The U.S. and China have officially joined the quest to develop fusion power, which proponents say could be an affordable, eco-friendly alternative to existing energy sources. The International Thermonuclear Energy Reactor is the largest global science project after the International Space Station. China, the U.S., Canada, the E.U., Japan, and Russia will spend $5 billion over 10 years to make fusion energy safe and viable. Fusion reactions — the kind that take place in the sun — produce energy by fusing light atoms such as deuterium and tritium to form heavier ones. In the ITER project, the heat created by those reactions would then be used to generate electricity. Both deuterium and tritium are abundant isotopes of hydrogen, and they promise a lot of bang for the buck: One kilogram of fusion fuel could produce the same amount of energy as 10 million kilograms of fossil fuel.