A cabin fire during a routine test killed all three Apollo 1 astronauts before liftoff

On January 27, 1967, Gus Grissom, Ed White, and Roger Chaffee were sealed inside their capsule for a launch rehearsal when a spark ignited the pure-oxygen cabin. The hatch design made it impossible to open in time, and all three died within seconds. NASA grounded human spaceflight for 20 months to redesign the capsule before trying again.

— Virgil "Gus" Grissom, Apollo 1 — NASA, 27 January 1967