How is the degree of vacuum in the Universe measured?

[Yuri B.]

Hello
It seems we refer to the outer space as "the absolute vacuum." I read recently that the deepest vacuum man reached on the Earth is about 1000 molecules per cm3. To obtain this there was needed energy sufficient to reach a pressure of 400 billions Bars.
Why space ships do not burst like the bubbles of soap up there ?

 

[Constantin]

Because their internal pressure is at most the normal atmospheric pressure. It can be less.
The shell of the ship can hold that pressure.

A similar situation is encountered for high flying passenger airplanes. Although they don't fly in vacuum, they do in low atmospheric pressure, so they're subjected to similar forces.

An "absolute vacuum", or a not so absolute vacuum, will result in a similar difference in pressure and in similar forces exerted on the shell of the ship.

 

[BillSaltLake]

If a vessel is pressurized at 1 bar (which I'll call P₁), and the outside pressure is P (< P₁), the stress on the skin is proportional to P₁-P. If P = 0.01 bar for example, the stress is 99% of what it would be in absolute vacuum (P=0).
I'm not sure what the 400 billion bars is about, but it has nothing to do with 1000 molecules/cm². By the way, 1 bar is ~4x10¹⁹ molecules/cm².

 

[Yuri B.]

Yes, achieving ultra deep vacuums is not qwestion of energy but rather purity of the process. Cannot find any more the article which stated the billions bars pressure to be comparable with absolute vacuum.

 

[Gfellow]

There is previous discussion on vacuums which might interest you:
https://www.physicsforums.com/showthread.php?t=360401