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How do astronauts breathe in space?

Astronauts on the International Space Station breathe air almost identical to the stuff we breathe on Earth. Dr Karl explains where it comes from.

According to the advertising blurb for the 1979 movie, Alien: "In space no one can hear you scream." This makes sense, because there's no air in space.

But inside the International Space Station (ISS), the American astronauts and Russian cosmonauts do breathe air almost identical to the stuff we breathe down here on planet Earth - same pressure and about 80 per cent nitrogen and 20 per cent oxygen.

But from where do they get their oxygen?

It turns out they get it by 'splitting' H 2 O with electricity. But, like everything to do with space travel, it's tricky.

The ISS has been continually inhabited by humans since November 2, 2000. They're mostly astronauts from the USA and cosmonauts from Russia, but there have been many other spacefarers from some 13 different countries.

When they go to sleep, the inhabitants of the ISS have to make sure that they are in a well-ventilated area. If there is not good airflow, the carbon dioxide they breathe out will accumulate around their heads.

On Earth, the expired air from our lungs is usually warmer than the ambient air. When you combine this difference in temperature with Archimedes' Principle, the warm air rises and the cooler air falls.

But this does not happen in the microgravity of space.

Before they discovered this, the spacefarers would sometimes wake up gasping for oxygen. On the other hand, it's better to wake up gasping, than to not wake up at all.

The ISS is huge — you can easily see it with the naked eye from most capital cities. It weighs about 450 tonnes. It's about 108 metres wide (about two Olympic swimming pools), about 73 metres long and about 20 metres high.

It orbits the Earth roughly every 93 minutes, travelling at around 27,700 kilometres per hour. It's surprisingly close in its low-Earth orbit — only some 330 - 435 kilometres above the ground.

Now, while my home solar cells generate only 4.5 kilowatts, the ISS's massive solar cells generate around 131 kilowatts — but only for slightly more than half of each orbit while the ISS is in sunlight. When it's in the Earth's shadow (some 35 minutes), electricity comes from its rechargeable nickel-hydrogen batteries.

It's this electricity that turns water into hydrogen and oxygen. There are two modules that do this. The Russian one is called 'Elektron' while the American one is called the 'Oxygen Generating System'.

The equation is as about as simple as you can get: 2H 2 O >>>> 2H 2 + O 2

While the oxygen is recycled into the atmosphere of the ISS, the hydrogen used to be dumped overboard. However it is now combined with carbon dioxide (which is breathed out by the inhabitants and captured by CO 2 scrubbers) to make water and methane. This happens inside a closed unit.

The equation for those dying to know is:

4H 2 + CO 2 >>>> 2H 2 O + CH 4

The water is recycled into hydrogen and oxygen.

Quite separately, nasty human aromas, such as methane from farts, and ammonia from sweat, are removed by activated charcoal filters.

So solar power enables the process of electrolysis of water, which in turn provides about 5.4 kilograms of oxygen per day for four people. However, it can be ramped up to 9 kilograms when there are six inhabitants.

The water recovery systems catch water from showers, sinks, and water vapour in the air, and then feed it into the oxygen generators.

Urine is also processed to provide more water.

Originally, the systems were supposed to be able to recover 85 per cent of the water in the collected urine. But the microgravity conditions of the ISS caused massive loss of bone density in the spacegoers, which led to very high calcium levels in the urine. This reduced the water recovery to 70 per cent.

Of course, there are backup oxygen systems. First, there are solid fuel oxygen generator canisters, similar to those on commercial jets. Each canister of lithium perchlorate will provide enough oxygen for one person for one day. And, as a back up to the back up, there are also some emergency cylinders of bottled compressed oxygen.

It's amazing just how complicated it is to copy what plants do for us for free …

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