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impurities are removed. A bed of charcoal soaks up smells. Fine filters trap floating particles. Cold plates like refrigerator coils condense moisture from the astronauts’ breath and feed the water into a tank. Carbon dioxide from the astronauts’ exhalations is more of a problem, and is usually absorbed in canisters of lithium hydroxide, then simply vented into space.
    In the Russian Elektron and American Oxygen Generation systems, new oxygen is created from water collected from the cold plates and from urine and waste washing. The water is split into hydrogen and oxygen by electrolysis, and the oxygen is supplied to the cabin crew, while the hydrogen is vented into space or made to react with waste carbon dioxide to produce methane and water, which can be used for washing, or for making more oxygen. Neither of these systems, though, has yet proved completely reliable, so keeping astronauts supplied with oxygen on long missions remains a problem. So you might be able to burn a candle in the space station, but it would prove a constant drain on the oxygen-generating systems because, unlike human breathing, it consumes oxygen as it burns without producing much water for recycling.
    There is a final twist to this question, because the International Space Station, like Mir before it, carries an emergency oxygen supply in the form of ‘oxygen candles’.Made of lithium or sodium perchlorate, these are flares that burn slowly to produce lithium or sodium chloride, iron oxide and oxygen. The candles burn for about six hours, and the International Space Station has 350 of them, enough to keep three people breathing for three months. So you can burn an oxygen candle in a spacecraft, and it could save your life! Even oxygen candles are not entirely safe, though. One burned out of control on Mir in 1997. Fortunately, no one was injured, although the space station was damaged. More tragically, it’s thought that an explosion of one of these candles caused the death of two sailors aboard the nuclear submarine HMS
Tireless
beneath the Arctic in 2007.

If I could fold this paper an infinite number of times, how many times must I fold it to reach the moon?
    (Physics and Philosophy, Oxford)
    The answer is about 43. You can work this out very roughly, knowing that the distance to the moon is a little less than 400,000 km and that a thin sheet of paper is about 0.1 mm, or 0.000001 km. You could double 0.000001 until you reach approximately 400,000 or halve 400,000 until you reach roughly 0.000001. The number of folds involved is actually surprisingly small because the thickness of the paper increases exponentially, doubling the thicknessincrease with each fold. I would take a little while to work this out mentally, without the aid of a calculator, but I just happen to know that it takes 51 folds to reach the sun from the earth, * and knowing that the moon is 400 times nearer than the sun, I can work out fairly instantly that it takes eight fewer folds to reach the moon. If you didn’t know, you’d simply have to work the answer out slightly more laboriously.
    Folding paper has actually been the subject of serious mathematical analysis for over half a century. Some of the interest, naturally enough, has come from those masters of paper-folding, the Japanese, and the basic mathematical principles or axioms for folding, covering multi-directional origami folds as well as simple doubling, were established by Japanese mathematician Koshiro Hatori in 2001, based on the work of Italian-Japanese mathematician Humiaki Huzita.
    Because of the exponential increase in thickness with each fold, it was widely believed that the maximum number of doubling folds possible in practice was seven or eight. Then in January 2002, American high school student Britney Gallivan proved this wrong in a project she did to earn an extra maths credit. First she managed to fold thin gold foil twelve times, and then, when some people objected that this wasn’t paper,