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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.

> The zero gravity is only possible in orbit where the forces cancel out.

Zero gravity is not possible anywhere. Zero Gs, on the other hand, is possible almost anywhere. Orbit is just the best place to experience zero Gs for any significant amount of time.

This is an interesting statement. Is there no place in the universe so distant from any large mass that 0 gravity or negligible gravity could be attained?

Force of gravity drops exponentially with distance so I feel like this is very possible.

Edit: after a little googling and finding out the shockingly huge mass of stars like our sun. I believe finding a spot in space with negligible gravity may be harder than I thought.

Lagrange points are cool in that the gravity force of two masses cancel out there, but it's really no different than how orbiting causes 0 g's even though gravity is still present

Yeah my first thought was Lagrange points are probably the closest thing to what you're describing.

If we were not orbiting the sun and were directly feeling it’s gravity, it would be about 1600 times weaker the gravity we feel on earth.

I don’t know how weak gravity has to be before you feel “weightless”, though. That’s a good question.

Negligible weight we could define as weighing <1kg just for an arbitrary definition.

>Some almost died from the lack of oxygen and low temperatures

Some did perish, e.g., Joseph Croce-Spinelli and Théodore Sivel in 1875:

https://www.atlasobscura.com/articles/the-balloonists-of-pere-lachaise-cemetery

Wait, so you're saying things in orbit are just endlessly falling? Does that apply to the earth and the sun as well? Is the earth's orbit just the earth falling towards the sun but missing? That's fascinating lol. Oh shit does this mean the moon is always falling towards the earth??

Yes. But everything is also moving fast perpendicular to the falling direction. Like it’s falling down, but it’s also going thataway so fast that it always misses the Earth. And there’s no air in space to slow you down, so you just keep going thataway and keep missing the Earth.

Yes, calculated to get just the right angle and speed to be missing. I don't think thats endless though. As in, most satellites will eventually crash down anyway. Even the international space station has to fire up its boosters sometimes to stay in the right orbit angle.

Then the sun has negligible gravity over you.

>Wait, so you're saying things in orbit are just endlessly falling?

That's exactly right.

Fun fact: If you want to go into space, the key problem isn't going high enough, it's going fast enough. If you don't go fast enough, you'll fall down and hit the earth. But if you go fast enough, you'll miss the earth and you'll be in orbit.

https://what-if.xkcd.com/58/

Yes and Issac Newton was the one to make a mental picture of the relationship of speed, trajectory, and gravity called "Newton's Cannonball." He hypothesized that there's basically a "sweet spot" in which an object traveling fast enough to not fall back to a surface but slow enough to not trail off into space. He hypothesized that gravity was a universal force and was the driver of planetary and satellite motion.

It's a pretty famous though experiment. It's even referenced in Issac Newton's episode of Epic Rap Battles of history.

Yes. Orbit is basically a fall that "misses" the Earth (or whatever body you're orbiting around).

Newton's Cannon is a great explanation of that.

https://i.stack.imgur.com/STNEW.jpg

If you launch an object, it will fall in a curved path. If you launch if faster, it will fall in a curved path, but with a larger radius. If you launch it REALLY fast, the radius of that curve will be such that it will go around the Earth.

That's how rockets reach orbit. They basically accelerate sideways a lot.

>I don't think thats endless though

It is endless in theory, if you're high enough.

ISS has to boost because, even though it's technically outside the atmosphere, there are still some air particles floating around up there, producing a tiny amount of drag which needs to be compensated every so often.

For a body to stay in orbit, it needs to have a certain speed. Drag is slowly taking that speed away. Boosting is adding that speed back.

I'm assuming planets and other celestial bodies don't experience drag in space, so those definitely will keep orbiting their star until it expands and swallow them then? Assuming nothing flying through space impacts them with enough momentum to shift the course

Well, in theory, if conditions don't change, an orbit remains unchanged forever.

However, in practice, in our Solar system for example, you've got planets orbiting the Sun, each at their own pace, and every planet is exerting some gravitational force on other planets as they pass each other by, making tiny changes in their orbits.

So, orbits do change over HUGE periods of time (but we're talking slight changes over millions of years), but they don't really decay.