system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are more exotic isotopes of hydrogen/helium preferred for fusion, so much so, that mining the moon is considered potentially easier than developing the tech to use regular hydrogen?
gpt
Regular hydrogen(H-1/protium) actually doesn't fuse well and is the limiting reaction in the sun.

To get two protons to fuse together you need them to hit and then trigger a beta+ decay to turn one into a neutron so they become deuterium.  If they both stay as protons they quickly fall back apart

So we instead using variants that are easier to get to hit and trigger fusion.  Deuterium (H-2) and Tritium (H-3) both fuse much more reliably and give a lot more energy per fusion event.  Deuterium fusing with Tritium is about 10x the energy per event of two protons fusing and becoming Deuterium

Fusing helium-3 together gives similar output per event but He-3 is stable and therefore wayyy more prevalent than Tritium which decays in about 12 years.

That said, the presence and quantity of helium-3 is largely irrelevant until we actually get stable functional fusion power and we're still a longggg ways off from that.