If space is basically a vacuum and void of atmosphere, how do rockets alter the direction and speed of space craft? In other words, how do they "push off" against nothing?
This is a very good question. Isaac Newton worked out the solution and published it in 1687 in his Principia Mathematica. It is phrased as Newton's 3rd law. I'll include all 3 below for completeness!
1st: A body will remain at rest or at motion with a uniform speed unless it is acted on my an external force.
2nd: The acceleration of a body with a force acting on it is that force divided by the mass of the body (F=ma)
3rd: Every action has an equal and opposite reaction.
So the third law basically says that if you shoot out stuff in one direction you will move in the other direction. This is how rockets work in a vacuum. They have a source of fuel which is heated up so that it expands and is pushed out of the rocket. In order to change direction in space rockets have to have little 'thrusters' on all sides (you need 6 in total to maneuver completely in 3 dimensions).
Newton's 3rd law seems contrary to our intuition because on Earth there are lots of sources of friction - providing much easier methods of propulsion! However, you might have seen it in action if you have ever blown up a balloon and then let go of it before tying it up. What pushes the balloon all around the room is the air you blew into it escaping.
Page last updated on June 25, 2015.