General information

Information about the groupmembers and the logbooks each week can be found here.

Problem statement

Setting

For years now, new initiatives to colonize Mars kept popping up all over the world. Examples include Mars One [1] and SpaceX [2]. It still seems far away, but when these plans eventually become reality, Mars will likely be covered with multiple colonies within the near future. While some companies and/or countries might try to work together to build one colony, others might end up establishing their own. This behaviour can also be seen in the construction of space stations right now. While the ISS is a collaboration between multiple space agencies, China is building their own space station in the form of the Tiangong program [3]. Because of political distrust, China is not allowed to collaborate on the ISS [4]. This distrust will probably continue when the time of Mars colonies is here. Some companies also might choose to build their own colony, like Bigelow Aerospace is doing with the Genesis program [5] when looking at space stations.

When these colonies are established, they will need resources to continue and expand. Delivering these resources from earth through rockets is time consuming and very expensive, so gathering as much of these materials as possible on Mars would be a better option. Ideally, these colonies would be build on locations that offer one or more of these resources, like water, building materials or minerals like copper and iron, at their site. However, these resources will eventually run out at the colony site and there is a possibility that not every resource is available at one location. This also could be a reason for having multiple colonies or a separate mining site. In this situation, trading or transporting resources between other colonies or mining sites becomes a solution. Just like countries here on earth trade with other countries to get their hands on things they can’t get in their own country, like for example oil, colonies on Mars could trade with each other to get easy access to the resources they need.

In order to build a colony, different factors to choose a certain spot for a colony have to be examined. To look at the different factors that determine what a good place to build a colony is, two different situations will be looked at. Firstly, there should be a good landing spot for the crew to arrive on Mars. Secondly, there should be a good location to actually start the colony. Ideally, these two locations should be on the same sport or really close to each other. For now, let’s assume the colony will be build on or around the landing spot, since too great a distance between them will create problems for travel. For a good landing spot, the landing area should be as flat as possible. According to [6] It also needs to be clear of rocks and boulders, since this would make landing dangerous and the landing area should not be too soft. It states that Mars has areas where the dust is several meters high, which is unfitted for landing. For building a colony, this also seems important. Building will be hard on areas that are too soft, since this would require better foundations. Non-flat and rocky areas can be not ideal for building, but can be worked around. [7] States that a good landing spot should be as close as possible to the equator, because this ensures that the solar arrays of the lander can deliver enough power at all times of the year and the temperature is high enough for the lander to stay warm. This factor is situational but can be really helpful when the colony will rely on solar power for generating energy. This will also decrease the power needs for warming the colony. Both previously mentioned articles say that the altitude of the area is also an important aspect. According to these articles, a lower altitude is better for landing. This means that there is more atmosphere above the land to slow down and steer for a good landing. [8] talks about food production which is another important aspect. At first the colonizers can live from provisions sent with them, but in order for them to live on mars indefinitely they have to grow crops. In order to grow crops there needs to be usable soil at the landing site and there needs to be enough water. Water is not only important for food production, but also for drinking or even as a fuel by turning it into hydrogen. Currently, the best way to get water on Mars is by gathering ice that is located underneath the surface on Mars. Ideally, this ice will be located as close to the surface as possible, since this makes it as easy as possible to mine. The image [9] below shows the depth of the ice on the surface of Mars.

As shown on the map, the ideal location for digging up ice according to NASA is noted with the white box. This is fairly close to the north pole however, which contradicts with the earlier mentioned preference of building the colony as close as possible to the equator. This already shows that there is not one ideal location to build a colony. Different countries/companies might choose a different location for their colony. Now, let’s assume some choose to build their colony close to the north pole, in the white boxed area, because they think water is more important than other factors. Now, other countries or companies choose to build their colony close to the equator, because they know other colonies can already easily mine water. They now value the better temperatures and sunlight more because they know they can buy and/or trade with the other colonies for water. This means that the citizens of the colony need a way of transporting water from colony to colony. Another possible option could be that a company chooses to build a colony close to the equator and a separate mining site for water. This way, they have the better temperatures and sunlight for solar power at their living area, the colony, and have a way to gather water on Mars. However, this again, brings the need for a transportation system between the colony and the mining site.

Problem

In order to trade or transport water from colony to and from another colony/mining site a way of transportation between the colonies is needed. Going out themselves is dangerous for the colony citizens. Radiation, low temperatures, dust storms and a toxic atmosphere [10] [11] [12] are all reasons to search for another solution than letting humans drive from colony to colony in person. Because of this, the colonies are in need of a transport system that does not require the citizens to go out themselves.

Solutions

There are several possible solutions for this:

Option 1: Building a transportation conveyor tube.

Option 2: Building a protected manned transportation vehicle. This vehicle must be protecting the people inside from radiation and temperatures.

Option 3: Building a remote-controlled unmanned transport vehicle with cameras attached which will be controlled by someone in the colony. There must be someone controlling the vehicle.

Option 4: Building a transport robot that would be able to autonomously deliver water from the one colony to the other.

Comparing solutions

We are going to dismiss options by comparing the most important properties needed to solve the problem:

Option 1 can only transport from two static locations, while option 2, 3 & 4 are more dynamic in this aspect. Because of uncertainty of the location of the colonies in the current development it might be best to leave this one out. Also, with the big distance between colonies this could be an extremely big project that is way too expensive and would still need human help in the dangerous environment to build.

The difference between option 2 versus 3 & 4 is that if we are sending a human on the transportation vehicle or not. As stated before sending a human on the vehicle would be dangerous and would require the vehicle to be of much higher quality to ensure safety of the human inside, which is most likely much more expensive solution than option 3 & 4. Also the added weight of the drivers and extra materials in order to protect the drivers will increase energy consumption of the vehicle.

Now we are left with a remote-controlled transportation vehicle versus an autonomous transportation vehicle. The remote-controlled vehicle will be controlled by a human. This means that the human is always aware of its position and situation allowing to help with overcoming obstacles. This means that decision making will be in human hands which the users might prefer over handing the driving over to an AI. However, the remote-controlled vehicle requires a driver. Since distances could be pretty far in between the colonies/mining sites it would be a better option for the users since they don’t have to spend that much time driving and/or routing.

Because the user will most likely prefer a transportation system that does not require a driver, we have decided to go for solution option 4: Building a transport robot that would be able to autonomously deliver water from the one colony to the other. This robot will autonomously drive to a colony or mining site and will bring back water for the colony.

Alternative

There is one simple alternative namely, using no transport at all. This is the case if every colony can obtain enough water, such that no transport is needed. The 2 most likely ways any colonies will have enough water will be whether it is near a water source or not. There are 2 methods that do not rely on location based water obtaining techniques namely recycling and obtaining water from the air.

