I would like to raise a point  the main article  does not cover: at some  point the kinetic  energy has to be dumped. As I have shown, this energy can be mammoth. For example, a hundred year trip would require a velocity 8.7% of light and the kinetic energy would be equivalent to 2,486 fusion bombs assuming a mass of the International Space Station. This incredible energy  has to be dissipated upon return to earth.  I am not concerned with how it done, only that if there is a failure and the spaceship plunges into the earth’s atmosphere carrying this much energy, there would be an explosion beyond comprehension.  Consider the  Tunguska explosion which occurred in 1908 in Siberia and which knocked down some 10-million trees.  That energy is estimated to be that of just one fusion bomb 

 No matter small the probability, it cannot be reduced to absolute zero.  Consequently, I cannot envisage politically any round  trip ever being allowed.