A range of designs have been proposed for space habitats. Some appear to be mostly artistic concepts, others are much more serious. They include:
(From Wikipedia http://en.wikipedia.org/wiki/Space_habitat)
- Bernal sphere - "Island One", a spherical habitat for about 20,000 people.
- Stanford torus - A larger alternative to "Island One."
- O'Neill cylinder - "Island Three", the largest design.
- Lewis One A cylinder of radius 250m with a non rotating radiation shielding. The shielding protects the micro-gravity industrial space, too. The rotating part is 450 long and has several inner cylinders. Some of them are used for agriculture.
- Kalpana One, revisedA short cylinder with 250 m radius and 325 m length. The radiation shielding is 10 t/m2 and rotates. It has several inner cylinders for agriculture and recreation.
There are other well-known structures from science fiction literature, including
- Rama (a 20x50km rotating cylinder) from Arthur C. Clarke’s novel, Rendezvous With Rama
- Space Station V (from the movie 2001: A Space Odyssey)
- Babylon 5
Of these, the most complete design is “Kalpana One, Revised,” which properly accounts for issues such as shielding and rotational stability. Most designs presume that it is best to provide windows to admit natural sunlight, but there are many reasons to prefer artificial light sources, primarily involving heat, but also the need for shielding. For adequate shielding from radiation and meteors, the outer walls of the habitat must mass about ten tons per square meter. While transparent quartz windows could be built of this thickness, most designs involving natural sunlight use mirrors to deflect sunlight around shields of stone. But the admitted heat is the real problem (discussed below).
In my previous posts, including Our First Colonies In Space, Life in an Asteroid, and Our Homes, the Comets, I assumed that we would tunnel into asteroids and comets, enclose and spin them for gravity if they were small enough, or build spinning structures inside them if they were too large.
But while writing a sequel to my short story Apophis 2029, I realized that the best choice was simply to build one or more space habitats from the raw materials of the asteroids and comets. I came to this conclusion because of considerations for effective use of space, the stresses of spinning large objects for gravity, and (most importantly) thermal dissipation.
People consume energy in their homes, workplaces, and travel. Much more important, food requires a large amount of energy in the form of light for growing crops. After extensive research on plant needs, high-intensity farming, and lighting technologies, I concluded that the minimum light levels needed requires 4 kilowatts of very-high-efficiency LED lights to grow the food for one person (assuming a primarily vegetarian diet – you need more to grow additional crops for livestock). Add to that the per-capita electric consumption in the U.S.A. of about 1.5 kilowatts, add a little more for contingencies, and I realized we need to plan on 6 kilowatts of energy consumption for every human aboard the habitat.
That’s not too bad, especially considering that readily available solar power can easily provide such levels and at a modest cost.
But energy consumption turns into heat, and heat must be radiated away. The bottom line is that we must allot 19 square meters per person of surface area assuming black body radiation at a temperature of 0 degrees C. It does not help to plant little radiators all over the surface, as they interfere with each other. All that matters is the apparent size of the habitat from a distance, and how closely it approaches the ideals of a black body radiator. Of course, we could use active cooling to heat radiators to much higher temperatures while cooling the interior, but I prefer passive techniques so that a failure of the cooling system doesn’t rapidly result in cooking the inhabitants.
There goes my idea that a million people could thrive in a cubic kilometer of comet. There is plenty of room, more than enough materials. Unfortunately, their waste heat would rapidly boil their home away.
Also, solar light has a large content of heat – and that excess, too, must be radiated away. Sunlight is not energy efficient for growing crops in a thermos bottle (which is what a habitat in space effectively is).
So, my revised plan calls for 20 square meters of surface per person. Also, to provide radiation and meteor shielding equivalent to the Earth’s surface requires 10 tons of shielding per square meter of surface – and thus 200 tons of shield mass per person (regolith is fine, slag works well and is dense, ice is best as long as it doesn’t boil away). But the needed surface area and shield mass per person are constants.
My earlier thoughts on structure did not consider rotational stability, and the folks that designed Kalpana One came up with some very strong arguments that a spinning cylinder is best, and that the width of the cylinder should be 1.3 times the radius. Thus, a cylinder of radius 100 meters (spinning at 3 rpm for 1 G gravity along the outer rim) should be 130 meters wide. That gives a 1-G living area of a little over 80,000 square meters, a total surface area of over 144,000 square meters, and thus a maximum population of 7,200. This structure provides 11.25 square meters (121 square feet) per person of 1-G living space. Is that enough?
It’s comparable to the space provided (per person) in many hotel rooms and cruise ships. But few couples want to live in a 242 square foot efficiency for long, although 28 sm (300 sf) studio apartments are common in many expensive cities.
There is no need to live only on the outer 1-G surface. Assuming 3-meter intervals, the next level up provides 97% of a G. Surely that is adequate. And now we have 22.5 square meters per person of available living space, equivalent to 450 square feet per couple – or 900 square feet for a family of 4. A third living level raises the per-person space to over 33 square meters – 675 sf per couple – 1350 sf for a family of four. Not spacious, but certainly comfortable.
