> more abundant fuel (on earth and the rest of the solar system) This is by far the most important reason in the long term. Between stars and even at our own

> more abundant fuel (on earth and the rest of the solar system) This is by far the most important reason in the long term. Between stars and even at our own outer planets where solar panels aren't reasonable fusion is the only long-term large scale energy source. It's the difference between being stuck as a Kardashev I or II civilization or approaching III. > Should we use fission right up until we have viable fusion? Of course, we should definitely be building more fission reactors. But I can't think of a single reason we'd continue using fission once we get to fusion. The power density and relative simplicity of fission (including mere thermocoupled) is still worthwhile for robotic probes or initial sources of power in distant places, but we'll be able to make our own fissionables indefinitely once we have solid fusion power.