The Sun is a giant nuclear reactor. Every second, it “fuses” together about 650 million tons of hydrogen atoms to make helium. Some of the hydrogen is converted to energy — less than one percent. That’s enough to keep the Sun shining brightly.

Scientists have been trying to replicate that process here on Earth since the 1950s. They use two forms of heavy hydrogen. One is common in seawater, while the other has to be manufactured.

Success could provide abundant power with no climate effects and almost no radioactive residue. But controlled nuclear fusion has proved elusive. There’s a long-standing joke that it’s about 20 years away — and always will be.

The big problem is temperature. For fusion to work in reactors on Earth, the hydrogen must be heated to about 270 million degrees Fahrenheit — 10 times hotter than the center of the Sun. It’s hard to achieve those temperatures. And it’s even harder to contain the hot gases, known as plasma. You need strong magnetic fields, which require superconducting magnets and other high-tech equipment.

Scientists may be making progress, though. A giant project in Europe should be ready to start experiments in a few years, and achieve stable fusion a decade later. And several companies are entering the field as well. Some will use new techniques to hold the fusion reactions. And others may try different processes.

Most of the projects report that success could lead to commercial reactors — in about 20 years.

Script by Damond Benningfield