Comment by hermitcrab

So a probe travelling between galaxies at 99%c can't hit anything bigger than 1.46×10−12 kg? It is ~2.5 million light-years between us and Andromeda. I don't know how much matter there is between the galaxies (does anyone?). But travelling that sort of distance without hitting anything above that tiny mass seems very unlikely. Also how is your tiny robot probe supposed to power any guidance or maintenance systems for that sort of duration of trip?