Propulsion through a Vacuum!

Hi science junkies! I’m geared up and ready for the AAS conference in Seattle, which means that before I leave, I finally have time to field a request made a while ago by my friend Maria—she asked me how rockets propel themselves through space if there’s no air to push against.

It’s an interesting question, and grows out of a common misconception that you need something like air to “push against” in order to move, like an airplane does with its wings to create lift. It’s true that movement through air is different than movement through a vacuum, but the general laws and principles governing the movement do not change. In order to understand how rockets propel themselves, all you will need are Newton’s laws of motion.

The first and third laws are the most useful here. The first law is the law of inertia, which states that bodies will remain at a constant speed (or at rest) unless acted upon by a force. This force can be anything…air resistance, gravity, your girlfriend after you tell her how she looks in that dress…any force that is not balanced by another will result in a change in movement. For an airplane, you not only have to push yourself forward and create lift, but you must also counteract air resistance, something rockets do not encounter (or encounter very little of, in reality) in space. That is why the majority of the thrust rockets create in order to get to their destination is done initially, as they’re leaving, or “escaping” Earth’s gravity. You’ve all seen rockets and spacecraft strapped onto gigantic fuel carriers…all of that fuel is used to propel the ship off of Earth and flying towards their target.

Let’s pretend we’re shooting to the moon. Because there is no air resistance in space, the spacecraft will be traveling in basically a straight line. Before the rocket is shot into space, scientists must calculate the exact location of the moon when the rocket gets there, because over the few travel days, the moon’s position changes. Here’s what the trip may look like:

If something happens, and there needs to be a small adjustment in the flight path, or the speed of the craft, then that’s where the thrusters come in. To understand how they work, we need to employ Newton’s third law: Action-Reaction. This may be the most famous of the laws, and the easiest to understand (even though if never stops us from getting angry and punching a wall). For a force applied to a system, there is an equal but opposite force applied. Going back to your angry girlfriend, if she slaps you on the cheek, she is applying a force to your face. At that moment, an equal force is applied by your cheek to her hand, and her hand will hurt a little. So you can have a little bit of solace in the fact that it will hurt her a bit too, even though telling her to lose some weight did enough emotional damage for the week.

But I digress. Thrusters shoot out fuel from the back or side of the vehicle, and an opposite force gets applied to the vehicle. You don’t really need something to push off of—by applying a force to the fuel, the ship gets a force back on it to propel it in whatever direction it wants. This is an experiment you can try at home! Sit in a computer chair on a floor with a small coefficient of friction (slippery), and throw an object across the room. You’ll notice that you and your chair will slide across the floor in the opposite direction. It’s the same idea, only the fuel is the object. An ice skating rink is another good place for that experiment. Just push off another person while on the ice…or play a game of catch. It’ll be harder than you expect.

The last thing I’d like to mention is the notion of using gravity as a slingshot. Another way to propel spacecraft through space is to use the gravity of the other planets as a slingshot. Here’s a VERY oversimplified picture of the idea:

Sorry Mars, you're just not that cool to be included.

The idea is that as you approach a planet, especially one like Jupiter, you get caught in its gravity. This gravity will accelerate you, and if you are moving fast enough, you will not stay in orbit, but instead get flung out. Scientists do the calculations so that you’re flung out towards your next destination. This method is not only effective, but it requires little to no fuel to take advantage of. AND as a bonus, you get a flyby of a cool object.

All of these things are more complicated, but this is a basic picture of how rocket propulsion works, as I understand it. Hope this was understandable, and that your girlfriend forgives you for calling her a cow. You do need to learn some more tact.

See you in Seattle, or when I get back!