The ISS is leaking through a hole in the Zvezda module that still hasn't been found.

However, as with all pressurized chambers, shouldn't all air instantly run out through the hole into the vacuum of space immediately after the hole formed, forcing the astronauts to put on spacesuits?

Or is it rather a valve that's leaking like in case of Soyuz 11 (where however all the air also run out quite quickly, killing the crew)?

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    $\begingroup$ If you drill a hole in a big plastic bottle using your smallest drill bit it'll take a while for water to leak out of that and air leaking into a vacuum arguably leaks even slower $\endgroup$
    – ikrase
    Commented Dec 20, 2020 at 13:29
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    $\begingroup$ Have you ever inflated a balloon, then let it deflate again? Did it a) happen instantly, or b) take some time? $\endgroup$ Commented Dec 20, 2020 at 16:56
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    $\begingroup$ Analogy: I open the faucet (tap) over my kitchen sink. The entire regional reservoir of two billion gallons immediately dumps into my house. Different viscosity, different fluid, same physical constraints. Matter has inertia, fluids have pressure gradients (not abrupt discontinuities), velocity and acceleration have time dependencies. $\endgroup$ Commented Dec 20, 2020 at 19:36
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    $\begingroup$ A better analogy would be to a slow leak in a car tire. Even though the usual pressure difference (2-3 atm) is greater, and the volume much smaller, it can take days or weeks to go flat. $\endgroup$
    – jamesqf
    Commented Dec 21, 2020 at 3:30
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    $\begingroup$ @Greenhorn: The "no exterior pressure" is irrelevant. All that matters is the pressure difference. $\endgroup$
    – jamesqf
    Commented Dec 21, 2020 at 17:16

3 Answers 3


It would take around 7 hours to depressurize the ISS through a bullet hole. See http://www.spaceacademy.net.au/flight/emg/spcdp.htm

Smaller holes will take longer. A tiny crack might take days to even cause a noticeable pressure drop. The ISS is not 100% leak-proof, so some small amount of air needs to be added on a regular basis to top it up. Further, the ISS spends roughly half of its time on the day side, and half on night side of the planet. This means it goes from pretty hot to really cold, causing metal to expand and contract. It's possible the small leak only occurs when its hot, or cold, so around half the time.


The pressure difference between the inside and outside of the ISS is one atmosphere. This is the same pressure difference between the bottom of a 10 meter tank of water and the outside at sea level or minimum legal water pressure in the UK

The behavior of air and water in detail is different but a first order approximation of fluid behavior VS hole size will be similar for a space craft leak and leaks in domestic plumbing (noting your actual mains pressure may be two or even three atmospheres) - undesirable over various time scales but not instantaneous even at quite substantial opening sizes (think shower head or open tap).

  • $\begingroup$ Doesn't it matter that there's ultra-vacuum outside, a pressure far beyond the Armstrong limit? $\endgroup$
    – Greenhorn
    Commented Dec 20, 2020 at 12:48
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    $\begingroup$ @Greenhorn pressure difference is the same so the mechanical loads are the same, which is counteintuative but how physics works. Also means they can test spacecraft (and airliners) just by pumping them up above ambient to the required level to simulate flight. There are a number of things like vapor pressure that DO make a difference with a hard vacuum (and having your sealing grease evaporate can of course cause a leak) but the 'how fast does fluid flow is consistant. $\endgroup$ Commented Dec 20, 2020 at 13:13
  • $\begingroup$ So basically it's like a spacewalk without a spacesuit, because all the astronauts onboard aren't shut up from the outside. $\endgroup$
    – Greenhorn
    Commented Dec 20, 2020 at 13:59
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    $\begingroup$ @Greenhorn a lot of scientists would love for creating "ultra-vacuum" to be as easy as you imply. Lots of science could be done more easily and cheaply. However, the more gas escapes, the less pressure there is to remove the remainder. $\endgroup$ Commented Dec 20, 2020 at 21:49
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    $\begingroup$ The difference between a pretty poor vacuum (call it 0.01psi) and ultra vacuum (0) is only another 0.01psi ... makes no difference to the rate of atmospheric pressure (14.6psi) air escape. $\endgroup$ Commented Dec 20, 2020 at 23:05

It is easy to conduct simple home experiments, or even do them as thought experiments to determine the answer to this question.

Think of a child's balloon. Inflate the balloon with air whilst holding the neck of the balloon with your fingers:

Boy inflating a balloon

Now relax the pressure on the neck of the balloon very slightly. You will find that the air leaks out of the balloon very slowly. It does not all escape instantly. It takes time. This is because the molecules of gas (air) cannot all move through the exit at the same time. They have to move towards it before they can exit. A bit like people rushing through an exit:

people trying to leave via the fire exit

The same is true for a space vehicle. With a small exit the air molecules will take time to pass through the very small hole. If the hole is small enough the lost air can be replaced. However, if the whole is large enough the the air will escape more rapidly:

Dave Bowman blowing the airlock door in 2001

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    $\begingroup$ @Greenhorn Big room, small hole. Very small hole. $\endgroup$
    – geoffc
    Commented Dec 20, 2020 at 12:17
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    $\begingroup$ @Greenhorn - may be worth chasing down the early mythbusters episode on airliner decompression to separate movie from reality in decompression effects, took them blowing an entire window out with a shaped charge to get 'explosion' type effects in the cabin and even there the plane was structurally fine other than the missing window. Smaller things like bullet holes just made a lot of noise. $\endgroup$ Commented Dec 20, 2020 at 12:23
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    $\begingroup$ @Greenhorn was thinking of linking that one, per the investigation section that was internal pressure tearing open a fatigue failure to make a big hole, not a pinhole that magically turned into an aircraft with a sunroof. Aloha airlines would have been fine if the structure around where the hole started had been sound - so yes you are right that if this leak is a weak point being cycled open and propagating down a seam it could fail in a similar very exciting way. If it is in otherwise sound material this is as bad as it will get. $\endgroup$ Commented Dec 20, 2020 at 13:02
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    $\begingroup$ @Greenhorn - my completely uninformed guess based on them not having found it and having chased pressure leaks in more boring circumstances is that they have a threaded fixture of some sort, possibly on a valve and air is working down the length of the thread and out - so cross sectional area fraction of a mm and a long path - easy enough to find if you can dunk the offending part in water and look for the faint trail of bubbles, not so much here! $\endgroup$ Commented Dec 20, 2020 at 13:21
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    $\begingroup$ @geoffc damn, you beat me to it. That in seven words is all the answer that's needed. $\endgroup$
    – RonJohn
    Commented Dec 21, 2020 at 1:24

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