According to the old quip, a practical fusion reactor will always be about 20 years away. Nowadays that feels a bit optimistic. The world’s largest plasma fusion research project, the ITER reactor in southern France, won’t begin fusion experiments until 2026 at the earliest. Engineers will need to run tests on ITER for at least a decade before they will be ready to design the follow-up to that project—an experimental prototype that could extract usable energy from the fusing plasma trapped in a magnetic bottle. Yet another generation would pass before scientists could begin to build reactors that send energy to the grid.