From a small hill in the southern French region of Provence, you can see two suns.
One has been blazing for four-and-a-half billion years and is setting. The other is being built by thousands of human minds and hands, and is, far more slowly, rising.
The last of the real sun's evening rays cast a magical glow over the other – an enormous construction site that could solve the biggest existential crisis in human history.
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It is here, in the tiny commune of Saint-Paul-lez-Durance, that 35 countries have come together to try and master nuclear fusion, a process that occurs naturally in the sun and all stars, but is painfully difficult to replicate on Earth.
Fusion promises a virtually limitless form of energy that, unlike fossil fuels, emits zero greenhouse gases and, unlike the nuclear fission power used today, produces no long-life radioactive waste.
Mastering it could literally save humanity from climate change, a crisis of our own making.
If it is mastered, fusion energy will undoubtedly power much of the world. Just 1 gram of fuel as input can create the equivalent of eight tons of oil in fusion power. That's an astonishing yield of eight million to one.
Atomic experts rarely like to estimate when fusion energy may be widely available, often joking that, no matter when you ask, it's always 30 years away.
But for the first time in history, that may actually be true.
In February, scientists in the English village of Culham, near Oxford, announced a major breakthrough: they generated and sustained a record 59 megajoules of fusion energy for five seconds in a giant donut-shaped machine called a tokamak.
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It was only enough to power one house for a day, and more energy went into the process than came out of it. Yet it was a truly historic moment. It proved that nuclear fusion was indeed possible to sustain on Earth.
This was excellent news for the project in France, the International Thermonuclear Experimental Reactor, better known as ITER. Its main objective is to prove fusion can be utilised commercially. If it can, the world will have no use for fossil fuels like coal, oil and gas, the main drivers of the human-made climate crisis.
There has been a huge sense of momentum at ITER since the success in the UK, but the people working on the project are also undergoing a major change. Their director general, Bernard Bigot, died from illness on May 14 after leading ITER for seven years.
Before his death, Bigot shared his infectious optimism for fusion energy from his sunny office, which overlooked the shell of ITER's own tokamak, a sci-fi like structure still under construction.
"Energy is life," Bigot said.
"Biologically, socially, economically."
When the Earth was populated by less than a billion people, there were enough renewable sources to meet demand, Bigot said.
"Not anymore. Not since the Industrial Revolution and the following population explosion. So we embraced fossil fuels and did a lot of harm to our environment. And here we are now, 8 billion strong and in the middle of a drastic climate crisis," he said.
"There is no alternative but to wean ourselves off our current main power source," he said.
"And the best option seems to be the one the universe has been utilising for billions of years."
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Mimicking the sun
Fusion energy is created by forcing together two particles that, by nature, repel. After a small amount of fuel is injected into the tokamak, giant magnets are activated to create a plasma, the fourth state of matter, which is a bit like a gas or soup that is electrically charged.
By raising temperatures inside the tokamak to unfathomably high levels, the particles from the fuel are forced to fuse into one. The process creates helium and neutrons, which are lighter in mass than the parts they were originally made of.
The missing mass converts to an enormous amount of energy. The neutrons, which are able to escape the plasma, then hit a "blanket" lining the walls of the tokamak, and their kinetic energy transfers as heat. That heat can be used to warm water, create steam and turn turbines to generate power.
This all requires the tokamak to contain serious heat. The plasma needs to reach at least 150 million degrees Celsius, 10 times hotter than the core of the sun.
It begs the question: How can anything on Earth hold such high temperatures?
It's one of many hurdles that generations of fusion energy seekers have managed to overcome. Scientists and engineers designed giant magnets to create a strong magnetic field to keep the heat bottled up. Anything else would simply melt.
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What those working on fusion have been trying to do inside their machines is essentially replicate the sun. The sun is a perpetual fusion factory, made up of a gigantic burning ball of plasma. It fuses several hundred tons of hydrogen into helium each second.
Plasma is the stuff 99.9 per cent of the universe is made of, including the stars, our sun and all interstellar matter. Down here on Earth, for instance, it's used in televisions and neon lights, and we can see it in lightning and the aurora.
