How It Works


Spacesuit oxygen tanks can only provide a finite supply


The atmosphere within the ISS and other spacecraft is kept pretty close to that experience­d on Earth – about 78 per cent nitrogen and 21 per cent oxygen at one atmospheri­c pressure. But spacesuits operate at much lower pressures – typically about 30 to 40 per cent of atmospheri­c pressure – to make moving around in them easier. At such low pressure, normal atmospheri­c concentrat­ions of oxygen are not sufficient to maintain life, so spacesuits provide astronauts with 100 per cent oxygen.

With such high oxygen concentrat­ions, astronauts re-entering spacecraft can easily suffer from decompress­ion sickness, a common hazard for deep-sea divers. Without adequate equalising of pressures and gas concentrat­ions, this can be fatal, so throwing yourself into the airlock of a spacecraft, filling the cabin with an Earth-like atmosphere and whipping off your helmet is probably not advisable. However, if your spacesuit’s oxygen supply has just run out, you’d probably have no other option. Similarly, donning your spacesuit and pressurisi­ng to 30 per cent of atmospheri­c pressure without first breathing 100 per cent oxygen for half an hour to rid the blood of nitrogen could be a death warrant.

The oxygen tanks of a typical spacesuit can last an astronaut around six to eight hours, depending on their metabolic rate and their level of activity. Another half-hour emergency supply is also common as a backup measure. It wouldn’t be unreasonab­le to find an astronaut cavorting around space in their spacesuit some time after they became untethered.

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