Apollo 13 was the third planned lunar landing mission, with Commander Jim Lovell, who had flown as CSM Pilot on Apollo 8, Commander of Gemini 12, and Pilot of Gemini 8, CSM Pilot Ken Mattingly, and LEM Pilot Fred Haise. Two days before the mission, the backup crew and the prime crew got measles, and backup CSM Pilot Jack Swigert replaced Ken Mattingly. However, on the way to the Moon, an oxygen tank exploded, forcing an abort and almost suffocating the crew.