Drawing of the space habitat

It appears that a number of people are misunderstanding my description of the space habitat (on the order of the "invasion fleet being swallowed by a small dog" scale).

So here's a picture, approximately to scale:

(Edited to correct drawing the moon orbit too small.)

The large circle is the earth's orbit. The circle in the middle is the Sun. The little circle on the left, that's the Moon's orbit around Earth. (Earth itself is a teensy dot in the middle of that circle, about as big as the line marking the orbit of the Earth around the Sun. I drew the Earth in, but you probably can't see it in the small image). The relatively large circle on the right, that's the space habitat sitting at a Lagrange point (or rather, sitting with a Lagrange point at its centre). It is so phenomenally big it might not actually be stable here (I have not done the calculations to check).

(By comparison, the original Ringworld would be represented by the really big circle. Much larger.)

Now the habitat goes around the Sun in the same orbit as the Earth, offset by 60 degrees. It rotates about its own central axis once every 24 hours, giving it a day-night cycle. The ring would be tilted a little, so it doesn't shade itself most of the time - in fact the ring will precess as it goes. The artificial gravity induced by the rotation would be about 1g (it would, I think, vary somewhat between night and day, because at midnight you're orbiting a fair bit nearer to the Sun than you "should" for the center of mass of the habitat, and midday you're a bit further from the Sun).

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.]