Three notables in residence - the crew of commander James Lovell, command module pilot Thomas K. Mattingly, and lunar module pilot Fred Haise - kept busy in the simulators and altitude chambers. While Lovell and Haise trained for two moon walks, Mattingly studied his photographic assignments which included the moon, sun, and other astronomical subjects. Training went smoothly, the hectic pace of previous launches seemingly a thing of the past. The situation changed dramatically, however, when NASA's Medical Director, Dr. Charles A. Berry, reported on 6 April that the prime crew had been exposed to measles. Backup lunar module pilot Charles Duke had a case of german measles (rubella) and Jeffrey Lovell, the commander's son, was down with the red measles (rubeola). Although the three astronauts were in good physical condition, blood samples were taken to determine their immunity. Initial tests showed satisfactory antibody levels in all three astronauts, but a recheck cast doubts on Mattingly's condition. Further tests indicated that Mattingly had no immunity and would likely experience the illness about the middle of the lunar mission.
At a press briefing on 8 April, Dr. Berry indicated that he would recommend against Mattingly's flying. NASA's preventive medicine program was questioned and Berry acknowledged the need to re-examine the subject. Previously crews had been restricted to essential contact during the last 21 days of prelaunch operations. This still included many people - training personnel, workers at the crew quarters, even younger members of the immediate families. The astronauts' schedule kept them in KSC's crew quarters much of the last three weeks, but risks were inevitable. Berry noted that some loopholes in the isolation program were necessary; others might be eliminated. He mentioned the likelihood of more antibody testing and immunization, even for such unlikely adult diseases as measles.27
Mattingly's health posed a difficult decision for NASA. Duke's illness ruled out the substitution of the alternate crew. Delaying the launch a month would lessen confidence in the space vehicle and add $800,000 in costs. Another alternative was to replace Mattingly with his backup, John Swigert. The longer time between missions had permitted extensive simulator training with the backup crew. Although a late substitution for the other two crew members was out of the question, the command module pilot was more on his own. A last-minute switch might work. Thursday morning, 9 April, a new crew of Lovell, Haise, and Swigert entered the flight crew simulators.28
The Flight Crew Operations Branch concentrated on situations that required rapid teamwork. First they tested the crew's ability to handle various abort situations. Then the crew practiced the mission's critical maneuvers: the translunar injection, transposition and docking, lunar orbital insertion, descent orbit insertion, rendezvous and docking, and transearth injection. Mechanical failures were cranked into each of the maneuvers, forcing Swigert to make corrections. One situation required a decision and response within two seconds.29 The major concern was communications between crew members. As Riley McCafferty, branch chief, put it:
From the standpoint of putting these three guys together and these three guys accepting each other and these three guys establishing confidence in each other, that wasn't our concern. Our concern was, did we have the proper communication, so when Jack Swigert said, "that's good," did they really know what "good" meant to Swigert versus what "good" meant to Mattingly?30The Flight Crew Operations Branch had striven for compatibility in training the prime and backup crews. With Apollo 13 came the first fruits of their labor. By Friday afternoon Deke Slayton and Riley McCafferty were convinced that Swigert could work with his new crew mates.
More important, Lovell was satisfied with the new arrangement. Paine, after discussing the matter with Lovell, Slayton, and other Apollo officials, gave the mission a go-ahead at KSC's prelaunch press conference Friday afternoon. Paine told reporters there was never any question about Swigert's ability as a command module pilot: "Jack literally wrote the book on the malfunctions and how to overcome them." NASA's concern had been whether the astronauts could work together effectively, and the 12 hours of intensive tests had removed all doubts. Slayton praised the crew training group: "They got the equipment on the line for the last 36 hours in A number 1 shape, came up with a beautiful plan, and we in fact did it. I guess we were all surprised also that the crew did integrate as well as they did." A reporter asked whether the change had caused extra crew fatigue. Slayton noted that the tests had not exceeded the normal work schedule. If the crew had not been ready by Friday noon, NASA was prepared to postpone the launch.31