>How much energy do you lose? Not entirely the right question. Usually in spaceflight you are overwhelmingly concerned with efficiency, since you have to carry

>How much energy do you lose? Not entirely the right question. Usually in spaceflight you are overwhelmingly concerned with efficiency, since you have to carry your fuel with you. But with laser-boosted light sails, you leave your engines at home. At reasonable distances, (100+ AU) most of the beam is wasted, but the spacecraft doesn't care about that. https://en.wikipedia.org/wiki/Solar_sail#Interstellar_flight https://en.wikipedia.org/wiki/Solar_sail#Interstellar_flight