Was Marx right? or How efficient are laser driven interstellar spacecraft?

The idea of a laser driven space vehicle capable of interinterstellar travel was raised 25 years ago. The original article by Marx [Nature 211, 22–23 (1966)] presented a relativistic calculation that contained conceptual and numerical errors. Here the inconsistencies of the original article are corrected. Marx’s calculations are generalized to the case where the laser is of variable power, and to the interesting case where the laser beam is recycled by means of mirrors. The obstacles to this form of interstellar travel appear insurmountable, but the calculations provide an excellent example of the application of relativistic ideas.