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The vacuum in interstellar space compares to the best vacuum achieved here on Earth as air under normal pressure compares to a wall of stone.
condition pressure in mbar applications/matter density
normal pressure 1000 respiration / at 20 C, 5 X 1019 molecules/cm3
rough vacuum 1000-100 vacuum cleaner, milking devices, vacuum packaging
intermediate vacuum 1-10-3 simple chemical procedures, metal coating, metallurgy
high vacuum 10-3 as above; light bulbs, picture tubes
ultrahigh vacuum below 10-6 chip manufacture, elementary particle acceleration below 1010 molecules / cm3
Earth's atmosphere at 250 km 10-7 109 molecules / cm3 at about 20° C
Earth's atmosphere at 400 km
Vacuum tube of LEP rings
10-9 107 molecules / cm3
Best vacuum reached today 10-13 103 molecules /cm3
matter density in interstellar space   1 hydrogen atom / cm3
"critical matter density"   10-7 hydrogen atoms / cm3
average matter density of visible matter in the universe   10-9 hydrogen atoms / cm3