The secret to all our energy needs is hidden in the stars.

Nuclear fusion is the process that powers the sun, releasing nearly four million times more energy than using coal, oil or gas and could mean an end to fossil fuel burning on earth forever.

Bringing that star power down to earth has been the dream of scientists for decades, but how far are we from that becoming a reality in our day-to-day lives?

It's tantalizingly close - fusion fuel is plentiful and easily accessible: deuterium can be extracted inexpensively from seawater, and tritium can be produced with naturally abundant lithium.

How does nuclear fusion work?

A couple of trucks full of nuclear fusion fuel could cover all our energy needs for millions of years and future fusion reactors will be much safer too, achieving ignition and maintaining the system is difficult so there's no risk of meltdowns, and no long-lived toxic nuclear waste byproduct either.

Nuclear fission is the process happening in our nuclear power plants today, splitting atoms to create energy, while fusion joins atomic nuclei together, smashing into each other under the force of massive heat and pressure, as happens in the sun, like two tiny stars colliding to form one. 

The light nuclei join together forming a heavy nucleus, but some of the mass converts to energy - lots of energy - four times as much as fission per kilogram of fuel.

In December 2022 US scientists at Lawrence Livermore National Laboratory (LLNL) in California achieved a historic first by successfully achieving nuclear fusion ignition, producing more energy from a reaction than the huge laser energy used to start it, leading to a net energy gain.

This breakthrough was a massive step towards fusion, mimicking the sun's power, and since then they have successfully gained "ignition" multiple times.

Once considered science fiction, fusion was proved possible, confirming a scientific dream decades in the making.

"Now we look into the future to make our second dream a reality, fusion energy for society," Carlos Hidalgo, Director of the National Fusion Lab, CIEMAT,  told CGTN.

"This is where we are today and this is going to be a huge revolution for society, for humanity - in nuclear fusion we explore the limits of science and the science of limits."

Why is nuclear fusion not possible yet?

CGTN gets access to the nuclear fusion test facility in Madrid, Spain, where temperatures can reach around 15 million degrees Celsius, making it the hottest place in Spain.

CIEMAT and Madrid's universities offer courses on nuclear fusion, so how far are we from nuclear fusion scaling beyond test facilities and powering our world?

"We are still a little bit far away," said Victor Tribaldos, Full Professor of Physics at Carlos III Madrid University. "We have quite a number of challenges to be solved. The most important is the integration of the science and the technology."

Some answers lie riddled in complex equations, others at the point where theory meets technology, and insiders say we are still decades away from nuclear fusion power plants.

AI and supercomputer clusters could help accelerate progress, but one other word is key according to experts: Collaboration.

Chin and the US have made great strides in nuclear fusion. China's EAST (Experimental Advanced Superconducting Tokamak) "artificial sun" can reach 160 million degrees Celsius (288 million degrees Fahrenheit) for 20 seconds, setting world records for sustained high-temperature plasma, far hotter than the Sun's core, demonstrating clear progress toward clean fusion energy.

But the entire world needs to come together to make this dream a reality.

"You have the EU, you have Japan, the United States, Russia, you have China, you have India, Brazil," Tribaldos said. "We require all the countries around the world to unite on a single project which is ITER."

ITER is a giant international nuclear fusion facility being built in southern France to prove nuclear fusion is feasible. Under construction since 2013 with billions of dollars spent, it's over budget, behind schedule, and still 10 years from completion.

A new generation of scientists, however, isn't giving up.

Jaime Romero Pica Piedra, a PhD student at Carlos III University in Madrid, is focused on plasma and said it's a "pretty interesting and exciting field," because of public funding, research and the growth of private businesses.

"I have already worked with some students from the United States and from China in different projects during my education. I think that there are a lot of opportunities."

Pushed for when we might see scale and mass adoption of this technology, Carlos Hidalgo, Director of the National Fusion Lab at CIEMAT in Madrid said: "Nuclear fusion will provide massive energy, environmentally acceptable energy for society, hopefully in the mid-century."