eli5: back then, before astronouts goes to space, how do we know about thing like nonexistent of oxygen & zero gravity?
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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.
The first thing scientists did when hot air balloons were invented was to fly as high as they could to record the pressures and temperatures. Some almost died from the lack of oxygen and low temperatures. If you continued the graph of pressure at various altitudes you would see that there were indeed very little atmosphere in space, almost non-existant. This was confirmed by theories of pressure and gravity that had been developed using heavier fluids in labratories.
And when modern artillery was developed a bit over hundred years ago the shells were flying high enough that the reduced pressure was very significant. You could say we fired cannon shells into space and had to calculate their trajectory through vacuum. This provided us with even further insight into how the upper atmosphere behaved and the exact low pressures there.
When it comes to gravity there is actually plenty of gravity in low Earth orbit. But it is not gravity itself you feel. You feel the force pushing up from the ground countering gravity. If there is no forces acting on you we say you are weightless and in free fall. When you are in free fall you do not feel the effects of gravity on your body and your surounding things. The ground is just approaching you at an alarming rate.
This is the case with astronauts as well, there is nothing pushing them so they are weightless in free fall. The reason they do not hit the ground is that they are moving very fast sideways and is therefore able to miss the Earth as they fall towards it. And once they missed the Earth and is on the other side they will fall towards the Earth from the other side but still have the high speed so they miss again.
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