Space Habitats

Resources

I am assuming that resources will be sourced as far as possible from nearby locations in the solar system for three reasons:

This section is based on current knowledge of extra-terrestrial resources and geology, which is still quite limited. The odds are that more resources will be discovered than are currently known.

Moon,Mars-or Asteroids?

The obvious source of most resources is the Moon, with Mars as second choice - but there is an economic problem. With its lower gravity, the escape velocity 0f 2.4 kms/s is only one fifth that of Earth. A linear motor launch ramp would only need to be 100kms long to reach escape velocity at 3(Earth)g, could be even shorter for transport of materials only. Indeed Gerard O’Neill suggested a prototype version of this to transport lunar regolith (the powdery surface covering of the Moon) as soil which he called a “mass driver”. In the airless atmosphere no expensive vacuum tube is required, as with Startram on Earth.

It would be easy to get launch costs down to only 10% of Startram on Earth. Assume that they can be got down to only 5% of Startram costs, that is $2.2/kg. Fine for people and high value goods, but at $2200/ton that is still way too much for the transport of bulk goods; by way of comparison a bulk carrier ship can transport stuff around the world for $100/ton or less. If this seems surprising, note that the Moon’s escape velocity is still five times the speed of sound on Earth (Mach 5). Being further away and with a higher escape velocity, Mars is even worse.

Is there a potentially much lower cost alternative? Yes- asteroid mining. There are 20,000 known near Earth asteroids, which cross the Earth’s orbit or come close - and that is only the one’s we can detect. Given that asteroid 100m long will contain a million tons or more of material, there are billions of tons of resources which can be mined. The key thing is that the amount of energy needed to deflect and adjust an approaching asteroid (or its products from mining) to the orbit and speed of space habitat should be quite small, so costs can be kept to the hundreds of dollars, not thousands.

These are of three types:

Essentially asteroid mining can supply all the metals needed, although some processes would be different to those on Earth e.g. aluminium from silicates not aluminium oxide. The iron in M type asteroids is mixed with nickel and cobalt; while they can be separated, used as is would make a strong and corrosion resistant alloy steel. Also carbon, phosphorus, silicon, and some hydrocarbons

The economics of asteroid mining are potentially boosted by the prospect of good grades in S and especially M type asteroids of precious and platinum group metals, and rare earths. On Earth much of these are inaccessibly locked in the Earth’s core.

Air

O’Neill suggested that oxygen could be liberated from the oxides in the lunar regolith, and that the habitat atmosphere could be pure oxygen. No - while OK in a medical emergency, breathing pure oxygen is bad for health over a protracted period, and would be a huge fire risk. In any case, you need nitrogen (78% of Earth’s atmosphere) to feed nitrogen-fixing bacteria in soil, essential for plant growth, and to produce artificial fertilisers.

Here is one area where the resource could be collected from Earth, cheaply and ready mixed in the right proportion. The point is that a collector ship from space could skim through the atmosphere, collect the air, and come right out into space again without incurring the large energy penalty of taking off from the Earth’s surface. But surely we should not deplete an essential part of the Earth’s resources? Even if 10,000 large habitats were built (enough to house the whole of the world’s existing population) the air extracted would account for 0.05% of the atmosphere, with no discernible effect on atmospheric pressure.

In any case, as we shall see, the cost of air is a minor part of the cost of building a habitat. If we wish to insist on not taking any resources at all from Earth, oxygen could be extracted from lunar oxides, but an easier alternative may be the huge amount of carbon dioxide present in the dense Venusian atmosphere. Venus may have gravity close to Earth, but all a collector vessel has to do is skim through the atmosphere, collect the carbon dioxide (CO2) rich air (efficient storage would then freeze it, as dry ice) and skim out again – little extra energy (ΔV) would need to be expended. A powerful laser beam would separate the oxygen from the carbon. The most problematic essential element is nitrogen. As with water, nitrogen is plentiful on Earth and in the outer solar system, not on the Moon, and amounts on Mars (as either gas or nitrates) do not seem plentiful, according to current knowledge. The best source seems to be Venus – only 3% of the atmosphere, but such is the high pressure of the atmosphere, that is more nitrogen than in the Earth’s atmosphere.

It should be noted that the air needed is almost all for the initial load for the habitat - with enough plant life inside it, normal photosynthesis should maintain all or nearly all the oxygen content of the atmosphere.

Carbon dioxide levels could be higher than on Earth, say 1000 parts per million compared to the current level of 400 ppm on Earth; on the latter higher CO2 is a bad idea, leading to global warming, but with the temperature artificially controlled on a habitat, higher CO2 levels would boost plant growth but still not affect the breathability of the air.

Soil The main thing is you don’t need much of it, at least to grow crops. The sensible way on a habitat is hydroponics, water trickling at just the right amount through troughs with suitable nutrients. Productivity is very high, but on Earth it’s expensive. In space, it’s the reverse, as the infrastructure can be built in when constructing the habitat. Some crops e.g. root crops need supporting soil, and for parks and woodland; it will be the by product of asteroid mining, especially from S type asteroids.

Water Water is the big one. Humans live in a totally water dependent environment. In a reasonably humid environment, 25% or more of the weight of topsoil is water, plus lakes, streams, drinking and washing, and a suggested water jacket around a habitat as part of radiation protection. Water would be efficiently recycled, but there is a large initial load required. There is plenty of water in the Solar System, but most of it is either on Earth or in the outer regions, Asteroid Belt or beyond, requiring long and costly journeys. Several of the moons of Jupiter and Saturn have sub surface oceans which together contain more water than in the Earth’s oceans - but they may also for that reason contain life. There is some on the Moon, but current evidence suggests not much and that would be needed for any Moon bases and mining camps.

If the water is taken from the Earth’s oceans the impact on total volumes would be even lower in percentage terms than for air (taken from melting ice caps, would not even need to be desalinated). Unfortunately a collector ship would need to go to sea level, and slowly; so the heavy energy penalty of escaping the Earth’s gravity could not be avoided, unlike collecting air.

The main source for early settlements would be C type near Earth asteroids, although it is in the form of hydrated minerals and would need to be processed. In the longer term, there may not be enough for the large needs of new habitats.

The other potential source is the planetoid Ceres, in the asteroid belt. Just below the surface is a deep layer of water ice, indeed Ceres may contain more fresh water than Earth. But it is a long way away, around 400 million kms from Earth, compared to 140 million kms for Mars, although in both cases it varies a lot depending on the relative position of orbits around the Sun. A round trip by a (probably unmanned) tanker to collect water could take a year. So surely the cost is too high, as would be bringing water from Mars? Not necessarily, because although Martian gravity is only 40% of Earth that still means an escape velocity of 5.0 km/sec (Earth 11.2, Moon 2.4) with attendant energy costs, while from little Ceres it is a mere 0.5 km/sec. The choice will depend on technology and logistics which cannot be determined fully at this stage, suffice it to say that there is plenty of water out there. ceres.jpg

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Small scale ecosystems