After an explosion in Apollo 13's service module cut off propulsion and power, the lunar excursion module (LEM) was used as a lifeboat to provide life support to the three astronauts, and the engine of its descent stage for course changes, until the command module could re-enter the atmosphere after a long coast around the moon.

MSN: Salvaging Science from Stricken Mars Moon Probe: A Scientist's View - Technology & science - Space - Space.com | NBC News