Power Systems for Establishing an Initial Self-Sustaining Human Settlement on

Mars: A Literature Review

Author: Parikshat Singh

Supervisor: Dr Guillermo Rein

Date: December 2014

Imperial College London

Department of Mechanical Engineering

Artist’s depiction of a potential, fully expanded, settlement - (Huffington Post, 2014)

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Abstract

Mars is often regarded as the final frontier of human space exploration. Plans exist for the exploration, profitable

exploitation and commencement of colonisation of Mars in the next 20 years. Due to adverse atmospheric surface

conditions, power is required to sustain human life on Mars.

This report will summarise and compare several power sources necessary to maintain life on Mars, in particular for early

missions endeavouring to bring about an independent, self-sustaining human civilisation without prior support, resources

or infrastructure.

Four power sources were found to be feasible on Mars. In order of their usage on Earth they are: nuclear, solar, wind and

geothermal. It should be noted that only a small quantity of literature for Mars based application exists.

To critically compare these power sources, power requirements for the initial settlement were identified. A consensus

exists that, before humans arrive to inhabit the settlement, breathable oxygen and rocket fuel for a backup Earth return

vehicle will need to be ready. This process, which involves delivering hydrogen to the surface and reacting it with the

carbon dioxide rich atmosphere, requires 60kW for 6 months. On human arrival, crew life support requires only 10kW of

power, additionally, in total, relevant systems would require 100kW for at-least 10 years. This review found that all four

power sources were capable of delivering the necessary power, each with different benefits and restrictions.

Nuclear fission was found to be the most feasible nuclear option as it produces consistent levels of energy irrelevant of site

location with a 4200kg reactor. However, political/public views may stall the launch of nuclear material from Earth. Solar

power was found to have the highest mass and volume specific power density. Nevertheless, attenuations in power due to

Martian dust storms and night time must be factored in, and, due to the 25000m2 area required, it may not be appropriate

for future settlement expansion. Wind power was found to be a conceptually feasible solution, although no rigorous

analysis has been conducted on the single proposed design. In addition, low power levels of 2kW/turbine and an irregular

power source may potentially create issues in providing the minimum life critical power. Wind and solar energy combined

is likely to yield increased power consistency. Geothermal power could potentially yield the greatest power levels,

between 1-5MW. Despite this, until manned surface exploration missions take place, evidence for geothermal power

cannot be confirmed.

A strong case can be made for nuclear and solar power as primary power sources as they best meet the criteria. Contrary

to intuition, if an optimal landing site is selected, solar power outperforms all other power sources, including nuclear, in

mass and volume specific power. Although for global access, nuclear is the sole feasible source.

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Table of Contents Abstract ....................................................................................................................................................................................... i

1. Introduction ........................................................................................................................................................................... 1

1.1 Motivation - Human’s on Mars ............................................................................................................................................................ 1

1.2 Report Aims, Criteria and Scope ......................................................................................................................................................... 1

2. Energy Sources Feasible on Mars ............................................................................................................................................ 3

2.1 Energy Sources on Earth ........................................................................................................................................................................ 3

2.2 Energy Sources Relevant to Mars ....................................................................................................................................................... 3

3. Mission Outline and Power Requirements ............................................................................................................................. 5

3.1 Initial Manned Missions – Mars Arrival Strategy......................................................................................................................... 5

3.1.1 In-Situ Resource Allocation - (ISRU Phase) .................................................................................................................................... 5

3.2 Initial Power Requirements ................................................................................................................................................................. 6

4. Feasible Power Systems for a Mars Settlement ...................................................................................................................... 8

4.1 Nuclear Power ............................................................................................................................................................................................ 8

4.1.1 Nuclear Fission............................................................................................................................................................................................. 8

4.1.2 Radioisotope Thermoelectric Generators (RTG’s) ..................................................................................................................... 12

4.2 Solar Power .............................................................................................................................................................................................. 14

4.2.1 Solar Dynamic Systems .......................................................................................................................................................................... 14

4.2.2 Photovoltaic Systems .............................................................................................................................................................................. 14

4.2.3 Space-Based Solar Systems .................................................................................................................................................................. 15

4.2.4 Factors that Affect Solar Power ......................................................................................................................................................... 16

4.2.5 Site Selection ............................................................................................................................................................................................... 17

4.2.6 Advantages and Disadvantages ......................................................................................................................................................... 19

4.3 Wind Power .............................................................................................................................................................................................. 20

4.3.1 Feasibility and Availability ................................................................................................................................................................... 20

4.3.2 Site Selection ............................................................................................................................................................................................... 22

4.3.3 Advantages and Disadvantages ......................................................................................................................................................... 22

4.6 Geothermal Energy ............................................................................................................................................................................... 22

4.6.1 Site Selection ............................................................................................................................................................................................... 23

4.6.2 Advantages and Disadvantages ......................................................................................................................................................... 23

5. Conclusion ............................................................................................................................................................................ 25

References ............................................................................................................................................................................... 26

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1. Introduction

1.1 Motivation - Human’s on Mars

Landing, exploring and living on Mars are considered the ultimate goals of human spaceflight and are the next milestones

in making life interplanetary, but do we need to go to Mars?

Scientific exploration of space has vastly accelerated technological development in the past. Challenging technical

requirements have led to spinoff technologies whose results can be seen everywhere in daily life. These range from solar

power and aircraft anti-icing systems to fire resistant materials. A second reason is in the profitable exploitation of space,

and in particular, Mars. Several companies in the private sector have clearly stated their intent to land on, and start a

human colony on Mars. Finally, setting up a self-sufficient Martian settlement will allow the human species to become

multi-planetary ensuring the survival of the human species in the case of a catastrophic disaster on Earth.

Nevertheless space exploration continues to have varying levels of support. The main issue is in funding space exploration

if current issues on Earth require our resources. In spite of this, a strong humanitarian argument can be made in creating a

Mars settlement, a form of life insurance for life.

