Future Mobility

How Much Energy is Needed to Send One Million Humans to Mars

While NASA is planning to send the first human explorers to Mars sometime in the 2030s, several non-governmental organizations, including at least one private company (SpaceX), are looking at more aggressive timelines starting in the 2020s. They are also setting a higher bar: rather than sending astronauts for a limited-duration mission, the goal is establishing a permanent human settlement.

My interest is in identifying new technologies that could be disruptive, and sending large numbers of people to Mars definitely qualifies. I have just completed a study entitled Energy and Resource Impacts of an Earth-Mars Human Transport System, which has been submitted for journal publication.

It describes the model I used to simulate the spacecraft and associated infrastructure needed to send one million people to Mars over the next century. In addition to estimating the mass, volume and numbers of spacecraft required, I calculated the energy and resource needs of such a system, in order to ensure that it would not put undue strain on resources both on and off of the Earth.

Mass estimates for sending humans to Mars

The mass and energy requirements overall are actually quite modest compared with what is consumed today globally. Building the fleet of spacecraft would require about 17 million tons of material, equivalent to 10 years of U.S. aluminum production, but spaced over about 100 years. The required propellant, or rocket fuel, needed to power these spacecraft would amount to approximately 100 times this mass.

I assumed rockets would use a combination of hydrogen and oxygen, or methane and oxygen, as propellant, which would mainly be produced on the Moon or Mars. The lower gravity of these locations would allow for large mass savings compared to making everything on Earth.

However, the choice of propellant is driven by available resources. There is very little carbon on the Moon, but it is likely that there is a lot of water ice. We could use some of that water for human needs, but convert most of it into hydrogen and oxygen for rockets, as well as oxygen to breathe. Methane and oxygen would be made from the abundant carbon dioxide and water ice on Mars.

Energy requirements for sending humans to Mars

My report estimated the energy required to manufacture spacecraft and propellant. Cumulative energy requirements would be 55 exajoules, or about four years of U.S. electricity consumption, again, a modest amount when spread over the course of a century. I even calculated the amount of solar photovoltaic (PV) capacity needed to generate this electricity, and found it was equal to 13 times the current global annual installation rate. Most of the solar power would be installed on Mars, not the Earth. So the impacts on Earth’s energy resources are small.

Limited water on the Moon

The study also found that between 18 and 78 percent of the water ice on the Moon might be depleted in providing rocket fuel for large spacecraft. This is a real problem. The water – if it is there at all – has probably been on the Moon for billions of years. If we build a base on the surface and use it to drink and bathe, we can recycle it almost indefinitely, but turning it into rocket fuel removes it permanently to space. It is irrecoverable.

As a consequence, the report warns that alternatives must be identified and explored early in the settlement process, to avoid undue resource depletion. One approach would be to build smaller, more efficient spacecraft. Another would be to find other sources of water, such as asteroids. A third way would be to develop alternatives to rockets, such as building a space elevator.

Space elevators as a propellant-saving alternative

A space elevator would consist of a cable extending from a gravitating body’s surface to more than 20,000 kilometers into space. Small electric “cars” would grip the cable and slowly ascend from the ground into orbit, where they could transfer their cargo to waiting spacecraft.

It sounds crazy, but this can actually be built on the Moon or Mars with existing technology. Building elevators on both bodies could reduce propellant energy needs by 69 percent. Propellant supplied from the Moon would be reduced by 51 percent.

Future challenges: Human settlement on Mars

To sum things up, many of the model’s parameters are not well constrained, so the results have a large amount of variability. There are still many questions, such as the assumed rate of population growth in the Mars settlement, the rate of people returning to Earth, or the masses of the spacecraft.

Once we see detailed spacecraft designs, we can constrain some of these parameters better, but other parameters, such as population growth, will have to wait until a Mars settlement begins to take shape to be better defined.

What do you think about the prospects for sending large numbers of people to Mars or other places to live? How will such developments change transportation and human culture? Share your thoughts in the comment section.

Please note that this article expresses the opinions of the author and does not reflect the views of Move Forward.


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