How do rockets work in a vacuum?

Short answer:

By propelling fuel in one direction, the rocket moves in the opposite direction (Newton's 3rd law of motion).


Long answer:

Rockets work by utilizing Newton’s third law of motion: For every action there is an equal and opposite reaction. This means that if you apply a force in one direction, an equal amount of force will be applied in the opposite direction.

Imagine a balloon filled with pressurized air. If you open the lip and let the air out, the air will accelerate in whichever direction the lip is pointed. That's the action force.

The reaction force is the balloon accelerating in the opposite direction.

If the lip is facing down, the balloon will accelerate upwards (although you'd need a stick attached to the balloon to stabilize it and keep it flying straight).

All rockets use the same basic principle: Shooting stuff out one end of the rocket will make the rocket move in the opposite direction. It doesn't matter if it's a child's water rocket or an ion-powered space probe, the physical principle is the same.

Many people think that a rocket wouldn't work in a vacuum because it has nothing to "push against". But it does push against something: the propellant. That's what the propellant is—something for the rocket to push away from itself.

If you're wondering how a relatively small amount of propellant could make a rocket go very fast, consider that:

Note: If you're rich enough to afford a vacuum chamber, it wouldn't be too hard to build a test rocket to see if it will work in a vacuum.