 Well, you guys asked for it, so today on Vigid Space, we're looking at reaction wheels. I've talked about reaction controls in an earlier video. This video right here. They're the small rockets on the side of a spacecraft that help it stay oriented by thrusting out a small amount of gas and the spacecraft moves in the opposite direction. Reaction controls can either be controlled by the pilot or by the computer. The problem with a reaction control system is that it requires a propellant, something that takes up both weight and space on a spacecraft, which are two things that a spacecraft rarely has to spare. So a lot of spacecraft and satellites use reaction wheels. This includes the Hubble Space Telescope, the Kepler Telescope, and the Cassini mission that's currently orbiting around Saturn. Reaction wheels are another way for a spacecraft to stay oriented in a vacuum. They can weigh anywhere between a few and tens of pounds and they're driven to spin by electric motors. The motors are powered by the spacecraft's own electrical power supply and are controlled by the spacecraft's onboard attitude control computer. It all comes down to the spinning motion. Reaction wheels move a spacecraft by the transfer of angular momentum and Newton's third law. That whole, for every action, there is an equal and opposite reaction thing. Get one reaction wheel spinning at a very high rate, think along the lines of 6,000 revolutions per minute, and then change that speed. The spacecraft will move in the equal and opposite way. If you speed up the wheel, the spacecraft will move one way, slow it down, and the spacecraft will move in the other direction. Spacecraft typically have four reaction wheels, three primary wheels, and one backup. And they're all offset at different directions so that they can all work together to orient the spacecraft around its three axes. So reaction wheels are actually a system that physically move a spacecraft to keep it oriented in the right direction, say, locked on a distant exoplanet. And reaction wheels are not to be confused with gyroscopes, which are a different way of keeping a spacecraft oriented. We're going to look at those on Friday, so be sure you come back to get those questions answered. So does that clear things up on reaction wheels for you guys? If not, leave your questions in the comments section below and I'll try to get to them. If you have other questions about spaceflight or other topics you have questions about that you would like to see answered, leave those in the comments as well. For daily vintage space content, including all kinds of old-timey pictures, be sure to follow me on Twitter as AST Vintage Space. And with new episodes going up every single Tuesday and Friday, be sure to subscribe right here so you never miss an episode.