This post was originally posted on blogspot.com on February 22, 2012
In another post I have presented arguments against returning to the Moon. Although more and more people are discovering that the prospect of mining Near Earth Asteroids is superior to Lunar mining activities, there are still people, like Newt Gingrich, who believe that humans should establish a base on the Moon. As Eric Drexler argues, returning to the moon is a waste of money.
Drexler presents several arguments why NEAs are superior to the Moon, but unfortunately many people are ignorant about this. From an economical point of view lunar mining is not able to compete with near asteroid mining, because the latter has lower energy requirements. Why are people still talking about mining and colonizing the Moon? Well, there are many reasons, most of them are covered by Drexler in his article, but there is just one reason (which is not considered by Drexler) for mining on the Moon which really deserves to be examined.
This reason is the fact that the Moon possesses a large amount of helium 3 in its regolith. Helium 3 is proposed by some nuclear fusion researchers as a fuel for Terrestrial fusion power plants. One problem for using helium 3 in terrestrial fusion reactors, is that helium 3 is very rare on our planet. It is estimated that the lunar soil contains enough helium 3 to power the world for several centuries (Lewis, p.138).
Due to their small sizes, Asteroids are less likely to possesses significant amounts of helium 3. So helium 3 provides a clear reason for colonizing the Moon. However, we should ask ourselves whether it makes sense to invest large sums of money in the lunar helium mining industry. We have to consider whether helium 3 can play a role in terrestrial energy supply.
One of the more fundamental problems with extracting helium 3 from the lunar soil, is the very low concentration. In other words for each ton of helium 3 we need to process one hundred million tons of lunar rock. This give us a concentration of 10 ppb (parts per billion). This low concentrations can be compared to concentration of minerals in seawater, this article shows that extracting uranium from seawater is not practically with current technology. This is primarily due to the fact that extracting an element from a low-grade substance (whether an ore or seawater) requires large amounts of energy: the lower the concentration, the higher the energy requirements.
One counter argument, is the fact that helium 3 as a higher specific energy than uranium. But on the other hand, because helium 3 is a gas with a very low density, while uranium is a heavy element, there is a great risk of leakage. In any extraction process a small amount of the material to be extracted is lost, in case of helium 3 mining on the Moon, without proper care much helium will escape the Moon in no time. Therefore one must invest much in preventing helium leakage. The question remains whether lunar helium 3 mining could ever be a profitable enterprise, personally I don’t believe so.
Due to high extraction costs and limited reserves on the Moon, some proponents of terrestrial helium 3 fusion have searched for other sources of helium 3. On Earth only dismantling of hydrogen bombs provides any significant supply of this, and only in very limited amounts. In order to power the global economy, we need much more helium 3. Some propose mining the surface of Mercury for helium 3, in a similar fashion as the Moon. They argue that since the first planet has a greater surface and a lesser distance to the Moon, it may possibly possesses large deposits of this fuel. Although Mercury’s magnetic field may have prevented the absorption of any Solar wind particles, so is currently far from certain whether helium 3 could actually mined there. Even if there is helium 3 on Mercury, there are basically two problems with this: 1. Because Mercury is closer to the Sun it is hard to send spacecrafts to and from it (due to the Sun’s gravity) and 2. Daytime temperatures are very high (although due to the planets slow rotation the night is many terrestrial days long).
For these reasons, not much people seriously consider helium 3 extraction on Mercury as a viable solution for our energy problem. In the space movement most attention, regarding helium 3, is payed to gas giants. Especially Uranus is popular. Of all the four gas planets, Uranus has the lowest escape velocity (Neptune’s is similar, but this planet is farther away) and is therefore more attractive for “mining” helium 3. John Lewis has shown that helium 3 can easily be extracted from the atmosphere of Uranus and be transported back to Earth.
Uranus has both larger reserves and concentration of helium 3 in its atmosphere, so extracting it is rather straightforwardly. Planetary scientist John Lewis has proposed to use refrigeration to extract (since helium has the lowest boiling point of all elements). So the question remains whether helium 3 extraction from Uranus’ atmosphere is likely to become important for terrestrial energy demand.
One of the fundamental problems with importing helium 3 from the outer solar system, is that it take six years to transport anything from Uranus to Earth. Lewis argues (assuming future power demand at 80 TW) that we need 4000 metric tons of helium a year, this amount requires a large fleet of space craft cycling between Earth and Uranus. Because it’s very unlikely that one spacecraft can bring back 4000 tons, and one spacecraft has to travel twelve years to return the precious noble gas.
So a very huge investment is needed to start-up such enterprise, while there exist many cheaper competitors for terrestrial power production, like solar, high altitude wind and geothermal power. Therefore our conclusion is that helium 3 will probably never play a role in terrestrial power generation.
You have certainly noticed that I have emphasized terrestrial consumption of helium 3, for space colonies the situation is different. For colonies within the inner Solar system there is a lot of Solar power available, while in the outer parts solar radiation is much lower. This is due to the fact that the solar constant (the amount of power per squared meter at a given distance from the Sun) falls the square of the distant from the sun, Saturn for instance is tens times further away from the Sun than the Earth and it receives only one percent the amount of power we does.
So for colonists in the outer parts of our Solar system deuterium-helium 3 fusion is much more interesting than terrestrial people. Colonizing the outer Solar system is something we don’t expect before 2100 and we of Republic of Langrangia believe humans should first colonize Near Earth Objects, where we can depend on cheap and abundant Solar power.