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On the Voskhod 2 mission, cosmonaut Alexey Leonov performed a spacewalk which ran into issues when Lenov's spacesuit had some sort of pressure buildup. From the wiki -

[Leonov] was forced to bleed off some of his suit's pressure, in order to be able to bend the joints, eventually going below safety limits.

How did Leonov bleed off the pressure? The way its worded makes me think he punctured his suit like in the final scene of The Martian... but that sounds kinda crazy

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    $\begingroup$ To clarify, Leonov's suit did not have "Some kind of pressure buildup." It was operating at its design pressure of 0.40 atmospheres. Presumably they knew that it would stiffen but believed Leonov would have the strength to power through; that proved to be a dangerously optimistic assumption. I wonder if they discreetly included the lower relief valve setting knowing it would likely be needed. Depending on how much ground testing they had done, that suit may have been a 0.40 atm EVA suit in name only. $\endgroup$
    – Kengineer
    Commented May 12, 2017 at 23:34

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According to Astronautix, Leonov's Berkut space suit had an open cycle environmental system with a relief valve, and he simply set the valve to a lower pressure setting:

Oxygen, used for both breathing and cooling, was contained in a metal backpack. A relief valve vented the suit into space, carrying away heat, moisture, exhaled carbon dioxide, and unconsumed oxygen. There were two relief valve pressure settings - 0.27 atmosphere or 0.40 atmosphere. Sufficient oxygen was carried for 45 minutes of depressurized activity. It was only worn on the Voskhod 2 flight, when Leonov had great difficulties in getting back into the airlock due to the suit's stiffness. It was only after switching to the lower relief valve pressure setting that, bathed in sweat, he was able to get the hatch of the inflatable airlock closed.

According to the Wikipedia article on the Sokol pressure suit -- the successor to the Berkut, sharing the same open cycle system and same relief valve pressures -- the low pressure setting is intended only for "extreme emergencies".

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  • $\begingroup$ Seems so! Isn't the minimum pressure where atmosphere of pure oxygen is ceases to be sufficient for breathing around 0.3atm? $\endgroup$
    – SF.
    Commented Mar 10, 2017 at 8:33
  • $\begingroup$ I think it's more like 0.2 (at least, composition of earth air is about 20% oxygen), but reading around I think decompression sickness rather than hypoxia was the danger for Leonov. $\endgroup$
    – Russell Borogove
    Commented Mar 10, 2017 at 15:28
  • $\begingroup$ @SF. There is, of course, a continuum. A pure oxygen atmosphere at 0.3 atm is actually a higher partial pressure of oxygen than sea level conditions. To contrast, the partial pressure of oxygen in Leadville, Colorado, at an altitude of 3.1 km, is less than half that. In this case, oxygen availability would not be the primary concern in the short term. There would be limits on exertion ability without producing hypoxia symptoms, but the primary concern would be from the decompression itself. $\endgroup$
    – Tristan
    Commented Mar 10, 2017 at 17:37
  • $\begingroup$ @Tristan: are you sure human ability to absorb oxygen remains unchanged relative to partial pressure, as total pressure changes, no deviation near the bottom limits? $\endgroup$
    – SF.
    Commented Mar 10, 2017 at 17:58
  • $\begingroup$ My understanding is that the chemistry involved in oxygen exchange is pretty much dependent on the partial pressure of oxygen alone. The presence or absence (particularly a sudden change thereof) of other inert gases will have physiological effects that may indirectly affect the process (pulmonary edema, for example), but it does not directly affect the chemistry behind respiration. $\endgroup$
    – Tristan
    Commented Mar 10, 2017 at 18:58

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