One of the great mysteries in astrophysics surrounds the origin of the highest energy particles ever observed. These particles, called ultra-high energy cosmic rays, come from space and smash into the Earth with so much energy that physicists have struggled to believe, let alone explain, it.

An ultra-high energy cosmic ray can have an energy of 10^20 electron volts. To put that in context, that’s a single proton with the same energy as a baseball flying at 100 kilometres per hour.

It might come as some relief to know that these particles are extremely rare. Physicists detect them on Earth at a rate of less than one particle per square kilometre per century. And that makes them difficult to study.

So physicists want to study more of these particles to work out where they come from and how they might form. The obvious approach is to build bigger detectors. The largest on Earth is the Pierre Auger Observatory in Argentina that covers an area of 3000 square kilometres, about the size of Rhode Island or Luxembourg.

Clearly, finding a significantly larger area of the Earth for a bigger detector is no easy task. So scientists are turning their attention to the heavens.

Their idea is to exploit an exotic physical effect that turns the entire Moon into a detector for ultra-high energy cosmic rays. Today, Justin Bray at the University of Southampton and a few pals outline the plan.

This is no pie in the sky project. Their plans are already drawn up and the €1.5 billion budget is in place. They plan to start construction of the necessary equipment in 2018 and be in full operation by 2025.

So what’s the big deal about ultra-high energy cosmic rays? The biggest mystery is how a single particle can have such high energy.

Physicists think there are essentially two possible mechanisms. The first is that the particles are accelerated in an electric or magnetic field. But nobody is sure where such extreme fields exist or how they might trap a particle long enough to accelerate it to these energies.

The second possibility is that the ultrahigh energy particles are created by the decay of a hypothetical supermassive exotic particle, perhaps dark matter or perhaps produced by topological defects early in the universe.

One way to pinpoint this mechanism is to discover the source of these particles, by finding where in the sky they come from. That is easier said than done because cosmic rays are charged and are therefore bent by magnetic fields as they travel. So the direction of arrival does not necessarily indicate the source.

Having said that, there is another effect that ought to prevent the highest energy cosmic rays reaching us at all. High-energy particles should interact with the cosmic microwave background radiation as they travel through space and this should cause them to lose energy. That suggests the highest energy particles were probably created within our galaxy since they could not have travelled intergalactic distances and remained so energetic.

So where does the Moon come into all this? On Earth, physicists detect these high-energy particles when they smash into the upper atmosphere triggering a cascade of other particles that rain down on the surface. This is how the Pierre Auger Observatory works— by detecting the daughter particles created in the cascade.

These cascades also generate another signal. The rapid acceleration and deceleration of charged particles produces radio waves. So another signature of the impact of an ultra-high energy cosmic ray is a brief burst of radio waves, known as the Askaryan effect after the Soviet-American physicist who proposed it in the early 1960s.

It is this signal that astronomers hope to pick up from the Moon. The idea is that ultrahigh energy cosmic rays should smash into the lunar surface generating a cascade of other particles and a short burst of radio waves less than a nanosecond long.

This effect is complicated by the fact that radio pulses are projected forward in a cone and cannot travel far through the lunar surface before being absorbed.

That means that astronomers will only be able to see the radio pulses from ultrahigh energy cosmic rays that graze the edge of the Moon coming our way.

So the gear they need to detect the signal is a highly sensitive radio telescope on Earth. These signals are so short and faint that the current generation of radio telescopes cannot pick them up.

But astronomers are about to start work on a much bigger and more sensitive radio telescope called the Square Kilometre Array, which will be built in South Africa and Australia at a cost of about €1.5 billion. This will give them access to more data about ultrahigh energy cosmic rays than they have ever had.

Although there are limitations on the lunar detector, it is still sizeable. Bray and co estimate that it will be equivalent to a ground array of 33,0000 square kilometres or about the size of Maryland or Belgium. That is more than 10 times larger than the Pierre Auger Observatory. And they say the Array should detect around 165 ultra-high energy cosmic rays a year from the Moon compared to the 15-a-year currently observed.

That suggests an exciting time ahead for radio astronomers and for the astrophysicists attempting to understand the origin of these mysterious particles. With any luck, they should soon be able to tease apart the extraordinary events that somehow create the most energetic particles ever observed.

Ref: arxiv.org/abs/1408.6069 : Lunar Detection Of Ultra-High-Energy Cosmic Rays And Neutrinos