I found a paper when I deepdived after reading "A deepness in the sky" but I couldn't find it just now, sorry. IIRC, it had to do with taking a velocity hit at

I found a paper when I deepdived after reading "A deepness in the sky" but I couldn't find it just now, sorry. IIRC, it had to do with taking a velocity hit at like 0.9c and then the best you can do is send that atom out at 0.999c out the back. And also the scoop size was gigantic to get mass in sparse interstellar space.