Totally understood. I didn't want to spend so much time on the precision. There was a novel some years ago about humanity gaining spaceflight and aliens then taking us as a threat, and they wiped out all but a few "specimens" by accelerating rocks the size of a car to a high percentage of light speed, aiming them at earth, and theoretically, it's plausible.
Either way, this is a "setup" story and I'm grateful for the c&c!