Ask a Teacher

We need to start with Newton's Third Law of motion: To every action there is an equal and opposite reaction.

  In the case of space travel, if a mass (m) of fuel is pushed out from the exhaust of a rocket, then the rocket will accelerate in the opposite direction the exhaust fuel went. Once the rockets are turned off, the space craft slows down like an airplane does here on Earth until it comes to a complete stop. However, unlike an airplane that must contend with Earth's gravity, the spacecraft will keep going in the same direction as it slows down. The astronauts will simply have to fire the rocket engines again to keep moving. While the engines are firing, the spacecraft accelerates. When it has reached its planned speed, the rockets are turned off. This means it is now in some sort of orbit around Earth or around the Sun.