# Using a fidget spinner to rotate in outer space

There is a question How do astronauts turn in space?, and related questions here and on physics.stackexchange, that detail how astronauts might maneuver by spinning their upper and lower body separately. Wouldn't it be easier to spin a fidget spinner, and to preserve angular momentum, spin your body in the opposite direction?

• There is a video somewhere of an ISS crewmember doing sequential 90 degree rolls while free floating, so appears they have sorted out an approach that works. Not finding the source yet. Dec 31 '20 at 12:42
• Does the astronaut want to take ten minutes to rotate once? Dec 31 '20 at 12:46
• @notovny Not if he is using a very large fidget spinner.... Dec 31 '20 at 12:50
• or he could spin it really fast.. Jan 2 at 20:59

This is exactly how it works and how the orientation of many satellites is controlled. For example, the Hubble telescope has 4 fidget spinners installed, pointing in different directions - although they are commonly referred to as "reaction wheels".

Unfortunately a typical fidget spinner is a bit too light to be really useful: We have to compare the moment of inertia of the spinner and a human. For the fidget spinner I found values around $$3\cdot10^{-5}\,\mathrm{kg\,m^2}$$ and a human in a stretched pose around $$3\,\mathrm{kg\,m^2}$$. I.e. a fidget spinner turns about 100,000 times faster than a human. If we can get the spinner to 10,000 rpm (which is rather high), the astronaut would turn once in ten minutes.

• Spinners can reach about 100,000 rpm before self-destructing, but that requires extra equipment. Dec 31 '20 at 17:54
• You had me at "bigger space fidget spinners" Dec 31 '20 at 23:13
• +1 I'm imagining one with a crank to spin it up, and a patent to go with it.
– uhoh
Jan 1 at 2:39
• @Flater A rotating body requires centripetal tension forces to keep from flying apart. Or, in terms of the "fictitious" centrifugal force, spinning causes a force pulling the object apart. These forces exist independently of gravity, and once they exceed the tensile strength of the object, the object will break apart. Jan 2 at 2:22
• Accumulation's point. At such intra-fidget forces, terrestrial gravity is mere noise. ISS microgravity is noise within noise. Jan 2 at 3:10

It's possible to use reaction wheels to arbitrarily alter one's attitude, but a major limitation with reaction wheels is that if an object has rotational momentum and one wants it to maintain a constant attitude, the reaction wheel will have to spin forever unless or until one gives up on holding a constant attitude or one can transfer rotational momentum via some means (e.g. by using rockets). If there's any friction between the reaction wheel and the space craft, then as such friction slows the reaction wheel down, it will transfer the rotational momentum from the reaction wheel back to the rest of the space craft.

• If the system has acquired unwanted rotational momentum, the combined system (reaction wheel plus remainder of space craft) will need to have that rotational momentum unless it can be transferred to something outside the system (e.g. rocket exhaust). As long as the system has non-zero rotational momentum, at least some parts of the system will need to be rotating. Jan 1 at 21:32
• @There is no way to alter the total angular momentum without some outside object. If you have rockets that can alter the wheel's angular momentum, you can just use them to alter the whole ship's angular momentum directly. Jan 2 at 2:26
• @uhoh if you're in a space station you can just grab one of the handles. Your station has a bigger fidget spinner than you do. Jan 2 at 17:13
• @Flater well having the wheel reducing the rotational momentum by itself using thrusters is similar to slowing it with respect to the space craft, and using the attitude control thrusters on the spacecraft to counteract. The difference is really that implementing everything in the reaction wheel, including some mechanism for detaching and especially reattaching is a lot more difficult - that's why it's not done. Normal spacecraft do that once in a while actually in order to desaturate the reaction wheels Jan 2 at 21:06
• @Flater on second thought - you could have "one way reaction wheels" - like a magazin of spare wheels, and when the one in use gets too fast, you just eject the fast rotating disk into space. That said - to my knowledge spacecraft (at least modern satellites) are designed to not lose anything to space, in an effort to minimize debris (hazard) Jan 2 at 21:12