Video: Any number of forces can mess up a spacecraft’s flight path on its way to where it needs to go, however spinning helps average those torques to add stability to the trajectory. But when a satellite does successfully reaches orbit, how does it stop spinning? There’s lots of ways to change the momentum of an object in space, but generally, the more moving parts something has, the more likely it is to break. And fixing something broken that’s 2000km above Earth is extremely costly. That’s why engineers devised a method that resembles yo-yos exploding out from the sides of the satellite to stop its rotation.
Angular velocity and moment of inertia are the two concepts at work here, and they have an inverse relationship. Spin around in an office chair while holding two heavy objects out from your body. Now bring your arms in towards your chest and you’ll notice you’ve sped up a bit. Weird right?
The yo-yo de-spin works by the same principle, only opposite. Two weights on long tethers fling themselves out from the satellite and fly away via exploding bolts, taking the angular momentum with them — which is good news for satellites, whose instruments usually don’t work so well when whipping around like they’re inside of a blender.
Of course, quick solutions like this add to the amount of potentially dangerous space junk already floating outside Earth, but you have to admit that it’s elegant in its simplicity.