# Can you turn in space without propulsion?

Say an astronaut is floating perfectly still inside their spaceship. Can they rotate themselves by stretching out their arms and twisting one way, then pulling in their arms to their sides and untwisting, and repeating? A bit like how a cat turns in the air when falling?

And if so, I suppose you could you build a spaceship that rotates by shifting and rotating some internal mass around?

• 2,3 and 4 are propulsive. I missed 1 when searching. 2 is disappointing, but inconclusive since there was underwear in the way. Mar 27 '18 at 15:31
• Ya these are not duplicates or anything like that. Just people flailing in free fall in various ways and different degrees of discomfort and/or embarrassment.
– uhoh
Mar 27 '18 at 16:23
• Gyroscope? link Mar 28 '18 at 7:58
• Also see Is there a way for an astronaut to rotate? at Physics.SE. Mar 28 '18 at 15:37

Yes, this is normally achieved using Reaction Wheels, they work by using conservation of angular momentum. This is also how cats right themselves mid-air while falling (Here is a great video explaining the basic physics). You could do the same thing in space by pinwheeling your arms to get yourself rotating and then stopping when you have turned sufficiently.

Here is a demonstration on the ISS (from this video):

• Consider linking to Slow Motion Flipping Cat Physics | Smarter Every Day 58, there's a great explanation for how cats do this, some nice zero-g simulation footage as well, and it stars GiGi the stunt cat.
– uhoh
Mar 27 '18 at 14:18
• See also these astronauts on ISS Mar 27 '18 at 14:34
• @MattChambers yes, you should watch the cat video to find out why Mar 27 '18 at 20:49
• The conservation of momentum is also why ballet dancers/ice-skaters speed up when they bring in the arms, the mass gets centralised and so in order to keep constant the product of the mass and it's angular speed the speed must increase. You can try the astronauts turning method on a free-rotating seat ("office chair") a favourite game of my kids when we're on rotating bar-stools or such. We tend to use the method push hands away from body, twist at waist, pull hands in, move hands to start position, repeat.
– pbhj
Mar 27 '18 at 21:22
• @pbhj I am definitely familiar with the rotating seat trick :) But I was concerned maybe there was some friction involved there too. Mar 28 '18 at 8:21

Just to add to Mark Omo's answer, there's a difference between reaction wheels and what cats (and astronauts) do. When you use a reaction wheel to change orientation, you accept that some part of the object being reoriented will end up rotated the opposite direction by a certain amount. For a reaction wheel, that will be many full rotations in the opposite direction and the wheels are rotationally symmetric so it doesn't matter, but abstractly rotation in one direction of the bulk of the object has been balanced by a rotation in the other direction of another part of the object.

But the little dance the astronauts did relied on changing their moments of inertia as a way of getting their entire bodies in the same configuration but at a different orientation. You can think of their hands as, collectively, one "expandable reaction wheel". Extended, they make a strong reaction wheel that counter-rotates the rest of the body significantly. Pulled in, they make a weaker reaction wheel, that can be brought back to the same position with a smaller effect on the rest of the body. No friction needed to change orientation. Some of the comments pointed this out (like pbhj), but I wanted to emphasize the difference from regular reaction wheels and the fact that friction wasn't necessary at all.

• This is exactly what I wanted to confirm, thanks. Apr 1 '18 at 11:08

Reaction wheels are the answer. Reaction wheel act as angular momentum reservoir. Where

Angular_momentum_of_wheels + Angular momentum_of_rest_of_the_spacecraft = constant

Now, by absorbing some momentum in the wheels you can get angular velocity in the spacecraft.

Also, one uses mangneto torquers, they arent used for turning nominally, but are used for dumping extra momentum gained by spacecraft due to external torques such as solar radiation pressure and atmospheric drag. Sometimes in case of redundant as well one main wheel failure the torquers are use in conjuction with wheels to turn. Though this method does not work for celestial bodies have no or weak magnetic field.

• There are no reaction wheels or magnet torquers within human bodies. The question was about the possibility of spacecrafts using the same methods to turn as astronauts do.
– Uwe
Mar 29 '18 at 15:42
• Reaction wheels and magnetotorquing are not the answer, since the question specifically asks about waving arms. Reaction wheels are ONE solution, but the question asks for confirmation of a different solution, as aptly demonstrated by the wobbly astronauts video. Apr 1 '18 at 11:12

Nobody really provided a bunch of extra links to stuff, so here's some additional information and Wikipedia links on technology actually implemented in shuttles:

https://en.wikipedia.org/wiki/Reaction_wheel

A reaction wheel (RW) is a type of flywheel used primarily by spacecraft for attitude control without using fuel for rockets or other reaction devices. They are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star. They may also reduce the mass fraction needed for fuel. This is accomplished by equipping the spacecraft with an electric motor attached to a flywheel which, when its rotation speed is changed, causes the spacecraft to begin to counter-rotate proportionately through conservation of angular momentum. Reaction wheels can rotate a spacecraft only around its center of mass (see torque); they are not capable of moving the spacecraft from one place to another (see translational force). Reaction wheels work around a nominal zero rotation speed. However, external torques on the spacecraft may require a gradual buildup of reaction wheel rotation speed to maintain the spacecraft in a fixed orientation.

https://en.wikipedia.org/wiki/Control_moment_gyroscope

CMGs differ from reaction wheels. The latter apply torque simply by changing rotor spin speed, but the former tilt the rotor's spin axis without necessarily changing its spin speed. CMGs are also far more power efficient. For a few hundred watts and about 100 kg of mass, large CMGs have produced thousands of newton meters of torque. A reaction wheel of similar capability would require megawatts of power.

This, combined with monopropellant and other such types of limited propulsion come together to create a full RCS.

https://en.wikipedia.org/wiki/Reaction_control_system

A reaction control system (RCS) is a spacecraft system that uses thrusters to provide attitude control, and sometimes translation. Use of diverted engine thrust to provide stable attitude control of a short-or-vertical takeoff and landing aircraft, below conventional winged flight speeds, such as the Harrier "jump jet", may also be referred to as a reaction control system. An RCS is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow control of rotation (roll, pitch, and yaw). RCS systems often use combinations of large and small (vernier) thrusters, to allow different levels of response. Spacecraft reaction control systems are used: for attitude control during re-entry; for stationkeeping in orbit; for close maneuvering during docking procedures; for control of orientation, or 'pointing the nose' of the craft; as a backup means of deorbiting; as ullage motors to prime the fuel system for a main engine burn. Because spacecraft only contain a finite amount of fuel and there is little chance to refill them, some alternative reaction control systems have been developed so that fuel can be conserved. For stationkeeping, some spacecraft (particularly those in geosynchronous orbit) use high-specific-impulse engines such as arcjets, ion thrusters, or Hall effect thrusters. To control orientation, a few spacecraft, including the ISS, use momentum wheels which spin to control rotational rates on the vehicle.

Also, for those who play KSP and wonder how realistic those wheels are:

The other big difference is that KSP's reaction wheels are absurdly effective compared to their real-world counterparts. Real-world reaction wheels generally can't send a craft from zero to "vomit comet" spin speed in 5 seconds flat the way even a single reaction wheel in KSP can.

Even more resources for turning in space:

https://en.wikipedia.org/wiki/Arcjet_rocket

https://en.wikipedia.org/wiki/Ion_thruster

https://en.wikipedia.org/wiki/Hall-effect_thruster

• Thanks for the links, and I'm sure its useful to someone, but this question is specifically about waving your arms. Apr 1 '18 at 11:07