How does a rocket propel itself in space/vacuum?
Action and reaction are equal and opposite. A lot of mass is ejected at high speed from the rocket engine; the force necessary to accelerate that mass reacts on the engine, pushing it ‘forwards’. If you were in space and you threw an object, the object would move, and so would you. That’s how a rocket works except it expels gas rather than throwing an object. The exhaust moves in one direction, and the rocket moves in the opposite direction. It is conservation of momentum. It would be analogous to some on skates throwing/pushing a mass, e.g. 10-20 kg. The skater would roll in the opposite direction. So in effect, the rocket is pushing against the propellant exhaust. Space in not quite empty, there are still a few particles per cubic cm. One could deploy a solar sail and capture the light and particles from the sun. Again the momentum of the light/particles would be transfered to the craft.
