In the past space travel was considered impossible because space is a vacuum. If space is a vacuum, what can a rocket push against? When a rocket’s engine is trying to make it move in space, hot gases are released in a downward direction. This results in an equal sized upward force on the rocket that pushes it ahead. Another way to state the 3rd law is to say that for every force there is an equal and opposite force. This means forces come in pairs! The two forces in a pair are called action and reaction. Every time you walk you push against the ground with a force and the Earth pushes back with an equal and opposite force. If you move forward, the Earth moves in the opposite direction! Since the mass of the Earth is so largecompared to your mass, the movement is unobservable. Action and reaction forces act on different objects, not on the same object. Therefore, the forces cannot cancel because they act on different objects.
In the past space travel was considered impossible because space is a vacuum. 