Space Habitats

Gerard O’Neill’s Big Idea

In the mid 70s  academic physicist Gerard O’Neill asked the question: how big could you build free standing structures in space, which you could rotate to provide artifical gravity (through centrifugal force)? The answer, it seems, is surprisingly large; using steel, up to 6kms radius. Hence came the idea of O’Neill style habitats. These can be of several shapes:

O'Neill Cylinder


The maximum rate of rotation which is comfortable is around 2 per minute, which gives a centrifugal force of 1g at a radius of 225 metres.

The idea was to fill the interior with lunar regolith (the powdery surface covering on the moon) , acting as soil, some 2 metres depth would allow agriculture and even trees. Can plants grow in this? It seems that they can, although for decent growth humus (manure) and nitrogen fixing bacteria would need to be added and, of course, water. Mars regolith is even better.


Power would be from solar panels, and with the sun shining 24/7, it should be cheap. Experience with satellites shows that abrasion from micrometeorites is containable, with a panel life of around 20 years. Early designs for habitats suggested transparent glass strips to let in sunlight, or by mirrors though end caps, but these would soon be made opaque through micrometeorite abrasion and weaken radiation protection. A simpler and better way is to pipe in sunlight through optic fibres, also makes it easy to set up an Earth type day with alternating light and dark periods.

Such habitats could orbit anywhere, for convenience near the earth or moon. A particular good area are near Lagrange points, which orbit the sun keeping a fixed distance from Earth and Moon. There are five of these, but three are in unstable equilibrium, so around points L4 and L5 are potential sites. However, because of that stability, cosmic dust may tend to accumulate there, although the recent Japanese Hiten probe failed to find any increase compared to surrounding space.

Potentially, these would be by far the largest structures built by man, and could accommodate some sizeable populations. For example, a pair of cylinders each 10kms long and with 3km radius would have an internal surface area of 376 Populated at 500 people/ (comparable to some parts of Europe, and south and east Asia, and half the value of Bangladesh) that is 188,000 people, equivalent to a fair size city, but leaving room for farms and wild nature as well as housing.

Would people want to live in a rotating can with no outside view? Some densely populated parts of the world such as N.W.Europe, Japan, N.E USA and southern California, are among the wealthiest and most desirable places in the world. – and most people there live in cities. You could choose whatever climate one wanted (it could differ between cylinders), and agricultural self sufficiency should not be difficult, with multiple crops a year.

Thus O’ Neill type colonies provide a virtually unlimited potential for human settlement in space, subject to some very big ifs:

These issues will be addressed in subsequent posts.

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Moon and Planets: Not Suitable for Settlement

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Protecting Habitats from Cosmic Radiation