Re: An aside...
That's exactly right. and I think exactly how the Breakthrough Starshot thing is intended to work. The other advantage is that you don't have to fit the thing that fires the pellets or laser beams into a spacecraft and you thus make one which weighs thousands of tonnes if you want (I presume the idea, for a Lunar one, would be to mine the materials for it up there rather than lift them).
It seems to me that a more plausible thing (than Breakthrough Starshot) would be to use the same techniques to drive spacecraft within the Solar system.