Recently, Elon Musk threw a Tesla car at Mars and somewhat carelessly, missed. How can you miss a planet? The answer is, not unsurprisingly, quite easily. Mars might be a planet, and planets might seem large, but they are staggeringly small compared with the solar system. But whatever else this achieved, it did draw attention back to thoughts of humans on Mars, and as an exercise, it is not simple to bring the two together. Stephen Hawking was keen on establishing a colony there, mainly as some sort of reserve for humanity in case we did something stupid with out own planet. Would we do that? Unfortunately, the answer is depressingly quite possibly.
So what is required to get to Mars? First, not missing. NASA has shown that it can do this, so in principle this problem is solved. The second requirement is to arrive at the surface at essentially zero vertical velocity, and NASA has not been quite so successful at that, nevertheless, we can assume that landing will be with a piloted shuttle, so this should be able to be done. So far, so good? Well, not quite, because when you get there you have to have enough “stuff” to ensure you can survive. If it is a scientific exploration, the people will be away for over two years, so at a minimum, they will need groceries for two years, unless they grow their own food. They will need their own oxygen and water unless they can recycle it. They will need some means of getting around or there is no point in going, and they will need some sort of habitat. If they are settlers they will need a lot more because they are not coming back.
The obvious first thing to for settlers to do is to have somewhere to live. We can assume that the ship that brought them will provide a temporary place, although if the ship is to be recycled back to earth and they came down in a shuttle, this is a priority. At the same time they must build facilities to grow their own food and make oxygen. This raises the question, how many people could actually grow food and guarantee to do it well enough not to starve in a totally different environment to here? I am not sure you can train for that, but even if you can, there will still need to be a lot of food taken as well as oxygen. However, let’s assume these settlers are really competent and they are raring to get on with it.
The first requirement would be enough area to do it, so they would need a giant glass house (or houses). That means glass, and metal to hold it, but there is worse. You have to pressurize it, because the Martian atmospheric pressure on average is only about ½% of Earth’s. That means you need a strong pump, but because of the aggressive nature of dust in the atmosphere much of the time, you need some form of filter. The air is about 95.3% carbon dioxide, about 2.7% nitrogen and 1.6% argon. If you want to recover the oxygen to breathe, you want to boost the nitrogen so that what is produced is breathable as air, and that requires a major gas separator. The best way is probably to seriously overpressurise it, so the carbon dioxide comes out as a liquid, and keep the rest. However, there is another problem: you need water, so that equipment will probably have to be made even more complicated so the water in the atmosphere can be recovered. The next problem is that if the glasshouse is to be pressurized, it has to be leak-proof. All the joints have to be sealed with something that will not decay under UV radiation, and worse than that, a deep footer is needed around the glasshouse. That means digging a deep trench, pouring concrete, and sealing the walls. Finally, the whole regolith inside the glasshouse has to be treated to decompose its strong oxidizing nature (but this does produce a small amount of oxygen) otherwise the soil will sterilize anything you plant, then you have to add some actual soil. Many of these operations would be best done mechanically, but they each need their own machine.
You may notice that all of these things costs weight, and that is not what is wanted on a space ship. So the question is, how much can be brought there? There is a second requirement. Every time you use a machine, you need fuel. That has to be electric, which means either batteries, which so far would require huge numbers to keep going all day, or fuel cells, but if fuel cells are selected, what will be the fuel? Note that two fuels are required; one to “burn” and the other to burn it in, as there is no oxygen in the atmosphere worth having. Either way, a serious energy producer is required because not only do you have to power things, but you have to keep your glasshouse warm. The night-time temperatures can drop below minus 100 degrees Centigrade. The most obvious source is nuclear, either fission or fusion, but that requires shielding and even more weight.
The above is just some of the issues. I wrote a novel (Red Gold) that involved Martian settlement. The weight of the two ships was twenty million tonne each, and each had a thermonuclear propulsion system that detached and could be used as power plants and mineral separation units later. The idea was that construction materials would be made there, but even if that is done, a huge amount of stuff has to be taken. Think of the cost of lifting forty million tonne of stuff from Earth into orbit alone. Why two ships? Because everything should be done in duplicate, in case something goes wrong. Why that much stuff? Because you want this not to be some horrible exercise in survival.
At this stage I shall insert a small commercial. Red Gold is a story of such colonization, and of fraud, and it includes a lot more about what it might take to colonize Mars. It is available on Kindle Countdown discounts from 13 – 19 April. (http://www.amazon.com/dp/B009U0458Y)