[perkee]

For a sense of the scale of this problem, the retro reflectors that we left on the moon return 1 photon out of the 10^17 we shoot at them and that's not even that reliable. This thing would be a mind bendingly further distance away and would not have any retro reflectors on it (oh man it'd be WILD if it did though!!).

https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment...

[Rooster61]

This is the experiment that put the idea in my head in the first place. And also the reason I prefaced the question with the fact that I KNOW it's going to need to be a ridiculously powerful laser. Making something detectable that's hitting something with an unknown albedo and angle of whatever the photons are impacting makes the requirements start to bump up against the laws of physics, much less what we are technically capable of.