Field of Science

The Moon, where the Helium-3 from the Sun is

Moon is a 2009 science fiction film about astronaut Sam Bell who is the solitary worker on the moon. Sam is at the end of a three-year stint on the Moon so the film begins as if it was the denouement of another quieter story. When an accident occurs Sam suddenly meets himself for the first time.

I am adept at finding flaws in science fiction films, but Moon nails a lot of science as well as could be expected. The most incredulous point about the film for me was the lack of a radio array on the far side of the moon, I mean why else would we go to the moon? There is a very good and scientifically feasible answer for this. The movie begins, as you can see above with the first seven minutes of the film, with a commercial by Lunar Industries:
There was a time when energy was dirty word, when turning on your light was a hard choice. Cities in brown out, food shortages, cars burning fuel to run, but that was the past, where are we now? How did we make the world so much better? Make deserts bloom? Right now we're the largest producer of fusion energy in the world. The energy of the sun trapped in rock harvested by machine from the far side of the moon. Today we deliever enough clean burning helium-3 to supply the energy needs of nearly 70% of the planet. Who would have thought all the energy we ever needed, right above our heads? The power of the moon, the power of our future.
When I saw this at the beginning of the film I was delighted that they had based the story on a kernel of truthful science. The energy source they are gathering from the moon is Helium-3 (3He), but they aren't exactly burning it for fuel as they say. Helium-3 is a light isotope of helium with two protons and one neutron which is suitable as a fusion fuel. I have done some research into the literature to determine just how feasible this 3He mining on the moon is with two specific questions in mind. Why use 3He? Why go to the moon?

An advanced fusion reactor would combine 3He and deuterium (2H) in a fusion reaction to produce a helium-4 nucleus (4He) and a high-energy proton. Energy is released as charged particles and that is what powers the world in this science fiction vision of the future. 3He fuel offers some advantages over other types of fusion fuel because it is efficient and because less radioactive byproducts are produced (it is often stated or assumed that fusion power does not create any dangerous materials, but in reality the reactor housing can become radioactively activated and remain so for a number of years. However, the time scale on which it remains active is comparable to human lifetimes and is overall not as dangerous as the byproducts of fission reactors). 3He is very scarce on earth. It is possible to manufacture 3He on earth though the neutron bombardment of certain elemental targets which results in tritium, which then decays to 3He with a half-life of 12 years. This is a complicated, dangerous and inefficient process so other sources would be required to make 3He a viable fusion fuel.

The cosmological abundance of 3He is paltry, but its abundance and chemical evolution is of interest to astronomers because it can be a tracer of various stellar phenomena so it has been studied for many years. The primordial cosmological ratio of 3He to 4He is ~1.4 × 10 -6, however this abundance can be thousands of times greater in the solar wind. The solar wind is primarily protium traveling at a velocity of ~450 km/s with a flux of ~6 × 10 10  ions/(ms). Of this flux there is ~4% He which has an unusually high ratio of 3He to 4He of ~480 atomic parts per million (Heber V. 2003). 3He should be abundant on the moon's surface of regolith where it has been deposited by solar wind over billions of years.  Hence, we could go to the moon and mine it. However, to gather enough fuel to power the earth at current energy consumption rates more than one Space Shuttle load and the processing of 4 million tons of regolith per week, on the lunar surface, would be necessary. Further, to really nail the science here I cite Fa and Jin who state:
The [lunar] inventory of 3He is estimated as 6.50×108 kg, where 3.72×108 kg is for the lunar nearside and 2.78×108 kg is for the lunar far side.
There is a bounty of lunar fuel available, but I wouldn't place my base on 'the far side of the moon' as Lunar Industries states they have done because it would be cold, there is less 3He, and it would be, well, lonely. In conclusion I have found the academic literature validates the idea that Helium from the moon could power terrestrial fusion reactors one day.

Moon, Helium 3, fusionApparently Sam Bell is working alone on the moon to cut costs for the company. In order to mine the necessary amounts of 3He with minimal overhead costs Lunar Industries has chosen a one man job; and based on the size of the lunar regolith harvesters seen in the movie 4 million tons processed per week would not be unfeasible. As the movie continues themes of alienation and societal deception emerge. I have discussed some science that the film never divulges, but in fact, the film never even mentions anything about 3He or the reasons why any of this is going on again. This is a strong point for the film which actually raises deep philosophical questions, which I could dive into, but I don't want to spoil it for anyone. It is a great film and not the craziest science, really:

Online resources:
Mining the Moon from Popular Mechanics
Lunar 3He and Fusion power by J. Santarius
Non-Lunar  3He Resources by L. Wittenberg
Moon for Sale from the BBC

FA, W., & JIN, Y. (2007). Quantitative estimation of helium-3 spatial distribution in the lunar regolith layer Icarus, 190 (1), 15-23 DOI: 10.1016/j.icarus.2007.03.014

Heber, V., Baur, H., & Wieler, R. (2003). Helium in Lunar Samples Analyzed by High‐Resolution Stepwise Etching: Implications for the Temporal Constancy of Solar Wind Isotopic Composition The Astrophysical Journal, 597 (1), 602-614 DOI: 10.1086/378402

1 comment:

  1. The science in the film was good enough to allow suspension of disbelief, and that's much better than most science fiction movies. I liked it.


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