# Could a habitat ring be spun-up and stabilized by electromagnets?

Using the following dimensions (and assuming any material characteristics necessary to maintain structural integrity), could a Standford Torus-style artificial habitat achieve the necessary spin to produce its centrifugal "gravity" and maintain a stable position relative to a fixed containment structure entirely using electromagnetism?

Mass: 10 million tons
Diameter: 1,790 metres
Habitable tube diameter: 130 metres
Rotation: 1 per minute

The containment structure is many times more massive; essentially as massive as needed.

The habitable tube is the surface any electromagnets would be installed along. Assuming a hub-and-spoke construction, figure only the outer half of the tube is available for this purpose.

• Yes, it can. Why to ask? Commented Jun 19, 2017 at 2:39
• Yes, electric motor can exist. What's your actual problem? Commented Jun 19, 2017 at 6:34
• If you can stand the "voice over" this video explains how we rotate things with electromagnetism. The only problem I can see may be the high magnetic field and whether a person can stand the magnetic field they're living in. A faraday cage wouldn't work to protect the inhabitants since the field would appear static to an observer rotating at a constant rate. I don't know the strength of the magnetic field required though. Commented Jun 19, 2017 at 7:19
• @Lio once it's spinning in near vacuum you need really weak fields to make it keep going. Nothing to stop it, right? With good control systems fields would be hard to notice, I guess. Commented Jun 19, 2017 at 7:55
• @Mołot Do not underestimate weak force fields versus non-existent. Best way is to shield it. Commented Jun 19, 2017 at 12:15

Yes, with major caveats.

If you look at the video Lio provided (in comments), you will see the basic principle of turning on and off electromagnets to cause a temporary pull toward another magnet.

However, the only reason the motor works as we expect is because the outer casing is mounted to something much more massive than itself; otherwise the torque of the magnetic attraction would move both the outer magnet and the inner magnet; in fact if the masses of the outer casing and inner rotor were precisely equal it may not spin at all; it might just jump back and forth between two positions. But because the outer casing is more rigid (resistant to being moved or rotated) and the rotor is free to rotate, enough impetus goes to the rotor to push it to the next stable position, and then momentum ensures it keeps going after that.

In space, the outer "fixed" part has no anchor; so both the fixed part and the spinner part are actually going to spin. Without any other frame of reference (like the sun, a planet, stars, etc), a person sitting on one will only see the other spinning. e.g. On the spinner, an observer would feel centrifugal acceleration (gravity), but would see the outer ring spinning above them. While on the outer ring, they would also feel some centrifugal acceleration, but would feel static and see the inner ring spinning below them.

To an outside observer (one whose position in space is not changing and is not a function of the two parts in question), both the inner and outer parts are rotating in opposite directions, with speeds relative to their respective masses (i.e. if the inner spinner is half the mass of the outer ring, the spinner is rotating twice as fast as the outer ring).

If there is a reference field the parties can trust is not rotating around them, like a visible sun, planet or field of stars, then both parties can deduce that they are spinning.

You can observe a form of this equal and opposite force phenomenon in a light table-top fan: when it is on the table becomes part of its anchor; and vibrates as the fan rotates; due to force applied to the casing which transfers to a force that slightly moves the tabletop.

So the caveat is; if you make it just like an induction motor, both parts will spin.

I have not engineered motors or rotors and don't know the latest tech, but I am not aware of any design that allows the outer ring to remain stationary without some kind of braking power. My wild ass guess is that any purely magnetic force must act on the spinner, and slow it down; due to the "equal and opposite" law, basically this is conservation of energy, or a no-free-lunch law.

So yes to spinning, and stabilization, relative to a fixed component, but the fixed component will not exactly be fixed to an outside observer, it will also be rotating, in the opposite direction. From the POV of construction, the centrifugal forces on the outer ring will cause stresses that must be dealt with in its design.

• Conservation of angular momentum is what you are explaining here. Commented Jun 21, 2017 at 20:36

Spacecraft have reaction wheels used for correcting spin until it accumulates too much, at which point they use rockets to apply a torque.

You can calculate the rotational moment of inertia by using a hoop shape as an approximation. Now you can find out how much angular momentum your hab will have at speed. Now, where will you put the equal and opposite amount needed to concerve angular momentum? You can throw it away by ejecting mass — e.g. a rocket. But using electric motors alone you must store it somewhere.

This could be two hoops spinning in opposite directions. It could be a counterweight on a long arm. It could be a very very fast small reaction wheel.

In principle, that is possible.

Can you use electromagnetism to dump the angular momentum into an outside object such as a planet or star? Not in the implied context. You would need enormously strong fields in your vicinity.