You’ve heard of the Manhattan Project — the Allied research and development program that resulted in two nuclear bombs being dropped on Japan and the end of World War II — and now it’s time to learn about one of its successors, Project Matterhorn, a Cold War program to control and harness thermonuclear reactions to create fusion power.

Nuclear fusion occurs when two atoms fuse together (usually hydrogen) to form a heavier atom (helium), and releasing a vast amount of energy in the process. This process can only occur at incredibly high temperatures, such as the center of a star (such as our Sun). Every second, the Sun fuses 500 million tons of hydrogen into helium, releasing about 5 million tons of gamma rays that eventually heat and illuminate Earth. For a long time (Project Matterhorn started in 1951), nuclear fusion has been considered a very desirable power source because the fuel is virtually free, and the process releases vast amounts of energy and no pollutants.