## Thursday, September 18, 2008

### Large space habitats

I was thinking about space habitats. It was probably triggered indirectly by the fact that I'm reading* a SF story at present (The Algebraist, Iain M Banks), though there's no direct connection to my resulting train of thought.

*when I'm travelling, I read. There's lots of waiting about and sitting in planes and stuff (13-14 hours across the Pacific, for starters) and I rarely feel well enough in flight to concentrate on actual work. Even if I only spend a fraction of that time reading, I will get through several books on a trip.

It occurred to me that there will be a particular radius of "ring"-type space station where one rotation in 24 hours would produce artificial gravity of about 1g (this will obviously be large, even without doing calculations).

If you put such a habitat at an L4- or L5- point on the earth-sun system, you'll get earth-level radiation from the sun (without the nice van Allen belts to protect you, unfortunately), with earth "days" and earth "gravity".

So how big is it? Well, I did the calculations, and I get a radius of 1.85 million km. This thing is huge - the earth-moon system would fit comfortably inside it.

It's not Ringworld-huge, by a long, long shot - on that scale, it's miniscule. But still, very very large indeed. If it were a thin ring about 100 km wide (it's about 11.5 million km around, so 100 km wide is indeed "thin"), it would have a "land" area roughly eight times that of earth (assuming people live only on the inner surface of the ring). For the present I'm assuming you'd have a series of interconnected domes or similar on the inner surface, which allows you to bring air, water and other resources in stages.

Assuming, that is, that I have done all the calculations correctly.

Many of the problems associated with a ringworld habitat go away - you don't need to worry about the orbit being unstable for example, though I guess if the mass gets large enough there may be some issues with the stability of the Lagrange points. The amount of material required is far, far smaller than for a ringworld - and the relatively more obtainable amounts of material mean much less hyper-engineering is required.

I haven't done any engineering calculations to work out stresses and such, so I don't know whether you could build a lot of the base structure from simple rock, or if you really need to go to strong metals or even unobtainium.

A nice little thought experiment, anyway. I don't recall seeing anything like this in a story. I'm not sure if that's because I don't read around enough or just that nobody has used an idea like this - but I cannot have been the first person to work this out, so I am curious to know if it has been used in a story somewhere.

[Edit: there's some nifty ringworld artwork to be found.]

Blake Stacey said...

You should know better than to dangle such ideas in front of people like me. . . .

Anonymous said...

Why would you want a period of 24 hours? Unless this thing is floating around a large non-rotating light source that only emits light from one side I don't understand what the significance of the 24 period is. Besides 24 as a counting number is so medieval (so it what if it is divisible by 2,3,4,6,8 and 12 we have calculators now!!).

valhar2000 said...

Because humans beings work on a 24 hour cycle of light and dark(among others, of course). If you make the thing rotate once every 24 hours, that is the first step toward emulating the day/night cycle.

By the way, I think you misundertood Efrique. The stattion wouldn't reolve around the sun; it would sit in an Earth-moon Lagrange point (so it always seem to be in the same place, as seen form the Earth), and it would rotate around its own center of mass.

Efrique said...

valhar2000: no, it's at the earth-sun lagrange point, not the earth moon one. It would follow the earth around its orbit (or lead the earth around its orbit). It is bigger than the earth-moon distance (by a long way) - you really can't put it there.

So indeed it does orbit the sun.

Efrique said...

anonymous: The rotation about its own axis will give a day-night cycle. So it would be very comfortable for people from earth - a day-night cycle a day long with gravity of 1g.

valhar2000 said...

Oh, okay.