It's the flux. To scatter any photons takes a lot of flux, and to scatter enough to see would take quite a lot. Of course I haven't done the math either, so I'm

It's the flux. To scatter any photons takes a lot of flux, and to scatter enough to see would take quite a lot. Of course I haven't done the math either, so I'm just guessing what it would do to the atmosphere that got in the way.