No ignition at the U.S. National Ignition Facility, home to the world’s largest laser.

When it comes to nuclear reactions, you’ve got your fission and your fusion. Both garner energy from mass according to Einstein’s famous E=mc2, but in a different way. Fission — the process at work in an atomic bomb or a nuclear power plant — gets the energy by splitting a relatively heavy atom* (heavier than iron) into lighter atoms and particles. Fusion, by contrast, combines two atoms lighter than iron into a larger atom and a whole lot of energy.

Most commonly, for example on the sun and in a so-called thermonuclear bomb, hydrogen** serves as the feedstock for the fusion reaction.

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