Another criticism is that reaching Mars is simply unrealistic. Despite this, studies have shown that current technology is

ready and able to deliver, support and sustain life on another planet. The most valid concern amongst these is the

economic one. Is low public funding enough the get humans to Mars? Although answers are unclear, the recent arrival of

(and collaboration with) the private space sector has significantly cut the cost of achieving space travel, allowing more to

be achieved on limited public funding.

The private sector has also announced plans to get humans to Mars, independent of public funding, with SpaceX and Mars

One being the largest names in this field. Although the mission strategy of Mars One has gained early criticism for being

too optimistic (Do et al., 2014), it is clear that the private sector may play a part in the colonisation of Mars.

It is important to note that only a small amount of publically assessable literature exists on the topic. Although it can be

assumed that national and international space agencies such as ESA, Chinese and Canadian Space agencies have authorised

research in this area, the documents are not in the public domain. So far only NASA has released research in this area and

hence this report will inevitably pull heavily from them and independent academic research.

1.2 Report Aims, Criteria and Scope

Mars has an unfavourable atmosphere. Air consists mainly of carbon-dioxide (CO2) and very little oxygen (O2). In addition

Martian dust is rife and atmospheric density is 1/75th of Earths.

Several resource requirements need to be met in order to create a self-sustaining colony including: propellant, air, food

and water. A preliminary literature search shows a widespread inconsistency in literature availability and depth,

especially in power generation. Comparisons among several power sources are often are severely lacking in depth or

detail. Where a comparative analysis of power systems exists, they tend to be confined to a set system type, for example

nuclear or solar power, whereas other power sources have little to no published literature. Currently no aggregated

documentation of energy methods or power systems exists.

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A Mars settlement and mission imply two distinctly different environments and methodologies. Mars missions require the

crew to bring power sources and equipment with them, with power likely being sufficient but not abundant. A settlement

is assumed to have base power systems to provide plentiful power to permanent residents. This colony is likely to be

completely self-reliant, with links to Earth existing to expand the colony, but not to sustain it.

This report aims to provide an overview of current power systems for use on Mars, with specific focus on technologies

feasible to missions establishing an initial self-sustaining human colony with no prior support, resources or infrastructure.

The criteria for this report is listed in below:

Current Technology The settlement’s power needs must be met by mature power sources that are currently

available.

Settlement Power This report will look into power systems that are suitable to power to an entire

settlement hub rather than vehicle power or other smaller systems

Period Power should be supplied continuously for at least 10 years.

Power Requirements Sufficient power should be delivered for 10 people.

Exploitability on Landing System must be able to provide full power within 3 days of landing.

No prior resources No prior support, resources or infrastructure - power systems must be able to support

the settlement independently.

Figure 1.1 - Illustration of the Mars One colony - (Mars One, 2014)

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2. Energy Sources Feasible on Mars

2.1 Energy Sources on Earth

Mars, although similar to Earth in significant matters, also holds key differences that limit the energy methods applicable.

Initially the current breakdown of power sources that fulfils demand on Earth is reviewed.

2012 World Electricity Generation by fuels (TWh)

Source Type Percentage (%)

Coal/Peat 41.3

Natural Gas 21.9

Hydro Electric Power (HEP) 15.8

Nuclear 11.7

Oil 4.8

Renewable Sources 4.8

Table 2.1 –Breakdown of World Energy Sources, 2011 – Renewable sources consist of Solar, Wind, Geo-Thermal, Bio & Waste

Heat – (International Energy Agency, 2014)

2.2 Energy Sources Relevant to Mars

With over 75% of energy coming from oil, natural gas and coal, it is clear that fossil fuels dominate Earth’s total energy

production (International Energy Agency, 2014). Deep buried organic matter such as fish and other small organisms,

which have been subjected to extremely high temperatures and pressures, over millions of years form fossil fuels.

Although evidence has been found for Mars once being a water bearing planet, we are yet to find signs of life and therefore,

fossil fuels and other forms of bio-energy must be ruled out.

The next single largest energy source is hydroelectric power (HEP). Due to Mars having no active hydrosphere or water

cycle, HEP along with all tidal power sources must again be ruled out (Clifford & Parker, 2001)

Following this, at just over 10%, lies Nuclear Power. Currently the world’s largest reactors, such as the Kashiwazaki-

Kariwa Nuclear Power Plant in Japan, use a form of nuclear technology known as nuclear fission. Nuclear fission and other

nuclear process will be examined in this review.

Finally renewable energies remain. Solar power has been a dependable power source for previous Mars Exploration

Rovers and is applicable. Mars’ atmosphere also has very strong winds and dust storms, making wind power appropriate.

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Mars has a warm core and has shown signs of volcanic activity in the past, giving credence to the theory that large

temperature differences exist at different depths in the Martian surface. This implies that geothermal power may be

possible on Mars. This review will also explore geothermal power sources further.

Energy sources that are not currently used on Earth (due to feasibility, difficulty or readiness levels) also exist. Two main

sources, which will also be very briefly discussed in this review for completeness, include nuclear fusion and space-based

solar power. Since this report’s criteria aims to explore power sources readily available for missions and settlements

today, it will not consider these two energy sources in detail. The full reasoning will be given in the relevant sections.

In total, the relevant energy sources, in order of use on Earth are:

1. Nuclear (Fission, Fusion and RTG’s).

2. Solar (Photovoltaic, Solar Dynamic and Space-based Solar).

3. Wind.

4. Geothermal.

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3. Mission Outline and Power Requirements

3.1 Initial Manned Missions – Mars Arrival Strategy

A Mars settlement will rely on a system of plentiful base power. Before this system is running, several Mars Missions are

required to deliver and setup the energy systems. It is important to consider a scenario existing where the systems that

deliver the largest power, or power in the most advantageous way, are not feasible due to mass, volume or other

justifications.

Mission strategies will largely dictate both weight and power requirements for energy systems, therefore a brief

explanation of mission strategy is given below:

Several strategies exist outlining the stages of manned missions. Most mission architectures are modifications on the Mars

Direct plan proposed by Zubrin (Zubrin, 1998). A major consensus exists that this is the simplest and most effective

mission set. Mars Direct, Mars Semi-direct and the subsequent papers by Zubrin detail how a Mars base, which holds

sufficient resources including food, habitation and fuel for a return journey is achieved. An update to Mars Direct plan

(James, Chamitoff & Barker, 1998a) uses the most up to date technologies is briefly detailed below for context.

