Apollo 1

On January 27, 1967, Apollo 1's crew--Virgil I. "Gus" Grissom, Edward H. White II and Roger B. Chaffee--was killed when a fire erupted in their capsule during testing. Apollo 1 was originally designated AS-204 but following the fire, the astronauts' widows requested that the mission be remembered as Apollo 1 and following missions would be numbered subsequent to the flight that never made it into space. Image credit: NASA

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I like the movie 'Apollo 13'. I always thought this was a sad occasion, even though they were doing dangerous things.