I'd think it would take considerably less than 18W to clear the atmosphere.. I read a paper that talked about a HeNe (probably large-frame) that was detected from a Space Shuttle.. can't cite the source, though.
yahh i cant wait to get my x10 expander....i have lots of wide open land with a mountain about 10 miles away that im going to shoot from...its at 4000ft. diff. going to do some long and short dist. tests....
I read somewhere on here about a guy shining his high mW 532nm, and spotting it 35 miles away. This was in the mountains although. Mountain top to mountain top.
Cold, very dry air.
Found it. Also talk of of a hobbyist's laser being visible from the ISS and the moon. http://laserpointerforums.com/f45/35-mile-laser-shoot-40481.html
I don't know for sure if you could see it from the ISS, but it seems possible. (they did the math formula's that said it's possible) In my experience, about 3 miles is the farthest I've seen the dot.
Sitting up on the peak of the roof, shining it at a known 3 mile distant tower.
Well space is actually a pretty complete vacuum, and unless you were to shine it though an area with fine, densely packed, debris, you wouldn't see the beam since a laser's beam is just the photons reflecting off of particles they encounter. The DOT however would be able to forever technically, assuming it doesn't hit anything. This is because photons are massless and travel at the speed of light, so short of hitting something or encountering a black hole, there's nothing to slow them down in outer space.