Alex McColgan

The first substance is known as helium-3.

You may have heard of helium-3 in sci-fi stories, as theories suggest it is the ideal substance for a clean type of nuclear reactor, with no radiation and no dangerous byproduct.

It also has uses in medicine and radiation detectors.

However, it is really rare on Earth.

It does occur naturally and can be found in deposits of natural gas for instance, but it's generally not viable to extract, as even in natural gas, there are only around 100 parts per billion.

So let's say we had 1 billion cubic metres of natural gas.

You'd only be able to extract around 15kg of helium-3 from it.

A lot of the time, that's not economically viable.

We can also produce helium-3 as a by-product of the radioactive decay of tritium.

The problem with that though is that tritium is a crucial component of nuclear weapons, and so when the world slowed down the production of nuclear weapons, helium-3 stockpiles also started to diminish.

Assuming we don't want more tritium in the world, it means we need to find another source of helium-3, especially if technology improves enough for helium-3 reactors to become a reality.

Fortunately, we have a world in orbit around Earth right now, which has been bombarded by helium-3 for billions of years thanks to the Sun.

Earth's magnetic field deflects helium-3 travelling with the solar wind around the planet, whereas the Moon, with no magnetic field for protection, simply absorbs it in the top layer of the ground, called regolith.

We aren't talking huge quantities, it has at most 50 parts per billion, but because it's all over the moon, not just in tiny pockets, it can be collected alongside any other mining operation.

It could also be used to power reactors on the moon itself, which would help a moon base be self-sufficient.

Some people think that helium-3 mining on the moon will not be viable, however China states that eventually mining helium-3 is one of the primary goals of their Chinese lunar exploration programme.

American, European and Indian scientists have all stated it is something they will consider further, and Russia is conducting a feasibility study on this right now.

Even private companies are eyeing up the possibility.

Because the parts per billion of helium-3 are relatively low, even in the moon's regolith, it would make sense that whoever was mining for helium-3 would also be mining for something else in the regolith at the same time.

But what else can be found in it?