Big Green Laser + Hobbyist Telescope + the ISS

Not that I would ever do this, and seeing how much trouble you can get in just shining one near a police helicopter this would be 10^10 times worse than that I’m sure.

But just to ponder hypothetically let’s say you have some 1+ watt green laser with pretty good divergence and you mount it to a telescope, spend some time sighting it in on some distant mountains or something.

Then you track the ISS's orbit and shine it up at it. How bright do you think it would be from up there?
Do you think they would actually see it?

I remember my dad did something like this for NASA, but it was with some huge lasers that they shined at a retro reflector mounted to a geosynchronous targeting satellite in a low earth orbit.
I’m just wondering if 1+ watts is bright enough to be seen from 100's of miles away.

No, I don't think it could. Divergence would be too large, even if it was only 0.1 mRad. Even if the divergence was 0, Rayleigh scattering would dispel the beam almost completely. You would probably need a laser in the kilowatt range for this to be effective, although I could be wrong.

-Mark

That's what they use to shine at the reflectors on the moon, one killowatt. I'm not sure how far that is from the moon, but they're both really far away. I would say ~1KW.