Final answer:
Deke Slayton was grounded from Mercury missions due to atrial fibrillation but served as Director of Flight Crew Operations during the Apollo program, influencing crew selection and training.
Step-by-step explanation:
Deke Slayton was originally selected as one of the Mercury Seven astronauts but was grounded due to a heart condition known as atrial fibrillation. Despite being grounded from flight, Slayton remained a crucial figure in NASA's space program. During the Apollo missions, he served as NASA's Director of Flight Crew Operations, playing a vital role in selecting astronaut crews for all NASA manned space flights, including the Apollo Moon landings. His contributions were influential in the success of the program, particularly in the intricate process of selecting and training astronauts for their respective missions.