You can calculate the distance to the horizon with a simple equation. The real equation involves trig, but this one gives a nearly identical answer.
Dist(in miles) = sqrt[height(in feet) X 1.5]
Multiple the altitude in feet by 1.5. Then take the square root of the answer. The distance to the horizon is in miles.
Since my eyes are 6 feet above the ground, I multiply 6 by 1.5 to get 9. The square root of 9 is 3. So the distance to the horizon for my eyes is three miles.
A near sapcecraft at 90,000 feet sees 367 miles to the horizon. Because of refraction, radio should reach a little bit further.