The plan exploits low energy Mars flights, available once every two years when the distance between Mars and Earth is

minimised; the average travel time is 6 months. The first launch sends an Unmanned Earth Return Vehicle (ERV) two

years before the crew launch. This modified architecture sends a modified ERV containing cryogenic storage tanks, spare

parts and vitally, energy generation equipment.

3.1.1 In-Situ Resource Allocation - (ISRU Phase)

In-Situ Resource Allocation (ISRU) is the process where resources on the host planet or body are exploited.

Rocket fuel is required, in the form of liquid oxygen and methane, for the ERV’s return trip, though launching from Earth

with sufficient fuel for a return journey is both costly and poses severe weight and volume restrictions. It is therefore

suggested that fuel is produced on the surface of Mars using its carbon dioxide rich atmosphere (Zubrin, 1991). The

process whereby oxygen and rocket fuel is produced is commonly regarded as a requirement over an option. This process

is a power intensive one and is described in order to later justify power requirements.

On landing, the ERV’s are completely unfuelled. They carry a payload of 6 metric tonnes of liquid hydrogen which is used

to create 108 metric tonnes of fuel and breathing oxygen, in the form of methane and liquid oxygen. This is used, if needed,

to return the crew to Earth. Approximately 10 metric tonnes of fuel is left for Martian Rovers.

The chemical reactions used to accomplish this are:

CO2 (g) + 4H2 (g) → CH4 (g) + 2H2O (v)

Equation 3.1 - Sabatier reaction used to generate methane (rocket and vehicle fuel)

Then water is then broken down into hydrogen and oxygen:

2H2O(l) → 2 H2 (g) + O2 (g)

Equation 3.2 – Electrolysis of water (to generate breathing oxygen)

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The oxygen is stored for breathing and compressed for use as a liquid fuel. The hydrogen is recirculated into Equation 3.1

to increase methane production. The process will take 6 months to complete and for the ERV’s to be fully refuelled (Moss,

2013; Zubrin, 1998).

3.2 Initial Power Requirements

Power required can be broken into three main categories: life support, mission requirements and ascent vehicle fuel

production. Life support encompasses a rich supply of oxygen, pressure containment and other such basic necessities to

support life. Additional energy is also needed to power rovers, scientific experiments and support other such objectives.

Biosphere 2, a closed ecological system, was setup to understand how to sustain a colony of humans in a fixed and air-

locked bubble. This system contained 8 crew members for the duration of two years. It concluded that approximately

100kW was required per person (Nelson & Dempster, 1996).

However it was argued that the report’s approximation did not include the power required for pressurisation or

production of initial oxygen for the crew. On the other hand, Biosphere 2 did not comprise the ability to extract resources

from the environment or exchange waste (such as heat, gasses or substances) with the external environment. Others argue

that the incorporation of this feature would significantly reduce the energy per person (Meyer & McKay, 1989). Meyer

estimates that the power required for oxygen production on a manned mission would be significantly lower, between 15-

40 kW and the power required for production of propellant (In-Situ Propellant Production – ISPP) would be 10-50 kW.

NASA’s most recent Design Reference Mission states a power of 92 kW is required for oxygen and fuel production in the

day, and then approximately 5 kW for storage. The crew will require 17-20 kW (Cataldo, 2009). A full breakdown for

day/night/dust storm is shown below:

Day kW Night kW Dust Storm kW Notes

Element

Habitat 12 12 12

ISRU O2 Propellant (Solar) 66 2-3 2-3 8 hours/sol1

ISRU O2 Propellant (Nuclear) 22 22 22

ISRU O2 Propellant (Solar) 5.7 0.5-1 0.5-1 8 hours/sol

ISRU O2 Propellant (Nuclear) 2 2 2

Logistics Module 1.5 1.5 1.5 Option 1 only

Ascent Stage 1.5 1.5 1.5

Rover Recharge 1.5 0 0

ISRU Crew O2 Cache 1.5 1.5 1.5 Maintain only

Drill 3 0 0 Power from Rover

Table 3.1 – Estimated power requirements for critical surface elements- Approximate totals are: day 100kW,

night 50kW, dust 50kW - (Cataldo, 2009)

For a settlement on Mars, Haslach gives a power estimation of 400kW, although no data regarding settlement size or

location is detailed (Haslach, 1989).

1 1 sol=1 Martian day

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Clearly several sources contradict on the exact power required. This may be a result of authors creating estimates using

different baseline parameters, such as number of crew members, missions to be undertaken and so on, however not all

authors have provided further information. Accurately providing a forecast for power on unchartered territory is a

challenging task, however strong consensus exists on the order of power required.

Initial missions’ power requirements are between 15-100kW (with Haslach as an outlier at 400 kW) and for an established

settlement, the power required would be in excess of 400kW. Thus an intermediate figure, appropriate for an initial

settlement, would be 100kW as the daytime peak power requirement.

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4. Feasible Power Systems for a Mars Settlement

4.1 Nuclear Power

Nuclear power arrives from either the fusing elements smaller than iron and nickel together (nuclear fusion) or splitting

atoms that are much larger in order to produce energy (nuclear fission). An alternative nuclear process is radioisotope

thermoelectric generators which exploits nuclear heat decay.

Nuclear power is feasible for space and extra-terrestrial use. Projects in the last 50 years have demonstrated power

production under reduced gravity loads. These projects, funded by the Russian and American space agencies, have scaled

reactors up from 0.5kW up to the SP-100’s 100kW; their power densities varying between 17-37W/kg (Zubrin, 1998;

Haslach, 1989).

Despite American projects remaining incomplete, they show no major obstacles and are a proof of concept for nuclear

power on Mars. The Russian space agency completed the TOPAZ-II demonstrating a 6kW reactor with a 3-year life.

