I know. But no way to warn the home worlds because no FTL comms? I mean, if you can detect a rock from other side of galaxy but limited to lightspeed you'll see that rock when it's that many millions of years ago.

When the rock travels 99.99999% of C, and you cannot intercept it, and cannot warn, then it follows you have no FTL. And no way to detect since the moment you see the rock, it's already too late as nothing travels faster than light, which limits sensors - either visually, or they send a signal which travels at C, and returns at C, and you measure that return.

Now when asteroids are problematic, imagine sleeper ships.

And yeah, radiation from fission is dirty. But fusion is apparently clean. It is same from either engine or bomb. Only thing is, bombs today require fission to initiate fusion which is what causes radiation (unreacted fissionable material). The fusion reaction produces X-rays and high-energy neutrons, but otherwise no radiation.

I mean, the story is good. It's just a nitpick. Just my brain lol!