces the mass and energy requirements of the ISAP system.
- Potential for a zero prebreathe protocol for EHA/EVA.
The higher the atmospheric pressure of the Hab, the stronger and therefore heavier it will need to be. This is contrary to our requirement of reducing the mass of the Hab as much as possible because of the significant challenge of landing such a heavy object on Mars.
Atmospheres in early spacecraft had low total pressure, low oxygen pressure, or both. However, not all of these atmospheres would be suitable for an 1.5 year stay on Mars. Research conducted in recent years has more clearly defined the limits for artificial atmospheres suitable for extended exposure.
The minimum partial pressure of oxygen required to support human physiology is considered to be 16kPa. However, for long-duration space missions, a minimum partial pressure of oxygen of 18kPa is recommended (Duffield, 2003). This is based on a previous study about planetary surface habitats (Campbell, 1991), which reviewed 33 different considerations related to atmospheric pressure and composition.
From a physiological perspective, an O2 pressure of 18kPa is perfectly safe. This is equivalent to about 1370m altitude (approximately the altitude of Kathmandu, Nepal), which does not even qualify as “high altitude” in mountain medicine (1500 – 3500m). Acclimatisation to reduced O2 pressure at altitude is characterised by an increase in pulse and breathing rate. Most people can ascend to 2400m (where O2 pressure is about 16kPa) without difficulty, however, altitude sickness may occur above this level. Astronauts can be conditioned for an O2 pressure of 18kPa by training in a hypobaric chamber, or at a moderate altitude (e.g. Black Mesa, US). In a microgravity environment there would already be increased strain on the cardiovascular system , and it would be preferable not to cause any further strain; however, the habitat is in a gravity environment on the surface of Mars, and although this is still a reduced gravity environment compared with Earth, the increased load on the heart will be mitigated.
The next design question is how much buffer gas to include. A pure oxygen atmosphere introduces an unacceptably high risk of fire, such as the one that occurred in the Apollo 1 Command Module. The upper limit of oxygen concentration with regard to fire safety has not clearly defined, but 30% is considered a reasonable upper limit (Campbell, 1991). This gives us a total atmospheric pressure of 60kPa, about 60% of Earth.
Buffer gas refers to the component of the atmosphere comprised of metabolically inert gases, which usually means nitrogen (N2), plus the noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe). The buffer gas portion of the atmosphere of Earth is almost entirely N2 (99%), with about 1% Ar and trace amounts of He, Ne and Kr. As described in the section on In Situ Air Production, because we’re making buffer gas in an economical way by simply using the Martian atmosphere with dust, CO2 and contaminants removed, our buffer gas on Mars will be about half-half N2 and Ar, possibly with trace amounts of Ne, Kr and Xe.
Nominal atmospheric concentrations of CO2 and H2O must also be determined. According to JSC 20584 (Spacecraft Maximum Allowable Concentrations for Airborne Contaminants), the maximum CO2 concentration is 0.7%. A CO2 concentration of 1% can cause drowsiness, with more serious symptoms occurring at higher concentrations. A typical concentration in normal spacecraft operations is 0.5%, which is a reasonable design goal. This gives us a CO2 partial pressure of about 0.3kPa.
With regard to water vapour, NASA specifies a RH (Relative Humidity) of 30-70%, i.e. an average of about 50%. Our target temperature is 295K (about 72°F or 22°C, which is optimal for human comfort and productivity), and the saturated water vapour pressure at this temperature and pressure is about 2.6kPa. Our average water vapour partial pressure will be 50% of this, or about 1.3kPa.
Any other gases present in the atmosphere should be present in trace amounts only.
Proposed design for Mars habitat atmosphere.
|Gas||Partial pressure (kPa)|
|Carbon dioxide (CO2)||0.3|
|Water vapour (H2O)||1.3|
|Buffer gas (N2/Ar)||40.4|
This atmosphere will produce differences in sound quality that the crew will be required to adapt to. The higher density of Ar compared with N2 will have the effect of lowering audio frequencies, including astronaut voices. Sound will also not be as loud or travel as far due to the reduced atmospheric pressure.
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:
- In soil
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.