The race is on to build fusion reactors that would provide limitless energy without nuclear waste or carbon emissions. Here's everything you need to know:

What is fusion?
Thermonuclear fusion is the nuclear reaction that powers the sun and all stars. It occurs when two nuclei of a lightweight element such as hydrogen collide at colossal speed, forcing them to fuse. Leftover mass is converted into enormous amounts of energy, according to Einstein's formula E = mc2. Unlike fission, in which atoms are split, fusion requires small amounts of ordinary fuel — the amount of hydrogen in a glass of water could provide enough energy for one person's lifetime — and does not create much radioactive waste, which is why it's been called "the holy grail for the future of nuclear power." Proponents believe fusion reactors could solve the climate-change crisis by providing inexhaustible energy with zero emissions and no chance of a meltdown. But the challenge of creating fusion reactions is enormous: Scientists and engineers essentially have to create a small star. Hydrogen must be heated to about 100 million degrees Celsius — six times hotter than the sun's core. At that temperature, hydrogen is no longer a gas but a plasma, a soupy mix of charged particles that is incredibly difficult to sustain. Scientists have been trying to contain the plasma using a tokamak, a doughnut-shaped structure with an extremely strong magnetic field, but thus far have been successful only for seconds.

Why attempt such a feat?
With the climate crisis intensifying, President Biden has pledged to eliminate all greenhouse-gas emissions from the electricity sector by 2035. Solar, wind, and hydropower will play a major role, but those technologies aren't feasible everywhere, so Biden's energy plan also includes nuclear technologies. One option is to build mini-fission reactors that would be much safer and cheaper to produce than the current generation of aging nuclear plants. But nuclear fusion would be far superior, as it avoids the vast piles of radioactive waste that must be stored for thousands of years. Governments around the world are pouring billions into the research, as are private investment ventures such as Breakthrough Energy Ventures, started by Microsoft founder Bill Gates.

Where does research stand?
The biggest project is ITER, a tokamak the size of 60 soccer fields that is under construction in France and is expected to operate in 2035. ITER, which means "the way" in Latin and originally stood for International Thermonuclear Experimental Reactor, is a joint effort of the European Union, U.S., U.K., China, Russia, Japan, India, and South Korea. Preliminary experiments are being done at a mock-up facility in Britain. But several retired fusion physicists, including Ernesto Mazzucato and Daniel Jassby of Princeton's Plasma Physics Lab, have described ITER as a boondoggle run by bureaucrats that is likely to waste its potential cost of up to $65 billion.

How far off is success?
The joke in the industry is that every year workable fusion is said to be 25 years away. But proponents now contend it really could be just five to 10 years before a fusion reactor could actually provide more power than it consumes, thanks to significant technological breakthroughs. Materials are now available that can withstand or prevent erosion of the container around the plasma, including reduced-activation steel and tungsten. And high-temperature superconducting magnets have been developed that can create vastly stronger magnetic fields and can be kept cool by cheap and abundant liquid nitrogen instead of rare liquid helium. That means fusion reactors much smaller than ITER can be developed.

Who is working on those?
One company, Commonwealth Fusion Systems, spun out of MIT in 2017 and with some $250 million in private capital backing it is building a tokamak the size of a tennis court that will cost a fraction of ITER. It plans to bring a prototype reactor online by 2025 that can generate about 270 megawatts, enough to power 100,000 homes. A British company, First Light Fusion, is using an entirely different method of confining plasma, inspired by a crustacean called the pistol shrimp. When this shrimp snaps its claw to stun prey, it creates bubbles that collapse so forcefully that the vapor inside briefly turns to plasma at 4,700 degrees Celsius. This mini-explosion creates so much noise that pistol shrimp colonies interfere with submarine sonar. Using a similar technique, First Light hopes to initiate its first fusion reaction this year and to demonstrate net energy gain by 2024.

Will these efforts pay off?
The example of Lockheed Martin is sobering. At great expense, that giant defense and energy firm has been toiling away at fusion for years with slow progress. Furthest ahead is South Korea, yet even its National Fusion Research Institute has only managed to maintain plasma at 100 million degrees Celsius for 20 seconds. Still, the consensus among researchers is that fusion is within reach. "I think it's not going too far to say that fusion is having its Kitty Hawk moment," Commonwealth co-founder Martin Greenwald told the Institute of Electrical and Electronics Engineers. "We don't have a 747 jet, but we're flying."

An alternative: Small fission reactors
Cutting-edge energy firms — including Bill Gates' TerraPower — are working on mini–fission reactors known as small modular reactors, or SMRs. Last fall, the Nuclear Regulatory Commission gave its first approval to such a device, greenlighting a design by Oregon company NuScale Power that would generate 50 megawatts of electricity. That's much less than the 1,000 MW of a traditional reactor, but utilities could link up to 12 together to power a medium-size city. Some SMR companies are using molten salt as a coolant rather than water, with no pumps and valves that can break; proponents say such a reactor can't melt down, so no large evacuation zone is required. Critics, though, point out that even small fission reactors cost billions to build, and they still produce radioactive waste that must be stored somewhere at great cost and risk. "Although SMRs have lower upfront capital cost per unit," says the International Atomic Energy Agency, "their economic competitiveness is still to be proven."

This article was first published in the latest issue of The Week magazine. If you want to read more like it, you can try six risk-free issues of the magazine here.