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Material science
‘It costs a million dollars per tonne to ship material to the Moon, so it would be much cheaper to generate the oxygen on the Moon for breathing
and for propulsion.’ Derek Fray University of Cambridge, UK
that cell at 960°C, there are no external heaters; it just uses the current between the electrodes.’
Sadoway explains that the maximum
temperature is in the centre of the cell, while at the walls the temperature can be below the melting point of the electrolyte. ‘You can have liquid electrolyte and liquid metal lying inside a frozen skull of electrolyte and metal. You don’t have containment issues, which is really elegant. People look at this and say it’s insane, you are operating at 1600°C. But it is not an isothermal reaction.’ Sadoway adds that when you look at the composition of lunar regolith it varies little. Scientists hypothesise that the Moon formed when a big mass of Earth was flung into orbit. ‘As this liquid blob solidified it did so in a way that it is graded in terms of melting points. So the outer surface of the Moon is nearly of uniform composition. I am willing to bet that as you drill down into the Moon you would find that there is a gradation in composition from the outer surface to the core,’ says Sadoway. ‘The composition at the surface is a few per cent iron, some silicon dioxide, some aluminium oxide and a little bit of titanium dioxide.’ The basicity of the material – that is the amount of free oxygen versus chemically bonded oxygen – varies little across the Moon, based on samples analysed so far. ‘For processes proposed for reducing regolith into oxygen with hydrogen or carbon monoxide, such variations have a huge impact on their efficiency and yields. With MOE, it’s not an issue.’
Sadoway reasons that a plurality of
$100,000 per kilo – the cost to move mass from Earth’s gravity to the Moon
metals results the longer the electrolysis is run. ‘Initially you make iron, then once iron oxide has been depleted, the reaction on the negative electrode switches to production of silicon. We determined that the most efficient operation would deplete about half of the charge, and that would leave you with a liquid ferro-silicate alloy,’ Sadoway explains. The silicon could be used to make photovoltaic (PV) arrays and iron can be used as a structural metal. Since the Moon has no atmosphere and no moisture, ‘plain iron is tantamount to stainless steel,’ says Sadoway, ‘as there are no agents on the Moon to corrode the iron’. That gives you PV arrays for energy and metals for building. Taylor comments: ‘These processes being developed will become important
and I think they are the way of the future. But I don’t think we will be able to take what we want to the Moon. We need to get a gas station there. You spend 85% of the mass of a rocket just to get away from Earth’s gravity and it’s all fuel. If we could stop on the Moon, we could then go on to Mars. We could make water, oxygen and other valuable resources.’
Moon rocks are mainly oxides, so it should be possible to build a reactor that provides a structural metal and oxygen gas
However, the cuts to NASA’s budget coupled with the success and interest in robotic explorers have led to a decline in interest by NASA for these lunar reactors. Sadoway comments: ‘We made really good progress and just as it seemed we were getting to successful output at a large scale, the funding dried up.’ Fray is more optimistic: ‘Enthusiasm for the programme diminished, but I think they are starting to look at it again.’ In response, NASA says it ran a competition in the mid-2000s for teams to generate breathable oxygen from simulate lunar soil. The challenge expired in June 2009, with the purse of $1m unclaimed. There simply wasn’t enough interest in the challenge and the funds were moved to another competition. In a recent project, NASA is seeking to use water on the Moon and Mars to produce oxygen and other materials (
http://www.nasa.gov/exploration/ systems/ground/
resolverover.html) but Fray points out that much of the water on the Moon is buried as ice in the polar regions, places where it would be extremely difficult to generate energy from solar cells. Some of the water lies in craters at temperatures of -230°C or less, which will make its mining and melting energy-consuming. Fray and Sadoway believe space policy has shifted; it is no longer fashionable to view the Moon as a jump off point. Space rovers and robots reign supreme, leaving some disappointed chemists in their contrails. If the winds shift again, ideas for oxygen reactors will be of interest. Whether they will come from the US or emerging space exploring nations like India or China remains to be seen.
Anthony King is a freelance science writer based in Dublin, Ireland
Chemistry&Industry • November 2013 35
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