Water can almost perfectly be recycled. However, this is only possible when recycling human waste water, like urine and washing water, not when the water is used for growing crops or industrial usage.[13]

Water can also be obtained from the air using humidifiers. One example is the Water Vapor Adsorption Reactor(WAVAR), an industrial-level dehumidifier, that could extract water from the atmosphere. This should be able to obtain some amount of water a day but this would not be enough when also water is needed for crops and such. This is also the reason it is not the first choice between getting water from the glaciers or soil. However, the WAVAR is a very energy efficient method, so it might still be useful under certain conditions.[14]

Using both techniques a colony would be able to sustain humans, but would not have enough water to also sustain water for growing crops and industrial use. Industrial use includes creating rocket fuel and usage for fabricating, processing, washing, diluting, cooling, or transporting a product. Large amounts of water are used mostly to produce food, paper, and chemicals.[15]

Users

Technological Difficulties

Aside from the financial aspect, there are some other difficulties in conducting a journey to Mars. The most prominent ones will be elaborated here.

Distance

Mars is the planet that is closest to earth in our solar system. However, the distance that would have to be covered if we launch a manned vessel to Mars is still significantly large. On average Mars is around 225 million km from earth, with 55 million km at its closest, occurring every 26 months. This journey would take around 150-300 days, according to [16]. This is a reasonable amount of time, but the trouble lies in fuel consumption, as a manned vessel would be larger than a unmanned vessel, due to supplies that a manned mission needs. This would lead to a large fuel consumption. However, technologies are present to cover this distance in space with a manned vessel [17].

Housing

Three other factors to take in consideration are the atmosphere, the temperatures and the storms on Mars. As for the atmosphere, oxygen could be produced using the carbon-dioxide that is present in the Martian atmosphere [18]. For example, greenhouses could be built with plants that convert CO2 into O2 or a device for converting carbon-dioxide into oxygen could be developed. The temperatures range anywhere from -125°C to +20°C [19]. These kinds of temperatures are acceptable in the building of a space colony, with good isolation and air-conditioning systems. There are also severe dust storms on Mars from time to time. This means that a rigid structure needs to be built to withstand such storms, building underground or in caves.

Supplies

Once a group of people is settled on Mars, a huge challenge will be to supply them in their needs. First of all, food is needed to keep the population alive. This could be realized by both transportation of food from earth and, ultimately, growing food on Mars. Also building materials must be transported through space. This requires new rockets that can carry heavy loads. NASA is developing these spacecrafts [20]. This means that this issue, although very cost-intensive, can be technologically overcome.

In conclusion, we can say that a colony on Mars is possible in the future, because all the technologies to realize it is there. It is only a matter of further development and major investments to make this happen. This means that there will be a need for Mars-exploring robot technologies in the future.

The main users of the robots will be a Martian space colony. This colony is non-existent today. However, the idea of a Mars-colony is widely researched right now. NASA, for example, has launched several Mars-exploration-robots in the past [21] and still does extensive research on the planet today. There is also an organization that focuses solely on the development of a human colony on Mars. This organization is called Mars One and it focuses on the selection of astronauts and the raise of funding [22]. Mars One claims that all technologies to get to Mars and to begin a colony there, are already present. Only a return mission is impossible right now, but that will not be needed if the crew of the mission will settle on Mars. This means that, in theory, a Mars colony is feasible within the coming 50 years [23]. This means that the navigation technology, that we will design, is certainly of use for this yet to be founded colony.

Other parties of interest for this technology would be organizations like NASA, ESA, Space-X, Mars One, and other space-oriented companies, as they will be the organizations that will put people on Mars.

Stakeholders

The users of a device that can navigate itself across a planet, in this case Mars, are the people that live on Mars. At the moment, there are no people living on Mars yet. This means we should look at the probability for a Mars-colony to be formed in the near future.

People on Mars

The people that live on Mars are certainly the most obvious users of a transportation robot that transports water on Mars. If Martian colonies are realized, the people that live on Mars will have to get enough water to survive. When this colony is not built on a water source, there will be a need for a robot that transports the water from mining site to the colony that needs the water.

The requirements that these people will have for a water transporting technology are listed below.

Transport as much water as possible in one journey

Travel as fast as possible

Easy to empty the tank

Fast charging of the battery

Device should have a lifespan of at least a few years

Besides the people that live on Mars, there are other stakeholders that play a part in the implementation of a transport robot, for example the companies that put the people on Mars.

Organizations

There are several organizations that have plans to establish colonies on Mars. The most prominent ones are elaborated below.

Mars One

People have always dreamed about relocating the human race to another planet, in case something would happen to planet earth. This idea generates even more interest nowadays, than it did, say, 50 years ago, with the current problem of climate change affecting the earth. Now, more than ever, people are looking for another place to live in the galaxy. One of the most prominent candidates for this migration would be Mars, because it is ‘close’ to earth in comparison to other planets. Furthermore, it is believed to have water, which is a major life source, in its soil. But are we ready to move there in the near future?

Mars One is an organization, that certainly believes that a Mars colony is possible. Mars One handles the selection procedure for astronauts that want to settle on Mars in the yet to be formed space colony. It is also responsible for raising funds in order to make the mission happen. In [24] they state that the formation of a space colony, instead of a visiting mission that has to return to earth, excludes most technological and cost-intensive problems. Namely, if people want to settle on mars, no return vehicle, return propellant or the systems to produce the propellant locally are required. This decreases technological challenges and reduces costs dramatically. Furthermore, they mention that all the technologies to send people to Mars and make them survive there are already present. This means that a Mars colony is certainly feasible within the next 50 years.

However, in [25] they mention that the funds to actually develop a Mars settlement mission are not present right now (1 million USD of the 1 billion USD required). On the other hand, donations and investments might rapidly increase when more research is conducted in this field, and people would get convinced that a Mars colony is feasible. Also, if the situation on earth gets worse, people might have no choice but to invest in these kinds of missions. This means that, if Mars One succeeds in its’ mission, they will have an interest in a transporting technology on Mars.

Space-X

Space-X is another (privately owned) organization that has made concrete plans for a Mars mission and a Mars settlement [26]. Their goal is to send a cargo flight to Mars as early as 2022 and they have set the target for a manned vessel in the year 2024. Although optimistic, these plans are very concrete and ambitious. This means that Space-X is an organization that could certainly have an interest in the technology that we are researching for our project.

NASA

NASA is a governmental organization that also has the ambition to put humans on Mars [27]. Their goal is to put humans on Mars within the next decade. NASA is the organization that is most likely to succeed in completing this mission, because they have put men on the moon before, so they have the technologies in store to conduct such a mission. Furthermore, this is the organization that has gathered most of the data of Mars that is present today, as they have sent multiple satellites and exploration vessels to the planet [28]. Therefore, NASA is a major player that could have an interest in the technology we are investigating.

Other organizations

There are other privately-owned companies that have also conducted research in the field of establishing a Mars colony, like Lockheed Martin and Boeing [29]. Furthermore, there are other governmental organizations, like ESA, Roscosmos, ISRO and the CNSA [30], that could also have an interest in the technology. However, the plans of these organizations are not as advanced as the plans of the organizations mentioned before. Therefore, they are less likely to have an interest in a transport technology in the near future.

The requirements that these organizations will have for a water transporting technology are largely the same as the requirements that the people on Mars have. However, there are some additional requirements that the organizations may have:

Production process should be as cheap as possible

The usage of the device should be energy efficient, and therefore cost efficient

The device should be easy to transport from earth to Mars, as it will most likely be produced on earth.

However these needs are considered to be subordinate to the people’s need in this project, so they will be added as preferences.

