Sky Lights: How We Knew Space Was a Vacuum (2021)
But if you’re looking for “absolute proof” it was in the mid-1940s when Germany fired V-2 rockets across the English Channel from Peenemünde. These rockets were the first to reach outer space (top graphic). Near the apex of their flight they were observed to move along a ballistic trajectory — motion in which gravity is the sole acting force. Ballistic motion can only happen in a vacuum.