NASA is an agency known for its rigorous process, planning and perfectionism. Astronauts receive years of training before undergoing their first spaceflights, spending hours upon hours simulating different adverse scenarios they might encounter over the course of a mission. Aside from having backgrounds as military pilots, Lovell, Swigert and Haise were also all boy scouts in their childhoods.
The point is: Everyone involved in the mission understood the value of being prepared and having contingency plans. This photo depicts the original crew of the Apollo 13 mission, which included Ken Mattingly as command module pilot. Mattingly was pulled from the mission last-minute due to a possible exposure to measles, a disease he never contracted and therefore was vulnerable to.
The two years of training Mattingly spent specifically for the Apollo 13 mission didn't go to waste, however. He may have been grounded, but his work alongside flight engineers modeling different return flight procedures through simulations proved invaluable in solving the problem of how to lead a successful reentry with an extremely limited power supply.
ANALYSIS: When (Part of) Apollo 13 Reached the Moon