Humans need space for living, working, and of course for growing food. We must allot some space for office space, work space, schools. A single level should suffice (11 square meters per person), partly because some people will work in the farms, or in their homes, or outside the habitat entirely (such as in the mines, the smelters, the steel mills, the solar power satellites, etc.).
Each person requires approximately 64 cubic meters for crops, but crops don’t require 3-meter ceilings. Allocating 2 levels for agriculture may be tight, but 3 levels is more than enough and provides some excess capacity for the production of meat, milk, and eggs.
We need a little more space for overhead: storage, aisles, conduits for air, water, sewage. So we add an 8th level for good measure. That still leaves an interior cylinder with a radius of 75 meters as a park or recreation area. It has 3/4ths of a G of gravity. The opposite side is more than 500 feet overhead – it will feel spacious enough, and 15+ acres of playgrounds, hiking paths, trees, and grass will provide a little bit of Earth in space.
But there’s no need to leave the end caps – the walls of our cylinder – as bare metal. We should build offices, low-gravity facilities (perhaps hospitals), hotels, etc. along those walls. Allocating 15 meters of depth along each end-cap for such purposes still leaves a hundred-meter-wide park, now with only 12 acres of usable space, 100 meters wide by 470 meters around. The lowest level of the end caps is a perfect place for shops and restaurants.
The above ramble describes the capacity of a 100-meter radius cylinder, spinning at 3 rpm to provide Earth-normal gravity. This spin rate is often considered the maximum for a rotating space habitat, as most people (but not all) can adjust to it. More people can adjust to 2 rpm, and essentially everyone has no problem with 1 rpm. So how much room do we get with these and larger structures? Can they be built?
This table shows the size, possible population, and mass (in kilotons or kT) of the external steel shell, the internal steel infrastructure, and the shield (total mass of steel shell plus rock). Note that once the steel shell reaches a mass of 10 tons per square meter, additional shielding is not needed. For a reference point, the total mass of steel in a modern aircraft carrier is about 60,000 tons, about 20% less than the smallest habitat. The dimensions given are of the habitable volume; the outer walls are assumed to be an extra 5 meters in thickness to provide the volume needed to contain the shield mass (but that extra external area raises the maximum population as well). The thickness of the outer steel shell is also given, in meters, and it ranges from 3cm (1.2 inches) in the 100 meter cylinder to 1.31 meters (4 feet) in the largest. The table also shows the percentage of the asteroid Apophis needed to build this structure, or alternatively the minimum size of a rocky asteroid large enough to build it. *Note that the largest structure would require a nickel-iron asteroid, as there is no rocky shield mass needed.
|Steel Shell (kT)||38||105||385||2,092||23,258||129,560||3,154,722|
|Steel Structure (kT)||36||71||168||519||2,584||8,117||68,166|
|Total Mass (kT)||1,653||3,273||7,769||24,018||119,664||376,078||3,222,888|
|% Apophis (27 mT)||6.12%||12.12%||28.78%||88.95%||443.20%||1392.88%||11936.62%|
It is clear that Apophis contains enough raw materials to build habitats supporting 125,000 colonists in up to 16 structures. It is interesting that a 1-kilometer nickel-iron asteroid (of which there are approximately 50,000 in the main belt) provides enough iron that (adding the resources of a small carbonaceous chondrite for carbon, oxygen, and water) a 9x6 kilometer cylinder could be built, supporting over 15 million people. Still larger structures may be constructed; steel has adequate tensile strength for structures large enough to support a billion people, but they become wildly inefficient, requiring nearly 10 times the steel per person.
I plan additional posts providing details on farming in space, on solar power satellites, and on the economics of life in space. It is clear that space habitats are feasible, and that commerce based upon tourism and the construction and maintenance of solar power satellites can pay for it. The obstacles are the difficulty of the bootstrap process:
- capturing an asteroid such as Apophis into Earth orbit
- Launching the tools to mine the riches of the asteroid, the tools to smelt its ores into steel and other valuable materials, the tools to shape that steel into the plates, beams, and girders needed to build things
- Launching the people to make it possible with enough consumables to get past the bootstrap.
- Designing and implementing closed-system recycling facilities capable of efficiently converting human wastes (and crop residues) into food, oxygen, and water.
Once enough infrastructure is in place, the colony should not need the addition of oxygen, water, food, or structural materials. High tech tools will be needed, including whatever is needed to construct solar cells, but the raw materials would already be in place. The Earth will export technology, tools, vitamins, pharmaceuticals, and people. In exchange, the Earth will receive bountiful energy from the Sun, with zero carbon footprint.
But that, too, will take time, energy, and especially people. In the long run, the demand for people in orbit is likely to exceed our capabilities of putting them there. And that, too, is the subject of a future post.