We have nuclear fusion, both from the sun and here on Earth (ITER, NIF, H-bombs). The main problems with human-started nuclear fusion are that (so far, iirc) it

We have nuclear fusion, both from the sun and here on Earth (ITER, NIF, H-bombs). The main problems with human-started nuclear fusion are that (so far, iirc) it consumes more energy than it produces, and we cannot sustain it for a long time. "The benefits" are currently very calculable, and direct economics of nuclear fusion is very negative. (The research benefit is unique though.) For nuclear fusion to be of incalculable benefit to mankind, we need insanely small nuclear fusion that is energy-positive and sustainable in a simple way. "Insanely small" means smaller than Moon-sized, when we know that Jupiter-sized is too small/light in nature. Sustained nuclear fusion for energy generation still faces daunting challenges.