When early rocket pioneers started strapping volunteers to missiles, how did they know those volunteers would need spacesuits? Of course, in both the Soviet Union and the US, animals subject we tested first, but in fact, we already had aircraft flying almost to the edge of space before that–and aircraft flew higher than humans can survive even during World War II. How did we prepare for this environment? How did we know we needed to?
Well in fact, people had been flying in balloons since the 18th century, and some later balloon flights led to deaths. But long before this, in 1644, Torricelli described the first mercury barometer, writing “We live submerged at the bottom of an ocean of the element air.”
Torricelli and his mentor Galileo knew full well that we live on a spherical planet, but Galileo gave an erroneous explanation for the difficulties in using the suction pump to draw water up a well even though Aristotle had known that air has weight.