Exactly that's the problem! The scientists haven't found a solution for a brake yet, like mentioned above.. that would be a VERY big problem if they don't come up with something to solve it, there would be absolutely nothing to stop the aircraft from crushing into another planet at full speed!
But obviously, this whole project is still very much a theory, and I'm not sure about the detailed numbers.
All I know is that this is an exciting idea, but there is still a LOT of work today if we want to see this come to life!