As awesome as that all sounds, generating fusion energy in itself isn't actually the hard part, several experts at ITER said.
Humanity has been pulling off nuclear fusion reaction ever since the invention of the H-bomb, after all. The main challenge is sustaining it. The tokamak in the UK, called the Joint European Torus, or JET, held fusion energy for five seconds, but that's simply the longest that machine will go for. Its magnets were made of copper and were built in the 1970s. Any more than five seconds under such heat would cause them to melt.
ITER uses newer magnets that can last much longer, and the project aims to produce a 10-fold return on energy, generating 500 megawatts from an input of 50 megawatts.
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But ITER's goal isn't to actually use the energy for power but to prove that it can sustain fusion energy for much longer than JET was able to. Success here will mean commercial-scale machines can start generating fusion in the future.
While the sun fuses hydrogen atoms to create helium, the JET project used two hydrogen isotopes called deuterium and tritium, which ITER will also use. These isotopes behave almost identically to hydrogen, in terms of their chemical makeup and reactions.
Both deuterium and tritium are found in nature. Deuterium is abundant in both fresh and saltwater – the deuterium from just 500ml of water, with a little tritium, could power a house for a year. Tritium is rare, but it can be synthetically produced.
At the moment, only 20kgs of it exist in the world, and demand amounts to no more than 400g per year. But at a yield of eight million to one, only tiny amounts of both elements are required to generate a lot of fusion energy.
Tritium is an exceptionally pricey substance: a single gram is currently worth around $30,000. Should nuclear fusion take off, demand will go through the roof, presenting the world's fusion masters with yet another challenge.
A 10 million-part project
From afar, ITER looks like a project ready to go. From up close, it's clear it's still a ways off.
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The construction across 39 building sites is incredibly complex. The main worksite is a markedly sterile environment, where tremendous components are being put into place with the help of 750-ton cranes. Workers have already put together the shell of the tokamak, but they are still awaiting some parts, including a giant magnet from Russia that will sit at the top of the machine.
The dimensions are mind-blowing. The tokamak will ultimately weigh 23,000 tons. That's the combined weight of three Eiffel towers. It will comprise a million components, further differing into no fewer than 10 million smaller parts.
This powerful behemoth will be surrounded by some of the largest magnets ever created. Their staggering size – some of them have diameters of up to 24 meters – means they are are too large to transport and must be assembled on site in a giant hall.
Given the huge number of parts involved, there's simply no room for error.
Even the digital design of this enormous machine sits across 3D computer files that take up more than two terabytes of drive space. That's the same amount of space you could save more than 160 million one-page Word documents on.
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Time is running out
The scale and ambition of the ITER project may seem enormous, but it is, at the very least, a proportional response to the mess humans have made of the planet. Since 1973, global energy usage has more than doubled. By the end of the century, it might actually triple.
Seventy percent of all carbon dioxide emissions into the atmosphere are created through humans' energy consumption. And 80 per cent of all the energy we consume is derived from fossil fuels.
Now, the Earth is barrelling toward levels of warming that translate into more frequent and deadly heat waves, famine-inducing droughts, wildfires, floods and rising sea levels. The impacts of the climate crisis are getting harder and harder to reverse as entire ecosystems reach tipping points and more human lives are put on the line.
The world is now scrambling to rapidly decarbonise and speed up its transition from planet-baking fossil fuels to renewable energy like solar, wind and hydro power. Some countries are banking on nuclear fission energy, which is low-carbon but comes with a small, but not negligible, risk of disaster, storage problems for radioactive waste and a high cost.
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But there are serious questions about whether the world can make this green transition fast enough to avert catastrophic climate change.
That's where fusion could be an 11th-hour hero, if the world masters it in time.
When the late physicist Stephen Hawking was asked by Time in 2010 which scientific discovery he would like to see in his lifetime, he pointed to exactly this process.
"I would like nuclear fusion to become a practical power source," he said.
"It would provide an inexhaustible supply of energy, without pollution or global warming."
