I'm not an engineer, so let me give you an example (with fake numbers) to illustrate my point. Keep in mind that the real numbers are likely staggeringly small.
Let's say a 10^-6 Torr vacuum has 5000 air molecules floating around in it and you need to extract 4995 of them to get down to 10^-12 Torr. Even a small leak will ruin your progress toward achieving a higher vacuum because we're talking about a tiny amounts of matter here.
On the other hand, if you want to maintain a breathable volume of air pressure, a few molecules escaping through your craft's seals won't be missed. You don't have to maintain the vacuum of space, you just have to bring enough air with you to replace any that escapes.
I'm not an engineer, so let me give you an example (with fake numbers) to illustrate my point. Keep in mind that the real numbers are likely staggeringly small.
Let's say a 10^-6 Torr vacuum has 5000 air molecules floating around in it and you need to extract 4995 of them to get down to 10^-12 Torr. Even a small leak will ruin your progress toward achieving a higher vacuum because we're talking about a tiny amounts of matter here.
On the other hand, if you want to maintain a breathable volume of air pressure, a few molecules escaping through your craft's seals won't be missed. You don't have to maintain the vacuum of space, you just have to bring enough air with you to replace any that escapes.