By Wayne Boyd
Air pressure works like water pressure. The deeper you go in water, the more the pressure because of the weight of the water above. Similarly, the lower you go in the atmosphere the more the pressure. At sea level, air pressure is normally about 14.7 pounds per square inch. This can be measured with an altimeter, which is used on aircraft.
Mount Everest is so high that climbers usually need to carry oxygen to go the summit. Commercial airlines go even higher.
When you get about 62 miles up (100 km) you’re at air pressure that is basically zero and you are above most of the atmosphere altogether.
The International Space Station orbits around 250 miles up, but there is still some, very thin atmosphere up there that eventually slows down the ISS, requiring it to be boosted once in awhile.
Eventually you come to deep space, and there are some molecules floating around out there, just not very many. So space isn’t completely empty, just mostly empty. Sometimes there are molecular clouds in space in regions where stars are forming, and the molecules floating around are a little denser.
If you went outside on a spacewalk on your way to Mars, took an empty bottle with you and opened it and then sealed it, once inside the spaceship you’d find your bottle had nothing but a vacuum in it. The free floating molecules out there are so sparse you’d not find any inside your little bottle.