In the context of the 21st century lunar race, in which the United States, China and Russia are actively involved, helium-3 is often mentioned. It is understandable: the huge spending on space programs must be given at least the appearance of some tangible practical benefit. Helium-3 is called an element that can solve all the energy problems of mankind for millennia to come, and the Moon is the most promising place where it can be mined. This is what sometimes causes the awakened interest of the leading powers in the development of the satellite. But today, helium-3 should be considered as the fuel of the future with great reservations. And that’s why.
Three Or Four
So, what is helium-3 and why is it, in fact, three? Here you need to remember the school chemistry course. All chemical elements in this universe exist as isotopes. Isotopes are variants of the same element that have the same nuclear charge (and therefore the same number of protons), but a different mass number (since the nucleus has a different number of neutrons). The properties of isotopes can differ – the most famous difference to non-specialists is that isotopes can be stable and radioactive.
The helium we are interested in has nine isotopes, but only two of them are stable: helium-4 (its nucleus consists of two protons and two neutrons) and helium-3 (its nucleus has two protons and one neutron). Natural helium is made up of these two isotopes. Why is it necessary to look for it on the moon? The fact is that of all the helium that is on our planet, 99.999863% is helium-4. Both isotopes, due to their lightness, constantly fly away from our planet into space. But helium-4 reserves are replenished because of the alpha decay of natural radioactive elements like uranium or thorium (an alpha particle is essentially a helium-4 nucleus). But helium-3 is not formed in this way, and its entire supply on our planet appeared billions of years ago, when the Earth was just being formed. Since then, it has been gradually escaping from the mantle into the atmosphere, and from there into space.
But on other objects of the solar system, things are different. Helium-3 is brought to them with the solar wind — a stream of particles from our star. The earth is protected by the atmosphere and magnetic field, so helium-3 does not enter here. But in the lunar regolith (the surface layer of the lunar soil), the isotope accumulates perfectly. As a result, over the billions of years that our satellite has existed, significant (compared to terrestrial) reserves of helium-3 have accumulated there. They are estimated at 0.01 g per metric ton, and in total from 0.5 to 2.5 million metric tons. It seems that this is not very much, but there are only 35 thousand tons of it on the whole Earth.
But why do we need it at all?
The fact that helium-3 is formed on the Sun, in which thermonuclear reactions take place, hints at the possibility of using the isotope in a promising thermonuclear reactor. Theoretically, a fusion reaction between helium-3 and deuterium (a heavy isotope of hydrogen) is possible. The use of such fuel has several advantages over the traditionally considered deuterium-tritium fusion reactions.
First, the problem of neutron radiation is reduced. When deuterium and tritium nuclei fuse, a large —neutron flux is released, which is not only dangerous for all living things and requires the installation of protection around the reactor, but also destroys the device itself. The neutron flux from the fusion of helium-3 and deuterium is ten times lower.
Secondly, the reaction product of helium-3 and deuterium is the isotope helium-4 and a proton. This proton can be caught by an electromagnetic field and used to generate electricity in a special generator.
Thirdly, such a reactor is safe. The fuel is not radioactive, and if an accident occurs, there are no emissions.
Fourth, because of such a reaction, a huge amount of energy is released. According to calculations, the fusion of 1 metric ton of helium-3 and 0.67 metric tons of deuterium is equivalent to burning 15 million metric tons of oil. If the estimates of the isotope content in the lunar soil are correct, then with the current consumption, helium-3 will be able to meet the needs of mankind for 5 thousand years.
Of course, not everything is so smooth. The temperature in the core must be above 109 degrees, otherwise the deuterium nuclei will fuse with each other, ignoring helium-3. In this case, due to radiation, the plasma will cool faster than it is heated due to the reaction. And this means that you will have to artificially turn on the heat, and it is quite difficult to do this at such temperatures. In other words, controlled thermonuclear fusion, in principle, has not yet been implemented on a scale that would imply commercial use. And the synthesis of helium-3 and deuterium is a task with an asterisk even for such a complex problem.
On the other hand, helium-3 production is also a technological problem. So far, mankind has been able to drive a car on the moon and plant the US flag there. To extract helium-3, it will be necessary to process millions of tons of lunar soil directly on the satellite (even if the isotope on the Moon is much larger than on Earth, its content is still no more than 0.01 g, or 0.0003 oz per metric ton).
In 2006, the then head of RSC Energia, Nikolai Sevastyanov, predicted that in 10 years (that is, in 2016) Russia would create a base on the Moon, and soon after that, helium-3 production would begin. The forecast turned out to be too optimistic.
Of course, in addition to production, there are also issues with transportation. We do not yet have a fleet of space tankers that could regularly make flights between the Earth and the satellite.
In general, everything looks so that the use of helium-3 as a fuel is not a prospect for the next few decades. And there, perhaps, we will not need to mine anything on the satellite. According to Doctor of Physical and Mathematical Sciences, Academician Lev Zeleny, if we master the industrially suitable technology of thermonuclear fusion, then we will be able to use boron, which is heaped on Earth, as fuel. And then helium-3 will simply become unnecessary for us.