Governments

Other major players in the Mars colonization field, are the governments of the different countries involved in the previously mentioned governmental organizations. In the case of NASA this will be the American governments, in the case of ESA the European union, and so on. These governments sometimes have different needs and interests than the organizations which they govern. For example, NASA cares greatly for the scientific knowledge that is gained through new space missions, while the government might care more if they are world leader in the field of space engineering. This sometimes leads to conflicting interests.

Moreover, a competition between countries can occur in the building of the first Mars colony. In the past, for example, we have seen a so-called ‘space race’ between the Russians and the Americans [31]. It could be that the colonization of Mars will follow a similar scenario. However, it could also be the case that governments will establish multinational Mars colonies, as they did in the past with the ISS (International Space Station [32]).

Some additional requirements, besides the requirements of the people and the organizations, that governments will have for a water transportation device are:

Easy to reproduce the manufacturing techniques (cooperation)

Difficult to copy technology (competition)

The requirements for cooperation are implemented, as this is the most likely scenario. This requirement is also treated as a preference.

Other stakeholders

There are some other stakeholders that would have an interest in the technology. One could, for example, think of mechanics that have to work on the device, possible organizations that are formed on Mars among the colonists, environmental organizations, manufacturers (although the manufactures probably have to answer to the organizations), suppliers of manufacturers and so on. The interests of these groups are largely in line with the interests that are previously mentioned. Moreover, the interests of these groups are much smaller than the groups that are mentioned before. Therefore, the interests of these groups are not accounted for in this project.

User requirements

A full list of user requirements is given below:

User Group	Requirement
People in colony	Transport as much water as possible in one journey Travel as fast as possible Easy to empty the tank Fast charging of the battery Device should have a lifespan of at least a few years
Organizations	Production process should be as cheap as possible The usage of the device should be energy efficient, and therefore cost efficient The device should be easy to transport from earth to Mars
Governments	Easy to reproduce the manufacturing techniques

Scenarios

Research question

This research will mostly focus on the energy system used in the water transport robot, this will include research on different energy sources which will be compared, power storage on Mars is also researched. The goal of this research is to find out if it is doable to provide such a water transport robot on Mars with enough energy for long distances. The main research question chosen is: Is it doable to provide a water transport robot with enough energy? To answer this question for the scenario described below a model is made.

Assumptions

To research different possibilities for power storage and power supply, it is first needed to know how much power the robot needs. For this a quantitative description of the situation is needed. Since this situation is still an idea and not the reality, this will consist mostly of assumptions. All the time related units will be based on the situation on Earth. In other words, one day means one day on Earth. Most of the assumptions are based on values experienced on Earth, so it makes sense to use this time scheme as well. Not that it matters that much, since a day on Mars is equivalent to 1.02749125 days on Earth [33], which is not that big of a difference. To start, it is assumed that there are 50 people living in the colony around the equator that needs water. When looking at the water map displayed in the setting section at the beginning of the wiki, it can be said the equator is about ⅛ of the Martian circumference away from the white boxed section which is about 2500 kilometers. So, the colonies are 2500 kilometers apart. The goal is to let the robot drive at a speed of at least 10 kilometer per hour, which will result at a travel time of 500 hours or about 21 days to drive from colony to colony and back. We also assume the transport robot gets the maintenance and cleaning it should need, like for example cleaning the solar panels of dust, at each colony.

In order to know how much water the robot needs to transport, the water usage of the colony needs to be established. Every person in the colony will use about 50 liters of water per day [34] for drinking, washing etc. From these 50 liters, it is assumed that 80% can be recycled at the colony [35]. However, water is also needed for other things. If the colony is growing its own food, water is needed to grow crops. A human eats around 2.5 - 4.6 kilograms of food per day [36]. For simplicity, it assumed that humans eat 3.5 kilograms of food per day. The water cost for food production of different types of food can be found in the graph below [37]. Since it will be likely that in this stage of the colony no animals will be on Mars, it is assumed that the only food produced on Mars is in the form of growing crops. Because of this, the average amount of water needed for 1 kilogram of food is about 1675 liters which is obtained by taking the average of the production cost of apples, maize, barley, wheat, soyabeans and rice. Here on earth, a part of the water used for hydrating the crops disappears in the earth, but on Mars, a greenhouse could extract all extra water from the soil and reuse it. Also, a part of the water inside the crops that is consumed will be turned into urine which is recycled at the colony. While based on nothing, it is assumed that 30% of the water needed for growing crops can eventually be recycled. This brings the total water cost to 1182.5 liters per day per person, which is 59125 liters for the whole colony per day. This is a lot, but it is just an assumption. It could be that a lot more of the water needed for crops could be recycled, which will bring the number down by a lot. But for now, this number is what will be worked with. Another option is that all the food is produced at the colony where the water is coming from, which will result in the need for another transport infrastructure for the food, or could be combined with the water transport. This will bring the water consumption down to 500 liters per day for the colony at the equator. This seems like a way better option, so it is assumed that food is supplied by another transport robot. This means the total amount of water that the water transport robot needs to supply is 500 liters per day.

Since 1 transport robot can be back at the colony every 21 days, and the colony needs 500 liters of water per day, it can be concluded that 1 transport robot should be able to carry at least 10.500 liters of water. While, for this project, it is assumed that this transport robot will always reach its destination, in reality, it could always go wrong. The transport robot could get stuck, lost in a dust storm or just get a malfunction. For this reason, it is assumed that 4 of these robots will drive from and to the colony. 1 Robot will dispatch every 5¼ day and is assumed to have a capacity of 5000 liters. This means that, assuming the colony itself has storage capacity enough for more water than consumed, even if 1 robot will fail to come back, the colony always has water enough to survive. For comparison, the Mercedes Atego 1317-A 4x4 Lindner-Fischer 2017, which can be seen in the picture below [38], has a capacity of 6000 liters. This means that the transport robot needs to be around this size. This model weights about 11900 kilograms. With a full tank of water, this means it will weight about 12400 kilograms. This means that it can be assumed that the transport robot will weigh approximately the same on earth. Since the gravity on Mars is about 38% of the gravity on Earth [39] this will result in a weight of about 4712 kilograms on Mars. For calculations however, mass is used, which will not change with gravity so it still has a mass of 12400 kilograms on Mars.

Now, the last thing to do is calculate how much energy the transport robot will approximately need. The energy needed to bring the transport robot from standing still to a certain speed can be calculated with the formula stated below.

In this formula, Ek represents the kinetic energy put into the robot when it reached this speed, m represents the mass and v represents the speed. As any car on Earth, the transport robot will experience friction while moving across the surface. This friction consists of the friction created by the ground on the wheels and the air friction. The friction created by the ground on the wheels can be calculated with the formula stated below.

In this formula, Ffr represents the friction force, m represents the mass, g represents the gravitational acceleration and c represents the rolling resistance coefficient. The friction caused by the air while moving can be calculated with the formula stated below.

In this formula, Ffr represents the friction force, C represents the drag coefficient, ρ represents the air density, A represents the cross-sectional area and v represents the speed of the transport robot. Both these friction forces will bring the transport robot to a stop when the engine of the robot stops putting kinetic energy in the robot. During the trip, these forces will put work on the robot which will require the engine to put new kinetic energy in the transport robot. This work is equal to the total friction force times the distance travelled. This means, that the total energy needed to move the transport robot from one colony to another at a certain speed can approximately be calculated with the formula below.

