WHERE DOES SPACE START?
There’s an easy answer to this question: space starts at the top of Earth’s atmosphere. The hard part is saying just where that ‘top’ is. The fact that the atmosphere doesn’t end abruptly, but just gets thinner and thinner, means there’s no hard and fast upper bound you can put on it. To some extent, it’s simply a question of coming up with an easily memorable number that’s in the right ballpark. To NASA and the US military, for example, space starts at an altitude of 50 miles. To the international community, on the other hand, it starts at 100 kilometres, which at 62 miles is a little higher. In the middle of the 20th century, a Hungarian-american aerospace engineer named Theodore von Kármán asked a simple question: At what altitude does the speed needed to keep an aircraft aloft through aerodynamic lift become so high that it exceeds orbital velocity? He did the necessary calculations, then rounded the answer to that memorable figure of 100 kilometres, or 62 miles. This altitude is now known as the ‘Kármán line’ in his honour.