How did the first astronauts/kosmonauts in the 1940s/1950s *know* that the sky isn't actually a dome with “little dots” (stars)?

Question

This may sound ridiculous, like a joke question, but I'm for real. I've been thinking a lot about things like this, but I always return to this question:

Those early "space explorers" who were actually able to use some kind of balloon or crude space craft to physically go up there and back to tell the story about what it actually is like up there... how could they know that they wouldn't just hit a "wall" up there or something? How could they know that there is a vacuum just by looking at the night sky with telescopes and stuff?

And, if they had telescopes or at least binoculars for so many hundreds of years, why did it take them so long to draw the conclusion that it's a big void of vacuum and we are on a marble in this space, rather than there being a big "dome" around us with little "dots" painted on by God, and right behind that wall, God is relaxing in a cloudy mist?

I mean, even in the 1930s, they had extremely advanced Earth technology such as cipher machines, mechanical computers, analogue computers, etc. Huge ships that could travel the sea, atomic-driven u-boats (I believe) and rather advanced air planes. But they had not gone to space.

As I understand it, with my limited knowledge (in spite of asking these questions all my life), they never sent up an "unmanned" craft to just film space or "record telemetry" -- the first time (other than that Russian dog in the 1950s?), they had a human "go up and verify things" in some kind of nightmarish little tin can and he managed to survive to tell the story?

And from that point, they started making more and more advanced "spacecraft" based on existing aeroplanes?

My point is, they just at some point in the 20th century came to the conclusion that the conditions are the way they are in our atmosphere/surroundings? They didn't "discover" it by having some film camera attached with a bunch of sensors fly up in balloon and then pull it down with a (very long) string, did they?

Was it simply that the various sciences involved in this had reached a certain point around the 1950s? It seems like it was as late as the mid-1960s before they really had people going up to do anything meaningful in orbit. I assume that countless people died as well in these experiments, possibly never announced to the public.

I just find it all to be very strange. They had cameras an telemetry, yet felt the need to have a dog and humans go up there (and quite late, IMO) instead of just... taking it safe and just relax down at the "ground control" and watch the data fly back through the ether to their radio masts.

Maybe this was like a "pride" thing? As in, they needed to physically go there with a human to "prove a point" or something?

I hope it's not unclear what I'm asking. Maybe it's a bit foggy, but this is the best way I could describe what I'm wondering. Essentially, how and when did they figure out all these assumptions that turned out to be true? What if that Laika dog had gone up there and cracked through the dome and met God, and then God had said:

Thou, Divine Dog, shalt become the new Messiah! And all of Earth's peoples shall bow before your paws!

Seriously. As far as I can tell, they had not "verified" this in any real way prior to sending up a dog there. And even then, the dog came back and did what? Bark? I assume that the Soviets had a camera running inside at least?

Also, pardon my "ignorance" and if the quote above offends anyone. I really do wonder about this.

Answer 1

The basic nature of our solar system, that the Sun is in the middle and that the movements of the planets around the Sun and the moons around the planets are based on an unknown attractive force similar to, but distinct from, magnetism had been known at least since Johannes Kepler's Astronomia Nova, published in 1609.

Sir Isaac Newton then, in the Philosophiae Naturalis Principia Mathematica (1687), described gravity, which not only allowed him to explain the motion of the planets, but also the motion of falling objects on Earth, thus providing a unified theory, whereas before, it was widely-believed that the laws governing physics on Earth, and the laws governing movement of the planets and moons, were distinct.

Crucially, this also describes how Earth attracts air, and that the attraction diminishes with altitude, thus air must be thinner the higher you go, and at some point, the attraction is so small, that for all intents and purposes, there is no air anymore. The fact that the atmosphere does get thinner would have been observable as soon as balloons and then aircraft started reaching altitudes higher than a couple of kilometers. Heck, it has been observed by the indigenous people of the Andes and Himalaya for tens of thousands of years; it is even observable in Denver, after all.

Everything that came after that (Albert Einstein's Special Relativity and General Relativity), is icing on the cake, and not really required for spaceflight.

So, arguably, all that was necessary to know that astronauts don't hit their head, was known in 1687, at the very latest.