Q: How do you turn/change directions in space?

Physicist: There’s more to this than you might think.  If you’ve seen a movie involving spaceships of any kind, then you’ve probably seen the wrong answer.  We’re used to thinking about airplanes (flying through the air) and walking (on the ground), so the basic intuition we have about how to move around (turning and starting and stopping) doesn’t apply in space.

In space it could barely matter less what shape your ship is, because it doesn’t interact or push off of air (left).  Some epic sci-fi franchises choose to ignore that fact (right).

Every possible motion always conserves momentum, which just means “if you want to move, you need to push on something else”.  Airplanes can bank in order to turn because they can push on air, and we can get up and walk across the room whenever because we can push on the ground.  But in space those luxuries are missing, since there’s nothing to push on.  Moving in space is the most frustrating damn thing ever.  Think about trying to maneuver on infinitely slippery ice.  Worse than that.

Space: Worse than this.

If you want to turn and face a new direction in space there aren’t a lot of options available to you.  One technique is to literally throw things and getting a push from the recoil.

Maneuvering thrusters firing on the back of the shuttle (left) to pitch the nose up and different thrusters just sitting around looking pretty on the front of the shuttle (right).

If you want to move to the right you need to chuck a bunch of stuff to the left, and if you want to turn one way you have to chuck a bunch of stuff in the other.  Flying around like an airplane in space doesn’t work at all (other than slowing down, this is a fair approximation).

However, if you just want to turn in space without moving (thrusters always push you around), you can “push on yourself”.  This is how the Hubble telescope points at stuff.  If it used tiny thrusters it would run out of fuel pretty quick (that thing is always looking at stuff) and it would pollute its tiny corner of space with exhaust.  Instead Hubble uses flywheels, which run on electricity, to turn.

Turn a wheel in space, and turn yourself.  Hubble has small flywheels (right) to turn, and tiny flywheels (left) to zero in on an exact direction.

Scattered throughout Hubble are an arrangement of little motors attached to basically nothing.  Just by turning on those motors, and by spinning them in one direction, the entire craft turns (slowly) in the other direction.

So, if you want to move in space, you’ve got to move something else, and if you want to turn, turn something else.