Science Puzzle
What Does a Rocket Push Against in Space?
On Earth, things that move push against something. You push your feet against the ground to walk, and a swimmer pushes against the water. Space has nothing to push against, not even air. Yet a rocket out there can fire its engine and speed up.
What does a rocket push against in space?
The Answer
Its own exhaust. A rocket throws hot gas out of the back at enormous speed, and the gas pushes the rocket forwards just as hard.
A rocket engine burns fuel to make huge amounts of hot gas and blasts it out of the nozzle at several kilometres a second. To throw the gas backwards, the rocket has to push on it. Every push comes with an equal push back the other way, so the gas pushes the rocket forwards. Isaac Newton set this out as his third law of motion in 1687.
Stand on a skateboard and throw a heavy ball forwards, and you'll roll backwards. You didn't push against the air; you pushed against the ball. A rocket does the same thing with a continuous stream of gas instead of one ball. If anything, rockets work slightly better in space, where there's no air in the way of the exhaust.
Even clever people got this wrong. In 1920 an editorial in the New York Times mocked the rocket pioneer Robert Goddard, claiming a rocket in space would have nothing to push against. On 17 July 1969, the day after Apollo 11 set off for the Moon, the newspaper printed a correction.
The principle: Action and reaction. When a rocket pushes its exhaust backwards, the exhaust pushes the rocket forwards with an equal force, so it doesn't need anything outside itself to push against.