Where does space start? Looking for The Final Frontier

 I've always loved the original Star Trek series but as you'll see from my poster, Captain Kirk wasn't my favourite.

However the news of William Shatner's trip into space this week had us asking the question "where does space start?". It turns out that it is probably governed by the Karman Line. To summarise, the lift generated over the wings of a plane is directly proportional to the density of the air but also to the square of the speed. The higher in the atmosphere you go, the less dense so to generate the same lift, you have to go faster. But faster means more heating due to air resistance. Theodore von Karman wrote a paper in 1956 in which he did the maths on this and discovered that somewhere over 80km up, a plane would overheat trying to generate enough lift. Above that height going that fast would put you into orbit. Now there seem to be two definitions. The Americans reckon space starts 80km up and the international view seems to be 100km. Space starts somewhere.