Recipe for a Space Habitat

To build permanent habitats for people to live in space will require several needs to be addressed:

• Living space providing adequate room plus radiation and meteor protection
• Gravity or its equivalent
• Oxygen to breathe
• Water to drink
• Food to eat
• Something profitable to justify life in space

Luckily, some of these are easily addressed, as certain asteroids have all the resources we need, including the majority of the asteroids in the belt – the carbonaceous chondrites.

STEP 1: Capture an asteroid into a useful orbit. The next great opportunity is the asteroid Apophis 99942, which will be in a location suitable for capture in 2029. See my post, Capturing Apophis for details. The good news is that Apophis contains 50 million tons of resources, including 10 million tons of iron, 15 million tons of oxygen,  12 million tons of magnesium, and perhaps 8 million tons of silicon. The bad news is that we presently believe that Apophis is an LL Chondrite, low in volatiles including water, carbon, and nitrogen, also relatively low in calcium, aluminum, and titanium. Available carbon is likely less than 0.2%, water less than 1%. Still, half of these values equates to 50,000 tons of carbon and a quarter-million tons of water.

How much water and carbon is needed? Studies of high-intensity farming techniques suggest that about a half-ton each of carbon and hydrogen are needed per person to grow crops for food and oxygen recycling. This is equivalent to about 3 tons of carbon dioxide and 5 tons of water, per person, much of which will be contained in the growing plants of our farms. Plants are, after all, carbohydrates. We also need significant amounts of nitrogen (for proteins), and phosphorus as well as various trace elements. But all of these are abundant in ordinary asteroids except carbon, hydrogen, and nitrogen. The most common asteroids, carbonaceous chondrites, contain these volatiles in abundance.

Still, it is clear that even a dry rock like Apophis contains enough raw materials to support as many as 50,000 people.

STEP 2: Establish a temporary beachhead. An empty shuttle tank works great.

An empty Space Shuttle SLWT External Fuel Tank has a hydrogen tank 8.4m by 29.5m (97x27’), and an oxygen tank 16.6m by 8.4m (54x27’); these function nicely as pressurized crew quarters. If the hydrogen tank is converted to recycling (growing plants and recycling wastes), its 1493 cubic meter volume could support about 36 people who reside in the oxygen tank volume (553 m^3), or about 15 cubic meters per person (2x2.5x3m).

You bury the tank for radiation and meteor protection. Five meters of regolith gives about the same protection as Earth sea level; we could get by with three meters on top.

STEP 3: Mine the asteroid and use a solar furnace (or other techniques) to smelt it into useful metals and free oxygen. Also, the first gases emitted when you heat up the regolith are CO2 and H2O. Save them. And note that all the leftover slag is extremely valuable as radiation shielding for the habitat we want to build, so don’t throw it away, either.

To yield enough water and carbon dioxide to grow food for one person, you’d need to process only 50 tons of ore from a carbonaceous chondrite like the Murcheson meteor, but more like 500 tons if Apophis is truly an LL chondrite. You’ll get a little excess carbon which lets you make steel instead of just iron. 100 tons of steel.  As a bi-product, you’ll end up with perhaps 25 tons of oxygen, which you’ll want to save, also. This is a good use for a few more empty shuttle external fuel tanks. We’ll be using oxygen as fuel, I suspect, in VASIMR type thrusters powered by solar energy.

STEP 4: Establish a farm in that empty hydrogen tank (or in several of them as the colony grows). My estimates of needed space are based upon 64 square meters per person of crop area, using high-intensity techniques, and using LED light sources. I also assume hydroponic techniques instead of soil, because it’s easier to recycle the root mass. We won’t use soil (even if it’s free and abundant) until we have a huge surplus of carbon and water to waste.

Note that we need to feed extra CO2 to the growing crops – humans don’t produce enough to grow everything we need, because of the small fraction of plant material that is edible. We’ll even be burning the dried crop residue to create CO2, or turning it into coke (carbon) to improve the efficiency of iron production.

STEP 5: Start building Solar Power Satellites. You’ve got the steel, and all the magnesium you could want, plus more than enough silicon. A square kilometer array of solar panels or collectors intercepts a gigawatt, yielding a net 200 megawatts to Earth. But why stop at a gigawatt? You’re building in outer space where it is simple to build large structures.

A circular array with a 1.6km radius would yield 4 gigawatts of power to be beamed to Earth from geosync orbit. Each generates $1B per year in wholesale electricity (at $.03/kwh). With no energy costs – just maintenance.

The steel & other raw materials for each one consumes about 1% of Apophis’ regolith. By the time you’ve built 50 of them, your revenue is $50 billion a year, and you’ve only consumed half of Apophis.

STEP 6: While you are building Solar Power Satellites in one factory, you can be building a large, permanent, self-sustaining habitat in another. Many designs have been proposed, and I, personally, like a rhombic triacontahedron. It is constructed from 30 identical rhombic steel plates. A simpler design (but more difficult to build) is a cylinder. Both would be spun for gravity.

For radiation and meteor shielding, you need about the same mass of shield as the Earth provides us: 10 tons per square meter. That’s about 5 meters of regolith, or 3 meters of slag (which is nice and dense). I realized that this much mass, spinning at one G along the periphery of a sphere (which is close to a rhombic triacontahedron) exerts an outward force entirely equivalent to a pressure vessel, whose characteristics are well known. To contain a force of 15 tons per square meter (3 tons of which is air pressure), a spherical pressure vessel 100 meters in radius only needs to be about an inch thick, masses about 25,000 tons of steel.

The limiting factor for population is likely to be heat dissipation. Using very high efficiency LED light sources to grow our food, and minimizing all wasted energy interior to the structure, we need about 20 square meters of cooling area per person (assuming passive cooling – the only safe kind). Thus, a 100 meter radius habitat, spinning at 3rpm for 1 G, has sufficient area for a population of about 6,000 people. It takes about 4 tons of steel per person to build the pressure vessel. Since the area per person is constant (20 square meters), and the amount of shielding per unit area is constant (10 tons per square meter), each person needs 200 tons of shielding. Thus the habitat for 6,000 people requires a 25,000 ton steel pressure vessel, plus probably that much again for internal structures, plus 1,200,000 tons of shield mass. This is 2.5% of Apophis – we could build 20 of these with the half left over from building 50 solar power satellites.

Spinning at 3rpm is too fast, you say? If the radius of our vessel is increased to 225 meters (yielding 1 gravity with a 2 rpm spin), our pressure vessel needs to be 5.6 cm (2.25 inches) thick, and requires nearly 300,000 tons of steel. But it now is large enough to support a population of over 30,000 people. And its construction consumes 12.5% of Apophis. Yes, we can still build 4 of these: one in Apophis orbit, one above geosync as the ideal place to maintain those solar power satellites, and I’m sure we can find places to stash the other two. How about L4 and L5?

Sorry, but if you want to spin at 1 rpm (radius of 890 meters), it takes all of 2 Apophis-size asteroids to provide the needed raw materials and would have living space to support about 500,000 people. But it takes more steel per person the larger you build it – there is no economy of scale, as larger and larger pressure vessels take more and more steel per unit area (and thus per person). But as long as we are doing the math, if you raise the radius to 4 kilometers, your steel shell must be a full meter thick, and itself provides all the shield you need.