Can you harvest the Moon for energy?

Note: these articles have been published in InfoChem, the supliment to Education in Chemistry produced by The Royal Society of Chemistry.

In the film Moon [1], Earths resources of oil and coal have dwindled and nuclear fusion provides the power. The fusion fuel is an isotope of He (32He or 3He: two protons and one neutron) which is mined on the Moon and then sent back to Earth. So is 3He fusion feasible and would it ever be cost effective to go 250,000 miles to the Moon to get it?!

Perhaps the most important natural fusion process is that taking place within our Sun but this is not an easy process to replicate in the lab because of the very high temperatures needed to overcome the repulsion of the positive nuclei. If you are going to attempt fusion there are actually other reactions that are preferable because they produce more energy. For example fusion of deuterium (21H, an isotope of hydrogen extractable from sea water) with 3He promises even more energy:

Equation 1. 32He + 21H → 42He + 11P + neutrinos + 2.9 x 10-12J

Unfortunately 3He is very limited on Earth but it is thought that the Moon may store useful quantities trapped in the dusty mantle covering the surface (the regolith). Roughly 1 ton of 3He might be extracted from every 100 million tonnes of regolith [2]. In the film we see machines running along the surface of the moon scooping up the regolith and heating it up to remove and store the 3He gas. As everything is automated it only requires one human to oversee the plant. In the film Sam (Sam Rockwell) plays the moody caretaker on the lonely lunar 3He extraction plant. His job is to collect the 3He storage tanks from the automated collectors and send them back to Earth via small spaceships.

In the film the tanks look about 1 meter long (h = 1m) and about 25cm wide (d = 0.25m). Modern cylinders can take pressures of about 300 bar (about 300 times the Earth's atmospheric pressure) but let us say that in the future the tanks can withstand 1000 Bar pressures, how much 3He are we talking about in one tank and what would it worth?

The tank volume is V = πr2h = π(d/2)2h = π(0.125)2 x 1 = 0.05 m3 which at STP would be equivalent to 1000 x 0.05 = 50 m3 = 50,000 dm3 of 3He. now 1 mole = 22.4 dm3 which means we have 2200 moles of 3He amounting to 2200 x 3 = 6600g = 6.6 kg of 3He per tank. Equivalent to 2200 x 6.022 x 1023 = 1.32 x 1027 atoms / tank.

Eqn. 1 give us the energy per reaction giving a total energy of E = 2.9 10-12 x 1.32 x 1027 = 3.9 x 1015 J per tank (Note: but some energy may be lost via neutrinos). Now the current world power consumption is about 16 Terra Watts which is 16 x 1012 J / sec = 1.38 x 1018 J / day. On this basis the world would need about 400 tanks per day but according to one estimate only one full space shuttle load of 3He would be needed to provide the USA's yearly energy needs [3]. 1 barrel ($80) of oil is equivalent to 6 x 109J so (ignoring the deuterium costs) a single tank of 3He would be equivalent to 640,000 barrels, or about 50 million dollars worth! It's early days to know exactly what the technical challenges would be trying to automate such a process on the Moon. However as resources get used up on Earth such a valuable fuel, even if it were on the Moon, would indeed start to look attractive.

[1] Moon, Sony Pictures Home Entertainment Inc. 2009
[2] Wiki entry for 3He:
[3] The Artemis Project


How teachers can use these articles in a lesson

Why Hollywood Science

Open University Hollywood Science web site

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Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ.

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