In this formula, E represents the total energy needed and d represents the distance. Some of these variables/coefficients are unknown, assumptions are made as presented in the table below.

Variable/coefficient	Assumed value	Based on
C	0.58	Drag coefficient of a Jeep Wrangler TJ [40]
A	7.5 m^2	Dimensions of Mercedes truck
c	0.06	Value also assumed by NASA [41]
m	12400 kg	Mercedes truck
d	2500 km	Distance colony to colony

Variable/coefficient	Known value	Based on
ρ	0.020 kg/m^3	Air density on surface of Mars [42]
g	3.711 m/s^2	Gravitational acceleration on Mars

Electric engines offer an efficiency of about 80% from grid to wheels [43], meaning that the energy that should be produced by the transport robot can be calculated with the formula stated below.

It is important to note that this formula does not account for everything and is just an approximation of the energy needed. A plot of the speed and the total energy needed can be found in the graph below. The noted point represents the wanted speed of 10 kilometers per hour, which is equal to 2.7778 meters per second. This speed results in a total needed energy of 8.63*10^9 Joule for driving.

Other energy needs could include keeping the water at a certain temperature. Temperatures on Mars range from -125 degrees Celsius on a cold winter day to 20 degrees Celsius on a summer day [44]. This means that for the most part, the temperature is under 0 degrees Celsius and the water will be ice. While energy could be used to keep the water under 0 always, it seems best to let the water alternate between these two states of matter, solid and liquid. If the tank is big enough to keep 5000 liters liquid water in ice state, it should not be a problem. This will cut the need to put energy in keeping the water in a certain state. Since the density of ice is 0.92 times the density of liquid water [45], the tank should be able to carry 5435 liters of ice. For simplicity, we assume the tank has to have a capacity of 5500 liters.

All in all, this means that if the transport robot moves with a speed of 10 kilometers per hour, it needs to be able to either storage at least 8.63 GJ of energy or needs to be able to, assuming it travels for 10.5 days one way, generate at least 9511.89 Joule of energy per second (9511.89 Watt). A combination of a power storage that is charged at the colony and a power supply during the trip could also be an option. The amount of battery capacity and the amount of energy production needed can be found plotted against each other in the graph below.

RPC list

Requirements	Preferences	Constraints
Needs to travel 2500 Kilometers from colony to colony	There should not be the need for too many robots (around 4)	Dimensions of robot (662x250x300cm)
Needs to supply 500 Liters of water per day	The robot should have a speed of 10 km/h

What does the robot already know?

The robot already knows the location of the colonies and already knows the terrain of Mars in between. Unexpected obstacles such as boulders, duststorms or cave-ins are not known however.

Plan

Research question

Main question

Is it doable to provide a water transport robot with enough energy?

The goal of our project is to research different possibilities for producing energy to see if it is doable to store/produce enough energy on the robot in order to be able to drive such long distances on it's own. In order to be able to answer this question, different sub-questions need to be answered. Also a model is created to calculate the maximum velocity the robot can drive given a certain design. This can be used to see what kind of design would work the best and could also be used by the manufacturers during the design process to see if certain designs would work at all nor not.

Sub-questions

Which possibilities are available to generate power on Mars?

First, the possibilities of different kinds of energy production methods are looked into; in our situation to see how much energy can be produced by each of them and to see if they are applicable to our robot.

Solar Energy

Radioisotope Energy

Hydrogen Energy

Biomass Energy

Fossil Fuels

How much power can the transport robot store?

Then, research will be done on the amount of power that the robot can store. This will be done to reduce the amount of energy that has to be generated on board.

Needed battery capacity

Maximum battery capacity that is possible

Given the generated and stored energy, is there an optimal velocity at which the robot can travel?

A model will be created in order to find the velocity at which the robot will travel. Furthermore, the ideal methods of power generation and power storage can be extracted from this model.

Inputs

Calculations

Which options are the best for our scenario?

Lastly, the optimal combination of power generation and power storage will be found.

Which power generation design could be used best?

Are we dependent on energy storage?

Milestones

Research on power storage

How much capacity does the battery need?

How much capacity can the battery have?

Can a battery be used at these temperatures?

Research different possibilities for power supply

Solar energy

How does it work?

What is the solar intensity on Mars?

What is the best angle for the solar panels?

How much solar panels can we put on the transport robot?

How much energy can we produce using solar energy?

Radioisotope Thermoelectric

How does it work?

Is fuel for this obtainable on Mars?

How safe is this?

How much energy can we produce using Radioisotope Thermoelectric energy?

Alternative Fuels

Are fossil fuels, biomass or hydrogen energy feasible options for power generation?

Comparing different energy supply methods

Which produces the most energy?

Which is the safest form of energy production?

Which is the easiest to produce locally on Mars?

Think about final design

Is only a battery enough? How much energy production is needed if we use the battery only as a buffer?

Can we go faster than 10 km/h with the battery and energy production?

Making the presentation

Deliverables

Research on users of the product

Research on the state of the art in this field

Making problem statement and scenario

In-depth research of the power storage

In-depth research of energy generation methods

Proper plan for power supply of the transport robot

Proper plan for power storage of the transport robot

Model

Conclusion

Discussion

Presentation

Task Division

Task	People working on it
Research on users and state of the art	- Rik, Zeph, Nick (Week 3 & 4)
Writing problem statement and scenario	- Zeph, Finn, Nick (Week 3 & 4)
Research on power storage	- Thomas (Week 5) - Rik (Week 5)
Research different possibilities for power supply (Solar Energy)	- Zeph (Week 5) - Stefan (Week 5)
Research different possibilities for power supply (Radioisotope Thermoelectric)	- Nick (Week 5) - Finn (Week 5)
Compare different power supply methods	- Zeph (Week 6) - Stefan (Week 6) - Finn (Week 6)
Think about final design (includes creating model and writing conclusion)	- Nick (Week 6 & 7) - Rik (Week 6) - Thomas (Week 6) - Zeph (Week 7) - Finn (Week 7)
Start on presentation	- Thomas (Week 7) - Stefan (Week 7) - Rik (Week 7)
Finishing Presentation	- Everyone (Week 8)
Finishing Wiki	- Everyone (Week 8)

Planning

Week	Working on	Deliverable finished at end of week	Mainly responsible for finished deliverable
Week 3 & 4	- Users and state of the art - Problem statement and scenario	- Research on users and state of the art (1) - Writing problem statement and scenario (2)	- (1) Rik, Zeph, Nick - (2) Zeph, Finn, Nick
Week 5	- Research on power storage - Research different possibilities for power supply	- In-depth research of both energy sources (1) Solar Energy (2) Radioisotope Thermoelectric - In-depth research of the power storage (3)	- (1) Zeph & Stefan - (2) Finn & Nick - (3) Thomas & Rik
Week 6	- Extra time for research on power supplies if needed - Comparing different power supply methods - Starting on model	\	\
Week 7	- Creating model and draw conclusions from it - Start on presentation	- Proper plan for power supply of the transport robot (1) - Proper plan for power storage of the transport robot (2) - Model (3) - Conclusion (4)	- (1 & 2) Rik, Stefan - (3 & 4) Zeph, Finn & Nick
Week 8	- Finishing Wiki - Finishing Presentation	- Presentation (Before Thursday 02-04-2020) - Wiki (Before Thursday 02-04-2020)	Everyone

State of the art

In the area of navigation on mars, there are several things that have already been tried. In order to understand what has already been done in the field of navigation on Mars, we need to have a look at the most recent, still working devices that have landed on Mars. In the past there have been a lot of rovers and other devices that have landed on Mars in order to explore the planet and its surface. However, the only two landers that are still active and in contact with earth are Insight (2018) and Curiosity (2012)[46]. All the other spacecrafts that have ever been on Mars are either broken, or have lost contact with earth in another way. There is also a future rover, scheduled to launch in 2020, which is called Mars 2020. Next, Insight, Curiosity and Mars 2020 will be elaborated.