Surface Nuclear Power: Fission vs Fusion

Nuclear fusion can produce three to four times as much as its fission counterpart and is the method that stars such as our

Sun use to produce energy. It is also a much cleaner process as it produces no waste and can use simple elements, such as

hydrogen isotopes, to generate power with little to no nuclear waste.

Nuclear fission uses a chain reaction to split large atoms and produces radioactive waste in the process. This nuclear waste

needs to be safely stored due to its extremely long half-life. Fission also requires refined nuclear reactants (such as

Uranium-235), which are expensive and radioactive. Despite the process not being as clean as nuclear fusion, it can still

reliably produce large amounts of energy in a compact, self-contained system. Once a reactant is provided, the system can

produce continuous power for decades. All the above examples such as SP-100 and TOPAZ II are examples of nuclear

fission power.

Nuclear fusion is a much more advantageous process, but it requires an extremely high temperature and density

environment to initiate. Fusion is currently being experimentally explored, but current technology is unable to produce a

system with a net excess of energy and will hence will not be pursued further in this literature review.

4.1.1 Nuclear Fission

Nuclear fission consists of two main processes: firstly the fission reactor process - a chain reaction of splitting large atoms

into several smaller ones, which generates heat; and secondly, the conversion of that heat into electricity.

If low operating temperatures are designed, simple metals such as stainless steel can be used, which is compatible with

Mars’ atmosphere (Cataldo, 2009). Another paper argues that it may be possible to exploit Martian landscape features for

shielding such as a deep crater, which could be used to potentially contain the reactors’ radiation for a crew base (Cohen,

1996).

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Figure 4.1.1 - Nuclear fission power plant– (University of California, Davis Chem-Wiki, 2014)

Several fission reactor concepts have been explored for feasibility on Mars and are here are classified by coolant. These

are: gas cooled, liquid-metal cooled, heat pipe cooled and salt cooled (Liquid Fluoride Thorium Reactors); with each

reactor type either being moderated or fast spectrum.

These each carry out exactly the same task in similar ways. A chain reaction is initiated by releasing thermal neutrons into

the reactor, which split the large fuel atoms. These break into two or three smaller atoms, and also release more neutrons

in the process. The rate of reaction is controlled by several control rods, which can absorb neutrons (Mason, 2006). Two

main shielding approaches were investigated: refractory wall and stainless steel.

Salt cooled reactors, such as liquid-fluorine thorium reactors (LFTR, pronounced “lifter”), were exempt from NASA’s

research, yet look particularly promising. It holds several key advantages over any other type of nuclear fission reactor

and hence is worth detailing further.

Firstly, it uses Thorium as fuel, which is abundant and cheaper than Uranium-235. It also can be scaled for both

established and initial settlements, allowing mass to be reduced for early stages of colonisation. Another key advantage is

that the reactor cannot “melt down” resulting in catastrophic failure. Current first and second generation (Gen-I/II)

reactors, such as the ones seen in Chernobyl and Fukushima have had major incidents in the past. When power is lost to a

Gen-I/II reactor, water is required to cool it down, but if water isn’t supplied in enough quantity or time, then the reactor

will melt down. A catastrophic failure on Mars would severely damage the habitability of the planet - the radioactive

material would be rapidly spread due to heavy winds and dust storms. However in a modern Gen-IV LFTR reactor, liquid

fluoride will simply drain away into a secondary container, making the process invulnerable to melt down. In addition

LFTR’s operate at atmospheric pressures compared to earlier reactors which can go up to 700atm (Smith, 2012). Finally

LFTR’s burn most of their fuel, creating very little waste. The waste can also be re-used as fuel for the next batch of nuclear

reactions (Moss, 2013).

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Figure 4.1.2- Liquid-fluorine thorium reactor architecture - (Smith, 2012)

It is clear that LFTR reactors hold promise, but, no direct comparison has been made between these and more

conventional reactor stages. In addition, no literature directly evaluates LFTR or Gen-IV reactors are feasible on Mars.

While no differences between Gen-I/II and IV reactors should result in these reactors being unsuitable, research should be

conducted to confirm their feasibility.

4.1.1.1 Thermo-Electric Conversion Options

These reactors all need to convert the thermal energy they produce into electrical energy. The three most commonly used

thermal conversion methods for Surface Nuclear Fission between 50-200 kW are: liquid-metal Stirling cycle, gas-cooled

Brayton cycle and liquid-metal thermoelectric process.

These three converters are detailed below:

Liquid Metal Stirling Cycle:

Stirling cycles convert thermal energy directly into mechanical energy by exploiting changes in temperature resulting in

liquid expansions or decompressions, which displace a piston, and therefore produces mechanical energy. The Stirling

converter, with a free-piston design, is connected to an alternator. This cycle uses high-pressure helium working fluid.

Gas Cooled Brayton Cycle:

The Brayton cycle is a closed cycle constant pressure thermodynamic cycle. The cycle pre-compresses the gas that is

followed by a recuperating turbine which turns a permanent-magnet alternator.

Thermoelectric:

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A thermoelectric converter directly converts a temperature difference to a voltage. The converter is made up of several

conductively coupled thermocouple arrays. The thermocouples are semiconductors using n-leg lead- telluride (PbTe) and

p-leg TAGS (Mason, 2006).

4.1.1.2 Optimum Nuclear Fission Proposals

A NASA study concluded that overall a Brayton cycle employing gas cooling with a refractory wall shield would be the

most mass efficient power source. However if a stainless steel construction was used, then liquid-metal Stirling was the

best option, this weighed in at 4200kg for a 30kW reactor (Smith, 2012; Mason, 2006).

4.1.1.3 Advantages and Disadvantages

The largest disadvantage is in its radioactive waste which it produces after a decade of use (Boston, 1996). Currently a

nuclear waste disposal strategy is unclear, with strong Martian winds and dust storms posing a significant threat. The

leading issue, however, is the public and political views on Nuclear since the Chernobyl and Fukushima disasters.

Resistance to launching a nuclear power device into space which could add significant lead-time in order to navigate legal

and political arguments.

