The launch and following day, April 12 went perfectly for the Apollo 13 crew. On the evening of the 13th, the astronauts did a routine pressurization of the lunar module Aquarius. Suddenly, a loud explosion was heard, and all three crew members scrambled into the command module Odyssey to examine the instrument panels.
Haise then contacted Houston:
Haise: Okay Houston-
Lovell: I believe we’ve had a problem here.
Mission control: This is Houston. Say again please.
Lovell: Houston, we’ve had a problem. We’ve had a main B bus undervolt.
The service module had two cryogenic oxygen tanks, and one of them had dropped to zero pressure. These two tanks, along with the cryogenic hydrogen tanks fed the spacecraft’s fuel cells, which in turn, powered the generation of electrical power, the oxygen for breathing and drinking water.
Aquarius became the crew’s lifeboat. The LM was designed for only two men, so it was a cramped living situation, and now all thoughts of a third moon landing were scrapped. As the Apollo 13 crew moved into Aquarius, they were 20 hours from the moon.