 Twenty-four NASA astronauts from the Apollo era flew on the Space Shuttle.NASA's Director of Flight Crew Operations during the Gemini and Apollo programs was Donald K. "Deke" Slayton, one of the original Mercury Seven astronauts, who was medically grounded in September 1962 due to a minor cardiac arrythmia – paroxysmal atrial fibrillation.Slayton was responsible for making all Gemini and Apollo crew assignments.