Insight

The Insight Mars lander was launched in 2018 and landed successfully on Mars. Its primary goal is to investigate the ‘inside’ of the planet. More specifically its "pulse" (seismology), "temperature" (heat flow), and "reflexes" (precision tracking) [47]. It is a stationary device, as it does not navigate itself across the surface of the planet. Instead, it measures the key characteristics of Mars from one place on the surface, with the goal to determine how small, rocky planets in our solar system have been formed. Therefore, this mars lander does not contribute to the state of the art of our project, as we will focus on the energy generation of an autonomous vehicle on Mars. Insight is a stationary device and therefore not of much interest for the project.

Curiosity

The Curiosity Mars lander was launched in 2011 and reached Mars in 2012. It is a car-sized Mars rover that will explore the Gale crater on Mars. The rover navigates itself across the surface of Mars by itself, which makes it an interesting device to look at for our project. Furthermore, its primary goal is to investigate whether or not life was ever possible. This means that it also looks for essential life conditions like water, oxygen, sulfur, phosphor and other minerals [48]. It uses very sophisticated techniques in order to collect samples from the Mars surface and to analyze them. To navigate itself across the surface, Curiosity uses so called ‘Hazcams’ and ‘Navcams’. These are hazard avoidance cameras and navigation cameras. These will be further elaborated.

Hazcams

The four hazard avoidance cameras are located on the lower front and back of the vehicle. They operate in black and white, using visible light to create a 3D-image of its surrounding environment [49]. It also works in tandem with software that allows the rover make its own safety choices and to "think on its own", which makes it autonomous. The cameras each have a wide field of view of about 120 degrees. The rover uses pairs of Hazcam images to map out the shape of the terrain as far as 3 meters in front of it, in a "wedge" shape that is over 4 meters wide at the farthest distance. The cameras need to see far to either side because unlike human eyes, the Hazcam cameras cannot move independently; they are mounted directly to the rover body.

Navcams

Mounted on the mast (the rover "neck and head"), these black-and-white cameras use visible light to gather panoramic, three-dimensional imagery. The navigation camera unit is a stereo pair of cameras, each with a 45-degree field of view that supports ground navigation planning by scientists and engineers. They work in cooperation with the hazard avoidance cameras by providing a complementary view of the terrain. [50]

Mars 2020

The Mars 2020 rover is basically a more advanced version of the Curiosity rover. It has as a goal to not only investigate if life was ever possible on Mars, but also to look for signs of microbiological life in the past. It will collect promising rock samples and store them for future investigation with equipment that is too large to take to Mars. Furthermore it will also look for ways to produce oxygen from the Mars atmosphere and for ways to collect water from the Martian soil [51]. It will use the same navigation instruments as the Curiosity rover, only a bit more advanced.

Power

The Curiosity rover uses a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) in order to generate energy. This is a device that relies on radioactive material to generate energy. It is a preferable energy source for Mars rovers, because the energy generation method does not require oxygen. Furthermore, Curiosity also has solar panels to generate energy. For the storage of this energy, two lithium ion batteries are present on board of the rover. These batteries have the highest known storage capacity, and are reliable in Martian conditions. For example, discharging is possible at the low temperatures that are present on Mars. These batteries are used for backup for the energy generators in times of peak demands. [52]

Research

Solar energy

Solar panels or also called photovoltaic modules are a collection of photovoltaic cells. These cells use sunlight as source to generate electricity.[53]

[54]

Performance

The performance of a solar panel depends on many different aspects. The most important aspects are the sunlight that hits the surface of the solar panel, the efficiency of the conducting material in the cells of the solar panel and the size of the surface of the solar panel. There are also many other aspects that determine how many energy is created like whether there is something blocking the sunlight, like an object or dust particles, and the temperature of the photovoltaic cells. The global formula to estimate the energy generated by a photovoltaic system is stated as presented below [55] .

In this formula, the following variables are used;

E = Energy (kWh)

H = Average solar radiation on tilted panels (kWh/m^2)

r = solar panel efficiency (%)

A = Total solar panel Area (m^2)

PR = Performance ratio, coefficient for losses (range between 0.5 and 0.9, default value = 0.75).

Average solar radiation on tilted panels (H)

To determine the average solar radiation on the panels, the sunlight intensity on mars can give a good estimate. Power received from the sun is often measured in solar irradiance, which is the power per unit area of electromagnetic radiation. To be precise, irradiance in general is defined as “the amount of light energy from one thing hitting a square meter of another each second” [56]. Solar irradiance is measured in watt per square meter (W/m^2). The solar irradiance on Mars can be calculated using the solar constant [57]. At a distance from the sun of 1 UA, which is approximately the distance between the Sun and Earth, the solar irradiance is equal to 1361 W/m^2. Since the average distance from the Sun to Mars is equal to 1.524 UA [58], the solar irradiance on Mars can be calculated as stated below.

To check this calculation, the average solar irradiance on Mars according to NASA is 586.2 W/m^2 [59]. Since one day on Mars takes 24.62 hours [60] it can be assumed that there will be approximately 12.31 hours of sun and 12.31 hours of darkness each day. Of course, the sunlight intensity will be at its peak at noon, which is when it will produce the 585.987W/m^2 calculated before. The sunlight irradiance during the rest of the day could be approximated using a parabola, which can be used to calculate the total energy production during one day [61]. This parabola will have its peak at 585.987 and will go through the x-axis at -22158 and at 22158, which is equal to the amount of seconds in 6.155 hours. This can be achieved with the equation stated below.

This parabola also has negative y-values, but these can be ignored. A plot of this equation can be found in the image below.

In order to calculate the total energy produced during one day, the area underneath the graph should be obtained. For this, the antiderivative of the function is needed, which can be found in the equation below.

Since the function is symmetrical, the total area under the graph can be calculated by taking the integral from x = 0 to x = 22158 twice, which results in 17312399.93 Joule per square meter per day. The robot is driving for 10.5 Earth days (252 hours), which is equal to approximately 10.219 days on Mars. Thus, there will be, approximately, 176915414.9 Joule of sunlight per square meter over the course of the 10.219 days on Mars.

