Final answer:
Ken Mattingly was replaced by Jack Swigert on the Apollo 13 mission due to concerns about possible measles contraction. The Apollo 13 mission, though it did not land on the Moon, was a 'successful failure' because of the safe return of the crew.
Step-by-step explanation:
The astronaut replaced on the Apollo 13 mission was Ken Mattingly. He was grounded by flight surgeons just days before the mission due to exposure to German measles, which they feared he might contract during the flight. Mattingly was replaced by backup Command Module Pilot Jack Swigert. The Apollo 13 mission is famous for its in-flight incident, where an oxygen tank explosion jeopardized the crew's safety and forced the mission to forgo its lunar landing. Thankfully, with extraordinary effort and determination, the crew returned safely to Earth. This mission is often cited as a 'successful failure' due to the safe return of the astronauts despite the mission not accomplishing its original goal.