In a 2015 talk with Sam Altman (Y Combinator) & moderator Andrew Ross Sorkin, Elon Musk explains fusion clearly: Fusion = slamming light atoms (hydrogen isotopes) → helium + massive energy (the Sun is “an ongoing fusion explosion”). Fission = heavy atoms decaying. He notes Earth lacks the Sun’s gravitational confinement, so sustaining fusion here is extremely difficult (reaction tends to die out fast). Musk favors solar + batteries as primary sustainable energy; fusion likely as future backup. His deep physics/energy knowledge consistently drives solutions for humanity: clean energy, multi-planetary species, safe AI, sustainable transport.