# Axis of rotation of a wheel space station [duplicate]

Is it possible for the axis of rotation of a rotating wheel space station in orbit to be aligned with the center of the Earth?

I'm wanting a situation where a person on the station could look out one widow and always see Earth, and able to turn 180° and look out into space, without seeing any indication of the Earth. This is important to the symbolism in the story.

Does this violate any aspect of the conservation of angular momentum?

## marked as duplicate by Nosajimiki, Measure of despare., Starfish Prime, JBH, Arkenstein XIIMay 29 at 22:56

• A rotating object tends to keep the rotation axis unchanged. You want to rotate the rotation axis as the object revolves around the Earth. This requires the application of an external force, which means constant consumption of fuel. – AlexP May 29 at 12:56
• Possible duplicate of worldbuilding.stackexchange.com/questions/146537/… – Nosajimiki May 29 at 19:58
• I had to stare at the Stanford torus question, but even though it addresses a somewhat larger issue than this one, it does completely address this question, as do the answers, making this a duplicate. – JBH May 29 at 21:41

With a single ring, you're out of luck (unless you cheat with rockets, though the very existence of your station implies fairly advanced rocketry and easy availability of power and reaction mass).

With two, however, you might be able to get your station to precess so that it will always point the right way. O'Neill worked out configuration whereby you strap two of his cylindrical stations side by side, contra-rotating, and carefully adjust their rates and spacings so you could get their light-collecting mirrors always pointed at the sun (useful also for solar power). There's a brief mention of how it might be done in their wikipedia page. You could mimic this arrangement with a pair of ring stations mounted on a static framework, bicycle-style.

I suspect you might also be able to do this with clever use of a big reaction wheel, though the mathematics of that are beyond me right now so I couldn't tell you how you'd have to arrange the various bits (eg. whether the control ring(s) could be coaxial with the habitat ring) in order to make it work.

Always pointing at earth seems unlikely to be an important, practical feature of a rotating station, so it would imply that the society that built such a thing has plenty of resources to spare for this sort of aesthetic or symbolic arrangement. Edit: as Gilad M pointed out in their answer, keeping your station facing towards the earth reduces your cosmic radiation exposure, which I hadn't spotted at all, though the benefit in high orbits will be limited compared to LEO.

Bear in mind that the view of the earth will rotate... in fact, it will rotate faster than the second hand on an analogue clock for your example stations, which may somewhat spoil the contemplatative or poignant nature of the view!

An alternative to using precession is to put your station in an orbit around the sun instead of the earth. Park up in the $$L_1$$ Lagrangian point, and you're always guaranteed an unobstructed view of the sun (useful for solar power) and the earth and as your rotation is now once a year instead of once a day (for your geosync station) the change in position of the earth is slow enough that your symbolic view will remain for months at a time (or alternatively, the effort you have to expend to remain pointed in the right direction is substantially lowered).

Also you get amazing views like this. (I basically made the edit just because I found this photo!)

The Earth-Moon $$L_1$$ point might be more justifiable, not to mention easier to get to and perhaps more useful, depending on your needs (though it won't have such a nice view). That gives you a ~4-week rotation period, so the view will be largely similar over the course of any given few days.

In short, an object can't stably spin along two axes like you're suggesting, but there's ample reason to stabilize it manually and get the situation you want.

If you have an object in orbit spinning along only one axis, that axis is by definition not rotating. This means that as it completes one orbit, from its perspective, the Earth will circle around it. The view from the windows would show Earth for about half the time, on a roughly 90-minute cycle for low-earth orbit. That's not what you want.

Let's set it rotating along another axis, then, at precisely the right speed to make it always face the Earth. Unfortunately, you'll run into trouble. This second rotation will be unstable as a consequence of the Intermediate Axis Theorem, meaning that every so often, left to its own devices, the station will flip along its third axis, too.

Here's a quick video showing the flip. You probably don't want your space station to do this.

So okay, keeping this space station rotating the way you want will not happen in isolation. But that doesn't mean we can't do it. Put small stabilizing thrusters outside the station, and you should be able to counteract the occasional flipping.

This costs energy, though, so you need a good reason for the space station's designers to add this feature. Fortunately, such a reason already exists: Radiation. You want to shield the people on board from cosmic rays, but encasing the entire station in a shield would be expensive and impractical. So instead, build the shield into only half the station, and rotate it along with the Earth. That way, the shield always points out to space, and the Earth blocks all cosmic rays coming from the other side!

• +1 for the shielding idea. – Starfish Prime May 29 at 19:31

a situation where a person on the station could look out one widow and always see Earth, and able to turn 180° and look out into space, without seeing any indication of the Earth.

It's called tidal locking, and is exactly what goes on with the Moon: a face always sees Earth, the other one never sees it. And the one seeing it, sees it always in the same position in the sky.

But in this case the two axis of rotation are parallel.

Tidal locking (also called gravitational locking or captured rotation) occurs when the long-term interaction between a pair of co-orbiting astronomical bodies drives the rotation rate of at least one of them into the state where there is no more net transfer of angular momentum between this body (e.g. a planet) and its orbit around the second body (e.g. a star); this condition of "no net transfer" must be satisfied over the course of one orbit around the second body

• When this is intentionally done with spacecraft it is called gravity gradient stabilization, and it requires the spacecraft to be elongated in the nadir direction. Just throwing out some search terms in case the asker is interested in this method. – ben May 29 at 15:53
• tidal locking won't work if you are spinning the station for artificial gravity, not unless you have multiple spinning rings attached to each other. . – John May 29 at 16:07
• @John, that's no issue. OP never mention artificial gravity in the question. – L.Dutch May 29 at 16:08
• There is not much point to having a rotating space station other than for gravity. – John May 29 at 20:20
• @John, all of the things we have sent to space, manned or unmanned, have been rotating. None of them was rotating to achieve artificial gravity. Rotation is used for gyroscopic stabilization. – L.Dutch May 30 at 5:19

You can get tidal locking of an orbiting satellite by putting long booms out, or by making the satellite long and narrow. But the only rotation of this satellite is the one synchronized to the orbit period.

If you are thinking of a spinning wheel (AKA 2001) then this will act as a gyroscope, and the axis of rotation won't change as it orbits the earth. (Ok. There will be minor precession effects if the top and bottom of the 'spinning top' aren't symmetrical, but I don't think these are large enough to maintain a constant axis of rotation pointing at the earth.

If you can live with a parabolic orbit, you could:

Put your station above the North Pole of the Earth. Rotate it on the same axis and in the same direction as Earth, too.

Put one window at the "bottom" of your station. It will afford you a view of the Earth. In some sense, you are looking south.

Put the other window at the "top" of your station. You won't see the Earth at all.

It'll all work... for a while.

Or: If this station were in a highly-eccentric polar orbit, it might be possible for one window to display the Earth for quite a while -- as long as a human lifespan, if you wanted. But the windows would swap roles twice an orbit, which may or may not wreck havoc on your symbolism.

Or: If you need a more stable solution, you could put the Earth at the centre of your station, but that might make your space station a bit larger than you were anticipating.

• ...and stopping it from falling to Earth will be...? – Keith Morrison May 29 at 20:03
• ...alright that could be a small issue. – Roger May 29 at 20:04