Nuclear fusion power holds key advantages to all other renewable sources. The primary advantage lies in its ability to

produce continuous levels of power, regardless of atmospheric conditions. Solar and wind are both hindered in this

feature. For the ISRU phase, solar would require a peak power production of 100kW assuming a site was selected which

could utilise 8 hours of daylight at 31°N latitude. A nuclear equivalent would only need to produce a peak of 30kW as it

would be able to operate throughout the night. The nuclear power system would also have the same size and dimensions

regardless of latitude whereas a solar systems surface area would directly reflect its latitude due to the solar intensity at

varying distances from the sun (Cataldo, 2009). This consistency in power produced can be seen in Fig 4.1.3.

Figure 4.1.3 – Mass specific power as a function of latitude – Nuclear shows clear consistency at all latitudes -

(Cooper et al., 2009)

Another key point of note is mass. Initial missions will require crew members to bring the system with them. Due to the

extremely high cost of space travel, extra masses and volume are of critical importance. It can be seen that nuclear power

is a significantly lighter system at approximately three times lighter than solar (Figure 4.1.4). For scaling up an existing

settlement or global access to Mars, nuclear is the only current feasible option (Cooper et al., 2009).

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Figure 4.1.4 – Estimated system comparison for solar and nuclear power – Shows the clear mass advantage held by a

fission reactor - (Cataldo, 2009)

Key disadvantages are in the difficulties of repairing nuclear devices on Mars, constant increased crew radiation dose

(which varies depending on shielding used), as well as operational “keep-out zones” due to radiation (Drake, 2009).

4.1.2 Radioisotope Thermoelectric Generators (RTG’s)

Radioisotope power systems provide continuous energy in much lower quantities by using thermocouples to convert

thermal energy generated by radiation into electricity. The power systems are commonly referred to as radioisotope

thermoelectric generators (RTG’s). A typical design is shown below:

Figure 4.1.5 - Radioisotope thermoelectric generators- Radioactive decay produces heat, which generates electricity

via thermoelectric converters - (Karacalıoğlu, 2014)

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Figure 4.1.6 - Longitudinal Cross Section of a RTG – The radioisotope is labelled as ‘heat source’ - (Cockfield & Chan,

2002)

Primarily due to the lack of availability of Plutonium-238, their typical power production rate is in the order of kilowatts,

making it applicable for small devices rather than entire bases. Despite having low power, RTG’s have key advantages such

as lower, easily blockable, radiation levels and therefore require minimal radiation shields. RTG’s also have an

approximately 90 year half-life. These features have resulted in several of the current and future Mars rovers to be

powered by isotope power system (Cockfield & Chan, 2002).

Although RTG’s are a well-established technology, they require a large amount of Plutonium due to their limitations in

efficiency of heat conversion systems. A 5kW unit requires 62.5kg of fuel (Cooper et al., 2009).

Other more efficient methods of heat conversion, such as a Stirling engines, need to be developed to drive improvements

in RTG technology (Cataldo, 2009).

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4.2 Solar Power

Solar power refers to any system that harnesses the Sun’s rays for power. In total three main sources of solar power exist:

solar dynamic, photovoltaic (PV) and space-based solar power (SBSP).

4.2.1 Solar Dynamic Systems

Solar dynamic systems use solar energy to heat up a working fluid into its gaseous form, which drives a turbine, producing

electricity. These systems, due to their relatively rudimentary methods, require simple components such as mirrors, pipes

and boilers which could be open to repair, and possibly, manufacturing on Mars in the future (Zubrin, 1998).

The most potent issue with solar dynamic is that it requires direct light (point sources) to be effective. Although the matter

can be combatted using mirrors to direct light, the system’s 15-25% efficiency will be severely lowered in the presence of

dust storms (Geels, Miller & and Clark, 1989).

Figure 4.2.1 – Solar dynamic power system architecture - Light is concentrated onto a receiver, vaporing the working

fluid and driving the turbine – (Secunde & Labus, 1989)

Several studies concluded that solar dynamic power is feasible for space related missions and has the highest solar

efficiencies. Furthermore secondary systems, such as gas turbines, are common among several other power sources, which

may in turn be used as backup power (Davis, J 1996).

4.2.2 Photovoltaic Systems

Photovoltaic (PV) systems hold arrays of PV Cells which compromise of a collection of P-N junction semiconductors. When

light is incident on the surface of the semiconductor, electrons move from the N-type silicon to the P-type silicon,

generating a current.

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Figure 4.2.2 – The Photovoltaic Effect – Incoming light rays release elections creating a current, this is the principle

for PV cells – (U.S. Bureau of Labor Statistics, 2014)

Their key advantage over solar dynamic systems is that they do not require a point source of light. Efficiencies vary from

13-15% for everyday silicon-crystal semiconductors, but can increase to 25.0±0.5% for current state of the art cells.

Gallium-arsenide (GaAs) cells are more expensive, but can produce up to 28.8±0.9%, again for the current state of the art

(Green MA, 2014). Tests have shown 5J-GaAs/InP bonded cells can exhibit 38.8±1.9%, although their space worthiness

(capability to function under cosmic radiation) has not been demonstrated.

These modern cells have shown to exceed the mass weighted system power density that can be found in nuclear power.

Silicon-crystal solar arrays, used in recent ISS (International Space Station) missions, have produced 66 W/kg.

Additionally solar arrays which produce 130W/kg have been demonstrated in the past. Thin film solar cell technology

promises specific powers from 1-15kW/kg (James, Chamitoff & Barker, 1998b).

Cooper considers two technologies: ultra-light arrays with 15% efficiency and a mass/area of 0.063 kg/m2 and high

efficiency arrays, similar to those on the ISS, with 20% efficiency, a mass/area of 2.5kg/m2, which feature tracking to

ensure flux remains perpendicular to the solar cell surface (Cooper et al., 2009).

4.2.3 Space-Based Solar Systems

Space-based solar power (SBSP) methods attempt to eliminate the problem of low light intensity on Mars’ surface. This is

achieved by collecting and converting solar rays into microwaves in orbit and beaming the power down to a collector on

the surface of the planet. The surface station then converts the microwaves back into electricity.

