How do rocket thrusters work in space when there is no atmosphere? — Greg, Los Angeles, Calif.
Marshall Brain Answered:
The reason why rocket thrusters work in space is because rocket thrusters don’t need the atmosphere.
A jet engine needs the atmosphere to operate because the atmosphere provides the oxygen that the jet’s fuel needs to burn. See How Gas Turbine Engines Work for details.
On the other hand, a rocket carries both the fuel and the oxygen that the fuel needs to burn. If it is a liquid fuel rocket, the oxygen is stored in the liquid oxygen tank. If it is a solid fuel rocket, an oxidizer chemical is mixed with the fuel chemical.
The rocket engine isn’t “pushing” against anything. A rocket is a reaction engine. All the chemicals that burn in the engine have weight, even after they turn into fiery exhaust. “Burning” fuel does not eliminate its weight – it simply turns it from a solid or a liquid into a hot gas. As the hot gas exits the engine, the engine responds in a “to every action there is an equal and opposite reaction” way. So the hot, fiery, high-speed, heavy exhaust shoots out of the engine in one direction, and the “equal and opposite reaction” occurs in the rocket, which goes in the other direction. See How Rocket Engines Work for details.