eli5: back then, before astronouts goes to space, how do we know about thing like nonexistent of oxygen & zero gravity?

Question

eli5: back then, before astronouts goes to space, how do we know about thing like nonexistent of oxygen & zero gravity?

In: 4

Answer 1

Just climbing a mountain already tells you that pressure goes down. The rate at which this happens depends on altitude, we could calculate that far above there is practically no air, and therefor no oxygen.

Gravity was pretty well understood by then, so we know how strong gravity in space is.
Also, it’s a common misconception that there is no gravity in space. At the altitude of the ISS, Earths gravity still is about 98% of sea level gravity.
The zero gravity is only possible in orbit where the forces cancel out.