SBSP has several key advantages: higher intensity light is available, it can be illuminated over 90% of the time and also is

not be effected by atmospheric gas filtering, dust accumulation or storms (Office of Technology Assessment, 1981). Figure

4.3 describes SBSP for Earth, the process would be identical on Mars.

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Figure 4.2.3 – Space-based solar power – Sunlight is concentrated to the solar satellite where the energy is converted

into microwaves and beamed to a large surface collector – (Totty, 2008)

To provide sufficient power, assuming the PV cells have a 25% efficiency, it is argued that the satellite would require a

surface area of at-least 68000 m2, which is approximately the size of the international space station (Criswell, 1996). The

biggest issue with SBSP is in its setup. Mars’ microwave receiver would be at least 1km2 (James, Chamitoff & Barker,

1998a). Both large setup times and ISRU manufacturing are required for a successful setup. Consequently SBSP is not

applicable for initial settlement power and will not be explored further.

4.2.4 Factors that Affect Solar Power

The Martian surface has several key differences to Earth. The leading factors affecting light intensity, and hence solar

power, on the surface of Mars are:

Latitude (Distance from equator)

Seasonal Variation (Due to orbital eccentricity and inclination)

Daily Variation (Day/Night cycles)

Suspended Aerosols (Particles suspended in the atmosphere)

Dust Storms (Local and Worldwide)

The intensity of light on the surface of Mars is lower than on Earth for a number of reasons. Firstly the distance between

Mars and the Sun varies between 1.4-1.67 Astronomical Units at perihelion-aphelion (closest-furthest orbit)2. Additionally

large amounts of Martian dust is present in the atmosphere that absorbs and scatters light, which either is scattered into

the atmosphere or back into space. It is important to note that direct solar intensity decreases with levels of dust, yet total

illumination is a complex function of the dust levels and the angle of the sun (Cataldo, 2009).

In order to study the effect of atmospheric conditions on solar power production, ranges of data over time from previous

and current Mars Exploration Rovers (MER) become particularly useful.

2 1 Astronomical Unit is defined as the distance between the Sun and Earth.

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Optical density (also known as opacity or optical depth) is a measure of opacity of an atmosphere. Data from MER’s

suggest that Mars typically holds an optical density of 0-1, with an annual average around 1 (due to high peaks of up to 6

during dust storms), (Lemmon, 2010).

Figure 4.2.4 shows a large dust storm heavily increasing the atmosphere’s optical depth from sols 240-275, with larger

opacity readings referring to lower visibility and light in the environment (Cataldo, 2009). These values were calculated

through the changes in solar power produced. Although power generated is reduced to 14% of the maximum during the

worst of the storm, it is worth noting that, from sols 290-360 the power was restored to pre-storm levels.

Figure 4.2.4 – Opacity Measurements from several MER Rovers of sols 240-360 clearly show increases in opacity due

to dust storms which diminish solar power - (Cataldo, 2009)

Constant dust flow in the Martian atmosphere combined with windy conditions resulted in dust accumulation, blocking

light to the array surface. Data collected from the several rovers show a power degradation rate of 0.1-0.2%/sol due to

dust build-up (Landis, 2005).

Although dust accumulation slowly degraded the solar units’ effectiveness, it was observed that on sol 418, a “clearing

event” restored power to levels 90%. This clearing event was under increased wind speeds on top of a ridge, with the solar

array at a tilt angle of 22 °, suggesting that the windy environment blew accumulated dust away. It is implied that the tilt

angle played a large part in the clearing event. Wind tunnel tests that closely simulated a Martian environment confirm

these theories. (Cataldo, 2009). Dust accumulation is an issue that can be easily combatted though manual or automated

cleaning of the solar arrays. The very large surface area required for solar power leads to over-sizing/redundancy

solutions to be ineffective.

4.2.5 Site Selection

Solar power implies strict restrictions to site selection. At increasing latitudes, daylight period reduces, for example, at

approximately 30°N the shortest winter day is 10 hours whereas at twice the latitude (60°N) the daylight hours are

halved. 58 potential sites were identified, with regions along the equator, between ±40°, receiving the most annual

sunlight. Latitudes between 15°S and 30°N will yield the greatest results and have been highlighted in Fig 4.5 (Drake,

2009; Meyer & McKay, 1989). Solar dynamic systems would be effective in the northern regions during spring and

18

summer (Zubrin, 1998). Conversely a region to avoid would be between 20-40° in the southern hemisphere (directly

underneath the highlighted belt-Fig 4.2.5) as most major dust storm originate from this area (Geels, Miller & and Clark,

1989).

Figure 4.2.5 - Map of Mars marking regions for potential solar powered sites –Highlighted central region is optimal

for solar power - (Drake, 2009)

Cooper modelled data to show 31°N latitude is the optimal latitude for consistent solar power. This model assumed no

atmosphere and an optical density of 0.4 (equivalent to hazy skies) (Cooper et al., 2009).

Figure 4.2.6 – Mars Solar Energy at 3 optimal latitudes – 31°N shows the most consistency through different seasons

in a Martian year and is therefore the preferred choice- (Cooper et al., 2009)

At this latitude solar power’s power/mass ratio (using Regenerative fuel cells) outperforms all other sources of power.

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Figure 4.2.7 – Mass and volume specific power for a 100kW average power system- Solar power with regenerative

fuel cells yield the highest mass specific power - (Cooper et al., 2009)

It is clear that sources all agree on the impact that site location has on both light and dust storm intensity. Cooper, the sole

author to challenge widespread views on dust storms, argues that although impactful, dust storms does not pose a risk to

life support. 100kW of power production, with a severe dust storm, reduces to a minimum of 10kW, which is sufficient to

sustain life. Cooper also argues that the use of an RTG may provide additional robustness to the system during dust storms

(Cooper et al., 2009).

