Space: the final frontier. Every new frontier has a new set of rules - things that don’t work the way they did back home.
The stars are different in the southern Hemisphere, the most deadly animal in the tropics is the mosquito, and dogs are more useful than mules in the Antarctic. But physical rules of thumb generally stay valid anywhere on earth, from merry old england all the way to the East Indies. Engines work by turning some kind of shaft, spinning wheels that push against the ground, hot air rises, and if you don’t keep pushing, friction will eventually stop anything that’s moving.
[caption id="" align="aligncenter" width="558"] We're not in Kansas anymore... Courtesy of Nasa[/caption]
The fundamental physics aren’t any different in space either, but those rules of thumb? Forget about them. Between the vacuum, temperature fluctuations, orbital mechanics, microgravity, and low friction many customary assumptions on earth go out the airlock. The last two conditions in space: microgravity and extremely low friction open up doors that are closed on the ground. The goal of spacecraft actuators is to take advantages of these unique environmental effects.
It’s only fair, because those same effects inject difficulty into many things that are easy on Earth. Gravity and friction overwhelm most electromagnetic interactions because they are small to begin with and weaken rapidly with distance. However, electromagnetic forces are relatively unaffected by the conditions in orbit, making them prime candidates for spacecraft actuators.