To get the most out of this energy, the solar panels should be positioned correctly. The solar panels will generate the most amount of electricity when the sunlight is perpendicular to the surface of the solar panel [62]. This means that, ideally, the solar panel will always be directed directly towards the sun using a tilting mechanism. An example on earth where a system tilts the solar panels so that they always face the sun at a 90-degree angle is in a operation in Piteå [63]. The helianthus smart solar panel is a solar panel that is able to track the sun using a microcontroller based embedded system [64]. This will ensure that the angle of the sunlight hitting the solar panel is always 90 degrees. Some form of this mechanism should be implemented in the robot, should it use solar energy.

Efficiency (r)

The most important aspect that will determine the efficiency of the solar panels is the material that is used as the conductor. A better conductor will cause less loss of energy when converting sunlight to energy. The most common materials used in mass production at the moment are; Crystalline silicon (c-Si), amorphous silicon (a-Si), gallium arsenide (GaAs) and organometallics (soluble platinum). Each type of material has its own advantages and disadvantages, mostly being efficiency against the production cost.

Crystalline silicon is most common at the moment for mass production. It has a conversion rate of about 20 - 25%. It is not the cheapest material but has a low enough cost to be considered the best material for solar panels in home installations.

Gallium arsenide is a better conductor than silicone. However, it is very rare, expensive and manufacturing gallium arsenide is dangerous since arsenide is poisonous. This could be a reason maintenance is less ideal for the users on Mars that will be tasked with fixing the robot when needed.

Organometallics (soluble platinum) is a metal conjugated polymer and an even better conductor than gallium arsenide. It is lightweight and relatively cheap to produce. [65]

Multi-junction solar cell is GaInP/GaAs/Ge multijunction solar panels with a high efficiency of over 30%. However it is more complex and more expensive than any other. [66]

While having the most efficiency seems like the best idea, money is probably a big deal for certain users. Think about the companies manufacturing and funding these robots, like for example NASA. for now, calculations will be done using the material with the highest efficiency, but the manufacturer could decide to pick another conducting material.

Table containing efficienty of materials [67]

Total solar panel area (A)

To calculate how many solar panels can be put on the robot, an estimation is needed of how much space is needed for a solar panel with length L, width W and height H. Since every solar panel needs the ability to turn 360 degrees and tilt about 90 degrees, each solar panel will have surface of L * W m2 and should have a height such that H >= 0.5 * L AND H >= 0.5 * W. Now it is important to determine what the best setup is e.g. what is the optimal placement of the solar panels on the robot such that no solar panel will overlap and we have maximum utilized surface of solar panels:

One large solar panel.

Couple of mid range solar panels

Many small solar panels.

The robot will have similair dimensions as the Mercedes Atego 1317-A 4x4 Lindner-Fischer. [68] with length: 6.62 m, width: 2.50 m and height: 3.00 m.

Having one large solar panel with a length and width of 6.50 m will give a huge surface of 42.25 m^2. However, it will most likely not be possible to have a height of 6.5 m and therefore not be able to tilt and turn fully. Also it is questionable whether the giant solar panel will be able to catch wind and make the robot tilt at high wind force. Moreover having a solar panel stick about 1,3 m of the front and back when they turn, since the diameter is around 9.2 m and 3,3 m of the sides which can be problamatic as well. Therfore this option will not be viable.

Having multiple solar panels with a lenght and width equal to the width of the robot (200 cm) we will be able to put 3 solar panels on the robot and have a total solar panel surface of 12 m^2. The height should be around 1 m in order for the solar panels to turn which is quite doable. However, the solar panel sticks 15 cm off the side when they turn, since the diameter of each solar panel is around 2.8 m and 1 m off the front and back. This could be problamatic as well when driving up or down a slope. However by sacficing some area and making the middle solar panel a bit higher, about 0,5 m and putting the first and last solar panel closer to the middle the solar panels will not stick too much over the front and back. This will however cost some effective area since the large solar panel will shade the 2 smaller ones. So if we lose 50 cm of the front and back, the the area will decrease by 0.5 m * 2 m * 2 solar panels * 0.8 (20% loss due to shade) = 1.6 m^2. So we will have a remaining solar panel area of 10.4 m^2.

Another solution for the solar panel width / length is to have many small solar panels with length and width 1 m. We can put them in pair on the robot, such that they only stick about 10 cm of the side, since it will have a diameter of 140 cm, and have height 0.5 m. This will enable us to put around 5 pair of solar panels and the robot, having the solar panels only stick 25 cm of the front and back. This gives us a total solar panel area of around 1 m2 area per panel * 2 solar panels per pair * 5 pairs = 10 m^2.

Therefore the best option is to use 3 middle size solar panels.

Performance ratio (PR)

The performance ratio is most difficult to determine and will require many assumptions. The losses for certain temperature, shade and dust can be determined with some precision, while others will depend on the implementation, wiring and manufacturing of the solar panels. Example of detailed losses that gives the PR value are: [69]

Inverter losses

Temperature losses

DC cables losses

AC cables losses

Shadings

Losses at weak radiation

Losses due to dust, snow...

Temperature losses

A higher temperature reduces the efficiency of the solar panels. [70] In contrast a lower temperature will increase the efficiency of the solar panel. The formula for calculating the loss of temperature is:

TL = TC * (TT - MT) [71] with:

TL = temperature losses

TC = temperature coefficient

TT = tested temperature

MT = modular temperature

The temperature coefficient depends on the type of conductor that is used for the solar panel. The temperature coefficient of GaInP/GaAs/Ge multijunction is -0.09%.[72] The efficiency is tested at a standard of 25 °C, therefore the tested temperature is also 25 °C. The modular temperature is the temperature of the area where the solar panel is placed. For now, we wil use the average temperature on Mars of -63 °C .Therefore the total temperature loss (or gain in this case) = -0.09% (25 - (-63)) = -7.92%.

Do solar panels work at -70 degrees? [73]

Shadings

Most clouds on mars exist of ice water and clouds are also formed during dust storms. These clouds can decrease the brightness of the sun up to 40%. [74] As such we will assume the average of performance decrease to be around 20%.

Dust

Dust can be a large problem for the energy output of the solar panels. We can assume that at the start of every trip the solar panels are cleaned and the loss due to dust is 0%. However, since Mars has a lot of dust on its surface and can have a lot of dust in the air after dust storm/ dust devils it can stick to the surface of the solar panel and dramatically decrease the energy generator to 30%. [75]

To increase the performance of the solar panel during or after sandstorms, an Arduino Uno microcontroller can be used to track the power generation and cleans the photovoltaic surface when it drops to 50% from its rate value. It then uses a wiping tool to clean the surface of the solar panels. This method was proposed for solar panels in Iraq since dust storms are quite normal there. [76] For the water transport robot it could be wise to set the bound for cleaning the solar panel lower such that it cleans the panels at a lower drop. We will assume that it is possible to have the module clean the solar panels when its rate value drops to 15%. This would of course mean that more wiping fluid is needed.

Another method is to implement MPPT for each module individually which can measure performance and fault detection at module level. This ensures that if a part of the solar panel becomes shaded or too dusty, the power output doesn’t drop to zero. [77]

If both methods described above are implemented, dust will most likely not exceed a loss of 15% and have an average loss of about 8%.

Other losses

Since it is difficult to determine what the other losses will be we will assume the average for each of he remaining losses [78]:

Inverter losses (7%),

DC cables losses (2%),

AC cables losses (2%)

losses at weak radiation (5%)

These combined give a loss of 16%.