4.2.6 Advantages and Disadvantages

The consensus is that solar power is a clean, cheap and abundant source of power. Arguably more importantly it has a

heritage in extra-terrestrial missions, where it has been often used as the primary power source.

As power and safety requirements get larger, so does the impact of factors such as: dust storms, season variations and

location. It is argued that solar systems can function in a hybrid mode, with secondary systems to create a redundant base

load power source (James, Chamitoff & Barker, 1998a).

Widely agreed upon, the key disadvantage of a solar system is variable power generation as a result of large seasonal dust

storms and seasonal changes. Solar power would also require maintenance due to dust accumulation.

Another disadvantage is in the large surface area required. For 100kW of power production, 25000m2 of lightweight,

foldable, solar panels are required. The deployment time is the sum of time taken to unload, unroll and to pin the solar

array down to prevent strong winds from blowing it over. In total it was calculated that 66 hours are required for a crew of

two, or 22 hours for a crew of six. Another power source would be required until the first 10% of solar panels are active

(Cooper et al., 2009).

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4.3 Wind Power

Wind power on Mars works identically to systems currently in use on Earth - wind in the atmosphere drives generator

turbines creating electricity. Due to its irregular power production rate, it’s solemnly used as a primary power source.

Along with solar and geothermal energy, wind offers a third sustainable method of producing clean energy. However Mars’

atmosphere has key differences to our current habitat.

Figure 4.3.1 – Wind turbine – Wind passing through blades causes them to spin, generating current- (US Department

of Energy, 2014)

4.3.1 Feasibility and Availability

Mars’ lack of a strong magnetosphere has resulted in a very thin atmosphere which is 1/75th the density of Earth. This may

intuitively lead to the conclusion that wind power is not a feasible source of energy on Mars, however it holds some key

advantages on a Martian surface:

1. Gravity is only 38% of that on Earth - components of lower weight.

2. Extensive surface relief - increased wind speeds.

3. Volatile and substantial temperature and pressure differentials.

The final two point leads to Mars naturally having much higher wind speeds compared to that of Earth (James, Chamitoff &

Barker, 1998a). Wind speed is a direct result of temperature fluctuations in the atmosphere due to inconsistent solar

heating. Wind speed also increases with height above the ground with the rate of increase steadily declining. This wind

speed variation has been modelled using a power exponent function and is shown in Figure 4.3.2 (Hemmat et al., 1998).

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Figure 4.3.2 – Wind Velocity Profile against local height – Shows wind speeds of 30m/s are attainable with a suitable

height, on the right, the turbine profile is shown to show how speed varies along the profile - (Haslach, 1989)

Power can be calculated as a function of wind speed and density using (Haslach, 1989):

𝑃 =1

2𝐴𝑐𝜌𝑣3

Equation 4.3.1 – Power produced by a wind turbine

Where P is power produced (W), A is swept area (m2), c is the power coefficient, ρ is atmospheric density and 𝑣 is wind

velocity (ms-1)

Although power decreases with atmospheric density proportionally, it is clear in Eq-4.3.1, that wind speed is the

dominating term. Calculations show that the wind speed required for the wind turbine to generate the same levels of

power as it would on Earth (assuming ρMars = 0.1665 kg m-3), is 30ms-1 compared to 6ms-1 on Earth (Zubrin, 1998).

Taking calculations further and assuming a terrestrial value of efficiency (c=0.4), a 200 m2 turbine will produce 2 kW with

constant 14ms-1 wind speed and 12 kW in a 25ms-1 wind (Haslach, 1989). Drawing from the power requirements section, it

is easy to see how several wind turbines could together produce the 100-300kW requirements. It is worth considering

that this efficiency, which may seem low, is in practice relatively high. The Betz limit on efficiency which is analogous to

the Carnot efficiency for internal combustion engines, has a fixed value of 59.26% for all turbines. Assuming an efficiency

of 40% implies that the turbine is operating at 67% of its theoretical limit.

Designs that have been put forward include one by Hemmat (Hemmat et al., 1998). The total weight was slightly over

940kg and would require a wind speed of 28ms-1 for an hour a day to be effective. Several design characteristics such as

ultra-light weight blades and towers were used alongside pre-tension guy wires. The study also concluded that if an

average of 30ms-1 for an hour was produced, a figure which is likely in a dust storm, then the wind turbines will be more

mass efficient than a photovoltaic setup. However engineering calculations such structural dynamics and fatigue were not

factored into the design. The design concluded that wind power could be used as an integrated power source alongside

solar energy, offsetting power attenuations during large dust storms or night time.

22

Despite this design appearing lightweight and robust, further research and analysis should to be conducted in order to

verify its feasibility. It should be noted at this point this is currently the only design that exists explicitly for Mars and

hence a detailed comparison cannot be made.

4.3.2 Site Selection

To meet wind speed requirements sites recommended include raised rim craters (circular patches on Figure 4.3.3) and the

northern polar region, which can in theory produce speeds of between 25-33ms-1. In addition basins such as Hellas basin,

the lowermost circle, has approximately a 40% denser atmosphere, resulting in the same nominal increase in power

(James, Chamitoff & Barker, 1998a).

Figure 4.3.3-Map of Mars highlighting potential wind powered sites – Lowermost circle is Hellas basin, which has

increased atmospheric density; other highlighted areas identify raised rim craters and the northern polar region,

which both have higher wind speeds - (James, Chamitoff & Barker, 1998a)

4.3.3 Advantages and Disadvantages

As mentioned, wind power holds the key advantages of sustainability, generating no waste products, and is easily

maintainable and scalable. Combined hybrid systems such as wind-solar have shown to be extremely successful in

producing uninterrupted power on Earth. It is also worth considering that the turbine-generator systems are common

with some nuclear fusion and solar dynamic systems.

The primary disadvantage of wind power is that it is an extremely variable resource, and its use alone can raise concerns

over guaranteeing sustained minimum power for life support. Power produced is very low, producing less than a nuclear

system would with a much larger size. Finally, the wind turbine would have to be at least 20 meters high to ensure

appropriate wind speeds are attained (James, Chamitoff & Barker, 1998).

