Land, Liberty and Latency

Some months ago I entered the Mars Society City State Design Competition, to design a city on Mars for 1 million people. There were many entries and mine was not among those chosen to be presented at the Mars Society conference in October. Therefore, I thought I would share it here.

As I had chosen Hellas Planitia for the site of the city, I decided to call it ‘Athens’. This giant crater has the advantage of low elevation, and thus a thicker atmosphere than much of the rest of the planet. This makes landing easier and provides a little additional radiation shielding,

My main focus in the design was radiation shielding; on the one hand, there is a great deal of hysteria in the popular science press about how dangerous cosmic radiation is for astronauts (to the extend some believe it would stop them going, which it would not) and on the other hand there is sometimes too much complacency in the space settlement community on the issue when dealing with a general population. The doses that children and pregnant women can be exposed to are far lower than those for radiation workers or astronauts.

Generally the solution to the radiation issue would be to build underground; either finding natural cavities such as lava tubes, digging out artificial ones yourself, or piling up dirt on top of surface habitats. Most cosmic ray shielding on Earth is done by the atmosphere – little of it is screened out by the planets magnetic field. At zenith (straight upwards) there is about 1000 grams of atmosphere per square centimetre to protect you. Replicating this with Martian dirt would require you to be covered in a 6m thick layer. It may be possible to use less rock if we discover that we do not need the full protection of Earth’s atmosphere to stay healthy, but in my design I was conservative about this.

The idea of underground living is seized upon by critics looking for a quick way to shut down the idea of Mars settlement; its viewed as a bleak and depressing fate waiting for anybody who disregards their arguments and goes to Mars anyway. Its one of these arguments, like the radiation issue, that critics tend to throw on the table as a showstopper then sit back with a satisfied grin, not considering that the problem they have just presented might not just be solvable, but already have several credible solutions worked out. My entry to the city state competition presented one such solution to the issue of radiation shielding without having to live the rest of your life in a dark unpleasant burrow.

The trick is that, although the zenith shielding in Hellas Planitia is around 22g/cm² compared to the 1000g/cm² for Earth, radiation approaching from shallow angles has to pass through more atmosphere. In the case of a particle coming in from the horizon, there is over 400g/cm² of shielding which is about what you get in an aircraft at cruising altitude on Earth. So, it is possible to build habitats that have thick roofs, but are to an extent open at the sides to permit natural light in.

Given our everyday experience of buildings, most people would envision a massive, 6 metre thick roof requiring extensive support. But this is because on Earth, we live in structures that have compressive loads. On Mars, because habitats are pressurised, tension dominates. Think back to the 1000g/cm² of air above your head on Earth. What supports it against gravity? Pressure of course. That same level of pressure could by definition support the same amount of shielding made of Martian regolith – because weight is weight, regardless of the substance. The atmosphere inside a Martian habitat is enough on its own to support a thick enough radiation shield – the external pressure is as close to zero as makes little difference in this calculation – and in fact due to lower Martian gravity has a substantial redundancy. This leads to the somewhat counter-intuitive design above. Obviously this would collapse during a loss of pressure; but that is something you want to avoid anyway and I discuss in the paper how to mitigate the problem.

The end result of this design is a domed habitat that has large windows which are much better at admitting natural light than cosmic radiation.

I explore a number of other issues in my submission; I am interested in how we remake information technology several light minutes from our existing infrastructure, as I am concerned we have currently reached a local maximum in the utility of our IT. I also look into the financial aspect of the city. I did not expect to win this competition – given the number of different aspects that needed to be covered it felt that it needed a team of people to produce a competitive entry. The assessment criteria also required I delve into areas in which I’m not formally trained. The main issues in my eyes are

• I think I am underestimating agricultural land usage. I’m a physicist not a farmer or an agronomist and it probably shows.
• The business model is pretty speculative, to say the least. I wouldn’t put my money in it as its described here. This bit needs more work, but also bear in mind this is a model that would be put in place after several less ambitious models for smaller settlements had already succeeded.
• The radiation calculations were limited by the fact I don’t have access to the standard codes that are used. I was forced to use other peoples results, which were mostly sufficient; however I couldn’t take into account neutron albedo. Cosmic rays striking the surface cause it to emit neutrons meaning there is an indirect component of the radiation. I suspect this can be controlled to some extent with choice of materials but I can’t prove it without the radiation codes being open.

There was no feedback on individual entries to the competition; from my perspective a disadvantage of doing this rather than submitting papers. I would therefore very much like to hear comments from any readers who have a criticism or something they would like to add. Download my submission below, and email me on peterhague@protonmail.com