Total energy

H = 176915414.9 J/m^2

r = 32.00%

A = 10.40 m^2

PR = 100% - ((-7.92%) + 20% + 8% + 16%) = 63.92%

E = 176915414.9 J/m^2 * 0.3200 * 10.40 m^2 * 0.6392 = 376344660.9 J

[79]

Thus, the total energy produced by the solar panels is 376344660.9 J during the 10.5 Earth days, which equals, on average, an energy production of 414.8419984 W (414.84 W).

Other important aspect for solar panels

Maintenance

We will assume that at the start of each trip the robot will be fully functional. This includes the robot having no dust on the solar panels, which means that the occupants should wipe the solar panels each time a robot leaves for transport. Ofcourse regular maintance should be neccessary, but most solar panels have a life of 25 years [80]. Moreover, the solar panels when unrepairable can be recycled. Circa 95% of the a solar panel can be recycled, depending on the materials used to create them. [81] [82]

Weight

It is important for efficienty purposes to add the weight of the solar panel to the total weight of the robot and thus increase the total energy the robot needs. Since there will be 3 solar panels with length / width 2m and height of 1m.

We will assume that the solar panel will weight 15 kg per square meter. [83] This makes each solar panel weight around 80 kg and the total weight around 240 kg.

Since the solar panels need to turn, there is also the need of the pilar that of 1m that will lift the solar panels. This will most likely be a pole with the same material as the frame for solar panels being aluminum. [84] Aluminum is a “lightweight, non-ferrous metals with good corrosion resistance, ductility, and strength.” [85] The thickness of the round bar of aluminium will be around 10 cm. Therefore the total weight of all 3 pipes will be around 65-70 kg in total, depending on the type of aluminium used. [86]

Thus the total weight of all solar panels will be around 310 kg.

Degration

Slowly a solar panel will lose efficiency and produce less energy. After 25 solar panels can produce only up to 80% of their original efficiency. [87] We will assume that the solar panels will be replaced after 25 years.

Radioisotope Thermoelectric energy

Workings of a Radioisotope Thermoelectric generator

A typical design of a radioisotope thermoelectric generator (RTG) consists of 2 main ingredients: fuel that will decay radioactively and a large set of thermocouples to convert heat into electricity.[88] A thermocouple is a set of 2 wires made of 2 different metals that are placed in electric contact at each end.[89] When two metals are placed in electric contact, electrons flow out of the one in which the electrons have a higher Fermi level. The Fermi level of the metal represents the thermodynamic work required to add one electron to the metal. The energy of an electron in the metal at the Fermi level is −W relative to a free electron outside the metal. The flow of electrons between the two conductors in contact continues until the change in electric potential brings the Fermi levels of the two metals (W1 and W2) to the same value. This electric potential is called the contact potential ϕ12 and is given by eϕ12 = W1 − W2, where e is the electrical charge of one electron (1.6 × 10−19 coulomb.) [90] If a closed circuit is made of two different metals at the same temperature, there will be no net electromotive force in the circuit because the two contact potentials oppose each other and no current will flow, however if the temperature of one of the junctions is raised relatively to the other there will be a current. Since the Fermi levels of the two metals have a different temperature dependance there will be a net electromotive force generated in the circuit. To maintain the temperature difference, heat must enter the hot junction and leave the cold junction, in a RTG this heat is produced by the fuel that will decay radioactively. The generation of a thermal electromotive force at a junction is called the Seebeck effect. The electromotive force is approximately linear with the temperature difference between two junctions of the different metals, these two different metals are the thermocouple mentioned before. The picture shown below [91] shows a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) which is a typical modern RTG.

The fuel is located behind the thermal insulation layer and the thermocouples are lined in modules throughout the sides of the RTG.

Fuel in a Radioisotope Thermoelectric generator

There are several possible isotopes that can be used as a fuel for RTGs, there are a couple of criteria that these isotopes have pass in order to be candidates. Dr. Bertram Blanke research on the development of RTGs evaluated over 1300 radioactive isotopes for the project, but only found that 47 of them had the suitable characteristics needed to be a possible fuel for RTGs. [92] These characteristics include:

Possession of long half-life for continuous energy production.

Ability to produce high energy radiation.

Large heat power-density.

Tendency to produce radiation decay heat.

Based on these characteristics, the most frequently used isotopes for RTG fuels include:

Plutonium-238 (Pu-238)

Curium-244 (Cm-244)

Strontium-90 (Sr-90)

Of these Plutonium-238 is the most cited fuel in resources about RTGs. [93] In fact because of the frequent use of Plutonium-238 for the use in RTGs, including its use in more than 20 space missions, there is a current shortage of this isotope. Since Plutonium-238 is a byproduct of creating nuclear weapons most of what NASA uses is left over from the cold war. [94] However this does not mean it can’t be produced anymore: In 2012, the Obama administration got Congress to go along with a plutonium-238 restart, under the condition that NASA pay to repair aging DOE infrastructure. When these repairs are complete, the Energy Department of Energy will start producing 1.5 kilograms of plutonium-238 a year. [95] Pu-238 satisfies all of the RTG fuel characteristic needed with high radiation output, mainly alpha decay channels and is thus safer to use for any Martian colonist, a very long half-life of 88 years, and a small fuel pellet packaged into the size of a marshmallow as seen in the picture shown below. [96] All these factors make Plutonium-238 the best isotope to use for RTGs on Mars.

Production of Plutonium-238 will not be possible on Mars in the early stages of colonization since the production process is really complicated, thus all the Plutonium-238 would have to be transported to Mars from Earth.

Amount of energy produced using a Radioisotope Thermoelectric generator

To determine how much energy can be produced by using a RTG, we need to know the heat energy generated and the efficiency of the energy conversion. The latter depends on the materials used and the temperature on the cold and hot side. The following material properties influence the efficiency: Electrical conductivity, Thermal Conductivity and the Seebeck Coefficient. [97] If these are known then the efficiency by which a material is capable of generating power can be determined.

Then, by using the temperature on the hot and cold side, the conversion unit can be calculated.

Once M and Z are known the ideal efficiency can be calculated.

These formulas could be used to determine the efficiency of a custom setup, but for existing setups such as the MMRTG the power output is already known.

In the Multi-mission radioisotope thermoelectric generator (MMRTG) developed by NASA for multiple space missions, PbTe (Lead Telluride) and TAGS is used, where TAGS material is a material incorporating Tellurium, Silver Germanium and Antimony. This material and the temperature inside the generator result in a thermal power generation of 1975 W and an efficiency of 5,6%. Concluding in a netto electrical output of 110 W at the start of the mission, falling to about 100 W after 14 years. The MMRTG weights 43.6 kg which means the netto energy density is 2,52 W/kg. [98] For every one kg added to the Transport robot would require an extra 695.8 kJ of energy, calculated with the formula shown below, or 0.767 W if the Transport robot would travel at 10 km/h. This means the net energy gain of a MMRTG would be 1.753 W/kg if the Transport robot would travel at 10 km/h.