Dust storms significantly cut power levels to a solar system due to the increase in atmospheric opacity but produce

stronger winds leading to increased wind power. Hence a strong case could be made for the use of a combined solar-wind

system.

4.6 Geothermal Energy

On Earth, geothermal power exploits natural heating effects due to volcanism, and is described in Figure 6.4.1.

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Figure 4.6.1 - Geothermal energy on Earth – Cold water is pumped below the surface to exploit natural heating effects, the

hot water vapour is then used to drive a turbine, producing electricity- (The Carbon Neutral Company, 2014)

Heat flux values on Earth are around 80mW/m2 whereas estimates on Martian values are estimated to be near 35mW/m2.

4.6.1 Site Selection

Plate tectonics focuses geothermal energy to specific areas causing sources come in pockets, which may not have an

obvious surface manifestation (Meyer & McKay, 1989). Amazonian volcanism (geothermal sources younger than 2 billion

years) cover approximately 28% of the surface with latitudes varying from 20°-220° W and 50° N-15° S. This region may

contain several volcanic features such as cryptovolcanic or subsurface volcanism, giving further evidence that geothermal

sources may exist (Fogg, 1996).

By taking geological measurements of crater size and young intrusions it can be seen that 4.5 million km2 of Martian

surface area is likely to have experienced volcanic activity in the last 250-700 million years. These regions are likely

candidates to have near-surface resources of geothermal energy (Fogg, 1996). Young volcanic features are also observed

which point towards underground geothermal heat sources (Zubrin, 1998), although no current data can verify the

hypotheses.

4.6.2 Advantages and Disadvantages

Geothermal power has its clear advantages: large amounts of clean and renewable energy. Energy production can vary

from 1-10MW. It can also be heated near underground aquifers, creating a useful by-product: water. The electrical turbine

generators for this would be common among solar dynamic, wind and some forms of nuclear power (Meyer & McKay,

1989).

However any further conclusive information will only be found as a direct result of surface exploration (Duke, 1985). All

sources align stating that geothermal sources will only be feasible after a settlement has sufficient resources and capability

to produce ‘on planet’ supplies such as pipes and drill-bits (James, Chamitoff & Barker, 1998a).

24

It should be noted that available and public literature was limited to three sources from 1985 to 1998. Since then, due to

shrinking supplies in the oil and gas industry, technology and experience has increased measurably. All three sources

agree that geothermal energy could provide large amounts of power to a well-established settlement with mass intensive

mining and manufacturing capabilities, but it is clear that for an initial settlement, geothermal power is not feasible.

25

5. Conclusion

Overall, a powered initial settlement on Mars can be established with current technology. In total, four power sources

were found to be applicable to meet a 100kW power requirement on Mars: nuclear, solar, wind and geothermal. This

power requirement was derived from mission strategy and ecological studies.

Nuclear power has a key advantage – a closed system is impervious to atmospheric conditions. It will produce a lower

constant level of power until fuel is depleted decades later; can enable global access; and is generally a lighter, more

compact system. The optimal proposal produces 30kW at 4200kg. In particular, LFTR fission technology has the greatest

potential, but has been exempt from most nuclear studies for Mars. The unavoidable disadvantages of fission include the

inability to repair on Mars and increased radiation levels to crew. The principle disadvantage lies in political viewpoints on

launching a large scale nuclear reactor into space (Moss, 2013). Public perception of nuclear power, especially intensified

by the events of Chernobyl and more recently Fukushima, is an increasingly negative one. Space agencies such as NASA

have already established nuclear power to be the preferred strategy. However, both the public and private sector, will

need to conform to political views which are dictated by public perception, and may have restrictions imposed on them.

RTG’s produce little power and use rare, expensive fuel. Therefore they are not feasible for full base power, though may be

useful for smaller levels of power generation such as for rovers.

Solar power has the longest heritage on Mars, with previous rovers often using solar arrays. It is also the most well

understood of the sources, with previous rovers, such as Viking, enabling us access to opacity and light intensity levels

over long periods of time (Figure 4.4). If effects from dust storms and accumulation are considered fully, 100kW can be

produced with 25000m2 of solar arrays, dropping to 10kW during severe dust storms. Several sources still conflict on how

much of an issue these concerns pose. (Cataldo, 2009) discusses the uncertainty of the Martian environment creating a

disproportional impact on the worst case scenario. Solar power, much like wind power, harnesses energy from the

occupants’ surroundings. Due to the infancy of our missions and lack of complete understanding of the Martian

environment, these sources point towards speculation of unforeseen circumstances or events that are not yet fully

understood, which may lead to a power cut-off. Nevertheless, it has been shown that solar power at the optimal latitude

can outperform nuclear power in mass specific and volume specific power, making them an attractive option for an initial

settlement, when energy systems must be transported.

Wind power, like solar, is a well understood and feasible source of power on Mars. However, rate of production is

inconsistent with low power levels of 2kW produced for strong 14m/s winds, using a 20m high turbine. Moreover, only a

single design feasible to Mars exists, with no comprehensive analysis on reliability or mass having been undertaken.

Despite these limitations, wind power may still show potential if used in conjunction with solar power if mass/volume

restrictions are altered.

Geothermal power could potentially produce from 1-10MW of sustainable and renewable energy indefinitely. These

sources cannot be confirmed without direct surface drilling exploration. In addition large scale manufacturing and

construction will need to be present. For these reasons, geothermal power is not applicable to produce power for initial

settlements. For well-established settlements both nuclear fusion and space-based solar power appear promising,

however, these technologies are too immature for current use.

In conclusion, nuclear, solar and wind are the most promising potential power sources for an initial settlement. Even

though very limited literature exists, a strong case can be made for either nuclear or solar power for a primary power

source as they best meet the defined criteria. Despite solar powers’ limitations, if the optimal landing site of 31°N is

selected, it outperforms all other power sources, including mass and volume specific power. For global access, only nuclear

power is feasible. This report recommends further research into the areas of combined solar-wind power systems and the

comparison of LFTR nuclear technology to more conventional nuclear power sources for use on Mars.

26

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