This is the debate raging now in response to the Bush 'vision' for NASA. In this analysis I shall steer away from intangible benefits of space colonies such as expansion of scientific knowledge and psychological renewal of human society, of which much has been made by other authors, not because I consider them unimportant but for two more hard nosed reasons; that both can be achieved by any space colony or even by an increased robotic presence in space; and the telling arguments against human space colonies are usually economic. "How is it going to pay for itself". Gerard O'Neil's vision did not get realised on precisely these grounds. The power they would have produced was not cheaper than what could currently be had, even without his expensive (though breathtakingly beautiful) orbital living environments burdening the project.
If human governments are going to vote the funds required to colonise the sky they will need to see a payback, or at least an affordable sunk cost. Reasons why we cannot afford not to colonise space in the long term (with humans) shall be dealt with separately 1 . It will be a stepwise ladder-climbing exercise with each rung making the next possible, which takes a lot of text to lay out. I shall take as an assumption here that it is agreed the ladder must be climbed 2 but that we are discussing how to climb it. Most agree though that the first to steps are Mars and Luna but in which order?
The purpose of this essay is to address the currently topical issue of whether Luna or Mars (or both) should be considered the next single rung or whether neither can work without the other and they must be tackled together. The cost of both at once is obviously more burdensome on current human tax payers though so it is last choice for that reason.
1) The first thing to consider in choosing our first colony is the difficulty of getting there & back. This of course effects the cost & so the final economic value of the resources which can be had from there. When I say do the maths though, simply comparing the surface gravities of Earth, Moon and Mars is misleading as it is far to blunt a measure of the energy costs involved.
| Body | Surface G (m) | Mass (kg) | Surface Radius (m) | Escape V (km/s) | Escape E (MJ per 10kg) |
| Earth | 1.00 | 5.97 x 10 24 | 6360 | 11.20 | 626.8 |
| Mars | 0.38 | 6.42 x 10 23 | 3390 | 5.02 | 126.2 |
| Moon | 0.17 | 7.35 x 10 22 | 1740 | 2.38 | 28.2 |
It has been mentioned that the goods shipped from Mars would come along a long supply line and could not be changed to cope with different conditions at the receiving end whereas Luna, being closer, could be more industrially agile. This is true as far as it goes but when the goods are commodities like metals, propellants, Oxygen, food and water this is not a problem. If commodities needed short agile supply lines we would never have had world spanning silk roads or spice routes in the age of sail. As for manufacturing agility on the moon, why would we support factories there to produce manufactured goods for Earth? More likely the goods would be made on Earth where labour is cheaper and conditions more benign (& proximity no great problem). Luna may support specialised industry because if its hard vacuum an light gravity but though it could compete with Mars for proximity to the main markets - it could not so compete with Earth itself.
2) The second thing to consider is sustainability. i.e. reducing the enormous cost of sustaining an off world colony using materials shipped up the Terran gravity well. Let us consider first, a Lunar colony. Let us examine what materials are available on the Moon:
Compounds | Lunar Basalt (Apollo 11) | Lunar Breccia (Apollo 14) | Lunar Regolith (Apollo 17) |
SiO 2 | 40.46 | 48.09 | 44.47 |
TiO2 | 10.41 | 1.51 | 2.84 |
Al 2 O 3 | 10.08 | 16.72 | 18.93 |
FeO | 19.22 | 9.53 | 10.29 |
MgO | 7.01 | 10.18 | 9.95 |
CaO | 11.54 | 10.67 | 12.29 |
Na 2 O | 0.38 | 0.73 | 0.43 |
Other | 0.90 | 2.57 | 0.80 |
It seems the Moon, with its low gravity has not only lost any atmosphere it ever has to space along with all its Hydrogen, the Hydrogen was lost before it had the chance to cool down and be fixed into the rocks. Certainly, there is plenty of Oxygen and metals but notice the glaring lack of Carbon and Nitrogen - the building blocks of life. Perhaps if these were present we could ship enough missing trace elements for our colony, such as Phosphorus and Sulphur up the well but even with a small number of inhabitants and efficient recycling H, C & N - they are needed in considerable tonnage. There is little prospect of such quantities being boosted up in rockets, shuttles or any near future technology, even if launch costs were halved or quartered. Support for every single person in a Moon colony is going to cost a fortune if done from Earth. Mars on the other hand has a wide variety of minerals, including the metals available on the Moon but it also has a Carbon Dioxide atmosphere also containing Nitrogen. It almost certainly has water also as a Hydrogen source while Luna water is purely theoretical and the only polar orbit probe sent to look for the Luna water (Luna prospector 1998) turned up only tiny (500ppm) concentrations of Hydrogen which may possibly have been in water form. Thus even the wet parts are so dry, extraction would be very costly and may not be feasible.
We can add to these difficulties the lack of an atmosphere, bathing the surface in hard radiation during the Lunar day (28 Earth days) which also results in massive day/night temperature differences (93-393K). While Mars atmosphere is thin it is sufficient to shield the surface from this radiation and it has a 24 hour day. As a result, Luna colonists would live in deep windowless bunkers well beneath the surface while Martian colonists could inhabit the surface of their world.
In other words, a Martian colony has the potential not only to support itself with minimal supplies from Earth but also may supply the expansion into the Solar system from a gravity well far shallower than Earths. It can support long term human occupation on its surface. Luna cannot do this. It may, by proximity to Earth, become important once there are Martian and/or asteroid colonies to supply traffic. It may be an important but expensively supported scientific and/or military base ...but it is not the first step into space nor even a required part of our first step.
3) Third is the resources that can be extracted from the colony. This factor determines whether the colony will pay its way in the end as well as whether it can contribute to or act as a drag upon our prosecution of other space objectives like the asteroids and gas giant moons etc. As has already been shown, Mars can supply what a Moon colony needs but not vice versa. The cost of supplying either from Earth would be enormous and would not become affordable until launch costs drop by at least two orders of magnitude (about a hundred times).
The top few centimetres of Luna Regolith is impregnated with He3 from the solar wind which may become an important Nuclear Fusion fuel in the future. This element is very rare in the inner system (including Earth and Mars). Thus strip mining lunar soil to supply the energy needs of Earth and other space colonies may be important at some point. The He3 is however present in very tiny concentrations (four parts per billion) so extraction may be a fairly marginal operation. He3 is abundant in the outer system however so the focus of a fusion economy would undoubtedly shift there before long.
Furthermore the materials needed to colonise the Asteroids, as must be done for the safety of the Earth quite apart from extracting the many mineral resources, are the same as for the moon: Organics (C, H, N), Oxygen, Propellants, reaction mass and metals. Mars can supply them and moon can't. Simple as that. Furthermore the ballistic cost of getting any material from Mars is in the same ballpark as it would be from the Moon. Thus we might eventually choose to source some metals from Luna and we may use it as a steady base for telescopes, weapons etc but there is no great economic need to do so & we cannot do just Luna if our eyes are on the rest or the solar system. Whereas not colonising Mars and getting what we need from Luna is just not an option (the stuff isn't there). It is all there on Earth but the shipping costs make Terran resources many of times as expensive. It is however all there on Mars and for competitive energy costs too. QED
- David
1,2 Should anyone disagree with these premises, I will post separate essays to address them.
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