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

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.

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.

Manuevering thrusters firing on the back of the shuttle (left) and different thrusters just looking pretty on the front of the shuttle (right).

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.

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.

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

  1. We can the principle of ” Simple Harmonic Motion (S H M) ” just like the pendulum of a clock to control the direction of a ship in Space…. The pendulum attached at the base of a space-ship must be able to change the direction of a Space-ship (i.e. left, right, top and bottom)…. The pendulum must always attain to its mean position, and whenever it is required to change the Co-ordinate in space it will shift its position from mean to extreme !

  2. Tony says:

    Alright, I’m going to be “that nerd” and defend Star Wars for a second.

    The Incom T-65 X-Wing starfighter is not only meant to operate in space, but in atmospheres as well. If you wish to criticize its design for being useless in space, you’d have to criticize the space-shuttle for the same reason.

  3. The Physicist The Physicist says:

    Consider me nerd-slammed. That’s a good point.

  4. Morris says:

    So, the Hubble’s flywheels must be located very close to the center of mass, right? It seems like any eccentric motion would affect its orbit.

    Concerning the Borg vehicle, aerodynamics are not a concern, but this is not the most efficient use of materials. To maximize contained space with a minimum of outer hull material, you would need to knock the corners off that thing. Maybe pressure vessels work differently in the Borg world.

  5. The Physicist The Physicist says:

    The location of the flywheels is fairly unimportant. Short of a rocket, the orbit is safe.

  6. Ryan says:

    If a fire-hydrant were to burst it could potentially lift a car…so if some kind of machine were attached to a spaceship’s rear that generated a strong water current, could this push that ship? If this is even possible, what machine could keep this water-cycle going?

  7. The Physicist The Physicist says:

    Unfortunately, hydraulics don’t provide too much advantage in space. You can certainly use water as the rocket exhaust, but the basic rule for exhaust, “faster=better”, holds.

  8. Mehrdad says:

    Oh damn. I thought those flywheels on Hubble were for stabilizing it gyroscopically. In retrospective, nothing is there to destabilize the sat anyway.
    Well have look around. Air, water, ground, all reaction mass/propellant. Living and working in free fall vacuum must be a serious nightmare.

  9. Xerenarcy says:

    fun fact, you can use flywheels as angular momentum banks simply by ‘turning a spinning flywheel’. the effect is basically that of a gyroscope.

    a sufficiently large flywheel, once spun up, can have substantial angular momentum. if you re-orient the flywheel while it is spinning, the angular momentum changes with it, and due to the difference in angular momentum there is a secondary torque acting perpendicular to both the re-orientation torque and the angular momentum of the fly wheel.

    you can set up a system of 2 pairs of co-spinning fly wheels (the pairs spin in opposite directions), and by changing the angle of the fly wheels you can manipulate the total angular momentum of the ship-flywheel system yielding rotation. you would need to keep the flywheels spinning at a controlled rate, as angling will likely slow them down.

  10. Andrew says:

    When you mentioned “pushing on yourself”, it doesn’t sound right and seems to contradict what you said before about conserving momentum.

    Reminds me of the “fan in the sailboat” physics problems.

    Can a person suspended in space with nothing to push off of, turn themselves around?

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