eli5: If space is a vacuum, how can rockets work? What are the thrusters pushing *against* if there is nothing out there?

Question

I’ve never really understood the physics of this. Obviously it works somehow — I’m not a moonlanding denier or anything — but my (admittedly primitive) brain continues to insist that a rocket thruster needs something to push *against* in order to work.

So what is it pushing against if space is essentially a void?

In: 7157

Answer 1

Imagine you have a row boat filled with bowling balls. If you throw a bowling ball out of the backwards out of the boat, Newton’s 3rd law says that there is an equal and opposite reaction which pushes you and the boat forwards.

In a space rocket, the bowling balls are atoms of fuel. You throw the fuel out of the back of the rocket. You get an equal and opposite reaction which pushes the rocket forward.