What Is the Pressure in Outer Space?

For those of you getting ready to hit "Post" on a comment reading "It's a vacuum, dipshit" or something similar, give me at least a paragraph to explain this one a little.  One of the rarely-discussed fringe benefits of doing semiconductor engineering is that you end up learning a lot about vacuum systems.  For various reasons (most of which have to do with contamination) almost every process associated with making and looking at things on the micro/nano scale has to take place in vacuum.  That means all the machines you use for said processes are basically vacuum chambers.  Eventually, one of the pumps or gauges or other expensive things connected to a vacuum chamber will break, at which point you'll have to figure out how to fix or replace it.  As a result, spending any time at all working in nanofabrication or related fields will get you well-versed in the finer points of the care and feeding of vacuum systems in a hurry.

These two things are not, in fact, equivalent even though we use the same word for both of them (vacuum swiped from www.bubblews.com, space-scape from www.outerspaceuniverse.org).

One of the first things you learn about vacuum environments is that "vacuum," at least in the real world, is a catch-all term that encompasses many, many orders of magnitude of sub-atmospheric pressure, some of which are much harder to maintain than others.  The vacuum created by your vacuum cleaner ("low vacuum") is about 500-600 torr, which is only a bit lower than the standard atmospheric pressure of 760 torr (my guess is that my monster Dyson Animal does a bit better than that.  Damn thing can suck the fibers out of a carpet if you don't watch it).  On the other end of the scale, the vacuum you need to keep a high-voltage electron gun from arcing is on the order of 10^-9 to 10^-11 torr ("ultra-high vacuum", or UHV), pressures so low that it honestly starts to make more sense to count them in particles per volume or something.   Most vacuum equipment isn't quite that extreme; typical base pressures on semiconductor gear tend to be in the 10^-5 to 10^-7 torr ("high vacuum") range, which is still pretty low but achievable without weird things like ion getter pumps.

Vacuum system status screen for my electron beam lithography system at work.  Pressures range from an astronomical 10^-2 torr in the pre-vacuum area (G6), to about 10^-7 torr (G4) in the main chamber where the magic actually happens, all the way down to 10^-9 torr (G5) near the 100 keV electron gun.

As we know, space isn't empty, although it's pretty close.  Earth's atmosphere actually extends a good fraction of the way to the moon, if you define "atmosphere" as "higher pressure than interplanetary space" rather than "where I can breathe without dying," and by the same logic the sun's "atmosphere" is present throughout most of the solar system.  So space in earth orbit, on the moon, out in the solar system, and deep in interstellar/intergalactic space are all going to be "vacuum," but very different degrees of vacuum.

Let's start with low-earth orbit.  Low-earth orbit is generally defined as "high enough to not fall out of the sky that fast," or between 200-2000 km above Earth's surface.  You're generally considered to be outside the atmosphere in the conventional sense at this point.  From a vacuum-systems standpoint though, the pressure here isn't particularly low-- about 10^-7 torr, or on the outside range of what's achievable with a single good vacuum pump.  The technical term for this part of the sky is the "exosphere"--not empty space, but nowhere near enough pressure to even begin to call it the atmosphere.

The exosphere extends almost an entire earth-diameter from the surface, apparently (image swiped from www.universetoday.com)

Head out to the moon and you'll unsurprisingly see the pressure drop quite a bit.  The moon being about 400,000 km from Earth (sidenote: holy shit that's far. We really went there? That's bananas), it's pretty much completely free of Earth's atmosphere; surface pressure is going to range from 10^-10 (night) to 10^-11 (day) torr, or right about at the practical limit of artificial "ultra-high vacuum" conditions.  That's about standard for interplanetary space in the solar system too, although it can vary a bit with solar wind flux.

That's pretty low, but even between the planets we're still sitting in an almost insubstantial cloud of "solar atmosphere."  We know this because as soon as you get well outside of the solar system, pressure drops again, to about 10^-15 torr in the interstellar voids of the Milky Way galaxy.  That's substantially lower than anything we've been able to artificially create; even the best ultra-high vacuum systems top out at about 10^-12 mBar.

Even the "empty space" of the Milky Way has some density to it though.  Calling it an atmosphere is probably pushing it a little bit, but the pressure out in the intergalactic void, light-years away from pretty much anything that could cause matter to gravitationally accrete, is even lower, about 10^-17 torr.

So the "vacuum of space" can refer to about ten different orders of magnitude of pressure, depending on what you're calling "space."  It's important to note at this point that for anyone but a vacuum-systems engineer and/or pedant, there's no practical difference between 10^-7 and 10^-17 torr; each one will cause you to die of freezing/explosive decompression at exactly the same speed if you're exposed to it without protection.  Given that, it seems pretty reasonable to just call it all "vacuum," but it's interesting that stars and galaxies have atmospheres just like we do, provided you're extremely loose with the definition of "atmosphere."

Wikipedia has a neat table of pressure orders of magnitude, which is where I cribbed a lot of this stuff from.  You can figure out how astronomers were able to calculate, say, the pressure in intergalactic space by following the reference links.