Safety of a Radioisotope Thermoelectric generator

There is a risk of radioactive contamination with the use of RTGs. Any fuel leaks could result in the radioactive material contaminating the environment. To reduce the risk of this, the fuel is stored in individual modular units with their own heat shielding in current space-exploration RTGs. In NASA’s RTGs each marshmallow-sized pellet of plutonium fuel used in an RTG is encased in iridium, this iridium cladding is designed to deform and contain the nuclear fuel incase of an impact. A pair of these iridium-clad fuel pellets are encased inside a cylindrical casing l made of a hard carbon-carbon fiber, this cylinder is then covered in an insulating sleeve made of graphite. Two of these carbon-carbon cylinders are then enclosed inside a casing of even more carbon-carbon fiber to form the basic ‘building block’ of an RTG. A typical RTG contains two to eight of these ‘building blocks’. [99] The main isotope used in RTG is plutonium-238. The alpha radiation emitted by the isotope will not penetrate the skin, but it can irradiate internal organs if the plutonium is inhaled or ingested. In the past there have been several accidents involving RTGs: [100]

On 21 April 1964 a the U.S. navigation satellite, the Transit-5BN-3, failed to achieve orbit and burned up on reentry somewhere north of Madagascar. The 630 TBq plutonium metal fuel in its RTG was injected into the atmosphere over the Southern Hemisphere where it burned up, and traces of plutonium-238 were detected in the area a few months later.

The Nimbus B-1 weather satellite whose launch vehicle was deliberately destroyed shortly after launch on 21 May 1968 because of erratic trajectory. Its RTG containing relatively inert plutonium dioxide was recovered intact from a seabed five months later and no environmental contamination was detected.

In 1969, the first Lunokhod rovers of the USSR were launched with another isotope polonium-210 on board. However, the rocket exploded and radioactivity was spread over a large area of Russia.

In April 1970 the failure of the Apollo 13 mission meant that the lunar module reentered the atmosphere ant burned up. the RTG it was carrying, which survived reentry, and its 3.9 kilograms of plutonium dioxide plunged into the Tonga Trench in the Pacific Ocean, where it will remain radioactive for the next 2,000 years. Subsequent water testing has shown the RTG is not leaking radioactivity into the ocean.

In 1996 Russia launched Mars 96, but it failed to leave Earth orbit, and re-entered the atmosphere a few hours later. The two RTGs onboard carried in total 200 g of plutonium and are assumed to have survived reentry as they were designed to do. They are thought to now lie somewhere 32 km east of Iquique, Chile. [101]

All of these accidents were not caused by the RTG, but the RTG made the environmental impact way more severe. The only possibility for such an accident with the Martian transport robot would be when transporting the radioisotope fuel from Earth to Mars.

Hydrogen Energy

Hydrogen energy is a rather new and experimental energy source and has many different applications like energy production, storage, and distribution; electricity, heat, and cooling for buildings and households; the industry; transportation; and the fabrication of feedstock. [102]. There are many benefits to using hydrogen as a fuel, namely it has a lower mass density than fossil fuels, has 3 times as much energy than fossil fuels per kg and hydrogen does not produce harmful emissions. However, hydrogen takes up a lot of space, cannot be found in nature, is difficult to store and is highly flammable. [103] Hydrogen energy needs a fuel cell in order to work. A fuel cell uses chemical energy of a fuel (often hydrogen) and an oxidizing agent to create energy. Installation will depend on the size available for the fuel cell. A fuel cell ignites the fuel with agent which creates a vapor of water and a lot of energy. Figure of a fuel cell: [104]

Assumptions

We will assume that there is a maximum of m^3 that can be made available for the tank and there is enough space for the rest of the fuel cell. The amount of available space for the tank will be set to 2 m^3.

We will assume that the fuel used will only be available at the colony with a surplus of water. This means that the robot will have to travel twice as far. This would also mean that the robot is restricted to always have to return to the starting colony, but for our scenario this does not matter.

Obtaining the fuel

The fuel cell needs a chemical energy in order to work. The most commonly used is hydrogen with the molecular formula H2. The problem with hydrogen is that it cannot be found in nature, or Mars in this case, but almost always found as part of another compound. However, it is possible to separate these substances such that hydrogen can be extracted. The most obvious compound is water (H2O). Our scenario already provides the water needed, since we know that the colony where the robot starts has an abundance of water. There are many different methods hydrogen can be extracted from water (H2O), splitting it into hydrogen and oxide: [105] Natural Gas Reforming/Gasification: Synthesis gas, a mixture of hydrogen, carbon monoxide, and a small amount of carbon dioxide, is created by reacting natural gas with high-temperature steam. The carbon monoxide is reacted with water to produce additional hydrogen. This method is the cheapest, most efficient, and most common. Natural gas reforming using steam accounts for the majority of hydrogen produced in the United States annually. Electrolysis: An electric current splits water into hydrogen and oxygen. High-Temperature Water Splitting: High temperatures generated by solar concentrators or nuclear reactors drive chemical reactions that split water to produce hydrogen. Photobiological Water Splitting: Microbes consume water in the presence of sunlight, producing hydrogen as a byproduct. Photoelectrochemical Water Splitting: Photoelectrochemical systems produce hydrogen from water using special semiconductors and energy from sunlight.

There are also some alternative methods of obtaining hydrogen that do not use water: Renewable Liquid Reforming: Renewable liquid fuels, such as ethanol, are reacted with high-temperature steam to produce hydrogen near the point of end use. Fermentation: Biomass is converted into sugar-rich feedstocks that can be fermented to produce hydrogen. Ammonia: Liquid ammonia contains more hydrogen by volume than compressed hydrogen or liquid hydrogen. It is not a greenhouse gas and has a high hydrogen density, which leads to NH3 being a favorable alternative to hydrogen.

Comparing methods

Since Mars, and our scenario of Mars, have different properties then the processes on Earth, a comparison of the methods is needed to determine what method is best for our situation. The alternative methods and the natural gas reforming/gasification all need some resources as fuel that are not found on Mars and are therefore unfit to use as method. There could be a material among these that could be found on mars on further research or created on Mars, but will for now not be considered a valid method. The prime reason therefore is that these resources would have to be shipped to Mars each time. The remaining methods are:

Electrolysis:

Pros: Only water and an electrolysis device is needed.

Cons: Needs to use energy from the colonies.

High-Temperature Water Splitting:

Pros: Only solar concentrators and water is needed.

Cons: Nuclear reactor most likely not feasible.

Photobiological Water Splitting:

Pros: Only needs water, green algae, sunlight.

Cons: Needs green algae.

Photoelectrochemical Water Splitting:

Pros: Needs water and sunlight as resources

Cons: A special semiconductors is needed.

Each of these are valid options for generating hydrogen on Mars. We think electrolysis is the best choice since the energy needed can be produced at the colony at easy using solar panels.

Comparison of Fuel Cell Technologies

All fuel cells operate using the same methods. The difference between each fuel cell is determined by what kind of electrolyte is used. The electrolyte is responsible the kind of chemical reactions that take place in the fuel cell, the temperature range of operation, and other factors that determine its most suitable applications.

Figure of different fuel cell types: [106]

Since our robot most likely does not need more than 250 kW, the best choice for the electrolyte is PEM. Also because it has a lower operating temperature then the others, it is more likely to perform near its upper efficiency. PEM fuel cells have an output range from 50 to 250 kW.