Quote Originally Posted by Heraclius View Post
If this is the case then it would need to stop before it runs out of fuel, because in space there is no friction so technically you only need to accelerate to 100kph and then cut the engines and you'll cruise at that speed until you hit something.

Not true. Space is not empty. There are enough free ions and microscopic particles to create some friction, though it would only be noticeable at relativistic speeds.