Sound, and suspect, methods for putting spacecraft in motion
My three-year-old has seen enough representations of spacecraft, both real and fictional, to know they fly by making fire and some sort of extended PWSHOO! sound. While this is true for the majority of rockets of blasting off the Earth, it turns out that the awesome amounts of combustion and structure-shattering noise aren’t strictly necessary, especially if your craft is already free of the planet’s gravity. Much more discreet forms of propulsion exist, assuming you don’t mind potentially breaking physics along the way.
The core concept at the heart of launching a rocket is Newton’s third law of motion: for every action there’s an equal and opposite reaction. To put this in terms as relatable to my primary audience as possible, this is basically saying that when you push something, it pushes back on you just as much. On the street, your mass and friction with the ground generally overpower the push-back you’d notice from say, throwing a ball, but thinking about how things move when floating in a swimming pool makes this concept a little clearer. If you and a friend are each on pool floats and you give them a good shove, you don’t stay put while they drift away. Instead, you both start moving in opposite directions. In this case, your friend became your thrust, slowly launching you through the water.
Forced by burning fuel
So to push a rocket around, the most common method has been to throw fuel out the back of the rocket as quickly as possible. The combustion of burning chemical fuels helps get more push out of less fuel, which is helpful since chemical fuel is generally pretty heavy, necessitating an even bigger push to overpower the pull of gravity.
Pushed by protons
Once a spacecraft is in space, big, dramatic chemical engines work fine, but they’re likely to burn their fuel before you can go a really big distance. To better sip small amounts of fuel over longer distances, NASA is looking to ion propulsion systems. Ion thrusters still rely on the idea of opposite and equal reactions to push spacecraft through space, but in an vacuum they can get away with much smaller pushes to nudge ships along. The pushing comes in the form of single positive ions that are sped up and launched by an electrical field in the engine. As these ions pop out the back of the spacecraft, it speeds up ever so slightly. Over time, this can actually get things moving at speeds over 200,000 miles-per-hour, making them ideal for traveling through the vast reaches of space.
Moved by microwaves?
If propulsion via a series of tiny protons seems crazy, the EmDrive engine that is being studied supposedly pushes out even less propellant than that. The engine is based around a cone-shaped container filled with bouncing microwaves of energy. However, theoretically, nothing ever leaves the container, so there’s no clear source of thrust, which is where the confusion sets in. Rather than a ball pushing back when it’s thrown, all the forces are supposedly contained in this engine, like bouncing a tennis ball off the interior dashboard of your car and expecting it to move forward.
The EmDrive has understandably been called the “Impossible Engine,” which speaks to the good reason to study it. NASA’s Eagleworks team recently found that a prototype did seem to produce a small amount of forward movement, raising a variety of new questions. There are various concerns and doubts about the engine, such as is heated components are throwing off the center of gravity and giving the appearance of propulsion. It’s also been suggested that the engine is actually releasing a propellant of some kind, and that we’ve just overlooked what that is. If none of these concerns prove true, however, even bigger questions would need consideration: what if the third law of motion isn’t actually true? All our other rockets might have some explaining to do.
My second grader asked: Did the Voyager 1 and 2 use ion engines? They went far enough to leave the solar system.
Having gotten some speed boosts from gravitational slingshotting around planets like Jupiter, the Voyager spacecrafts operate with two kinds of fuel. Hydrazine is a liquid that can be sprayed into space as a propellant, usually for small heading adjustments now that speed isn’t an issue. To keep electronics running, both satellites also have some plutonium-238 dioxide, which decays and provides heat to onboard generators. As the plutonium’s decay slows, less electricity can be generated, and so systems have to be shut down one by one, probably closing up shop around 2020.
Source: Why Scientists Think That 'Impossible' Spaceship Drive Might Just Work by Jason Torchinsky, Jalopnik