Also they have trouble stopping in Tau Zero so they have no choice going further than they planned. It's one of the best sci-fi novels of all time, read it!.

That Bussard Ramjet, though, is thoroughly discredited. It can't possibly work. Hydrogen hydrogen fusion is a terribly slow nuclear reaction and if you had to stop the gas to give it enough time to react, you'd end up stopping the rocket not accelerating it. In fact, the most credible use of that kind of magnetic scoop is as a brake!