As NASA planned for longer stays on the Moon after the first few Apollo flights, they had to consider a number of new issues, one of which was what to do if the astronauts cannot get back. Typically the longer a spacecraft is idle the less reliable it becomes, so after a stay of two weeks on the Moon the Lunar Module ascent engine or other essential systems might fail to function, leaving the astronauts stranded on the Moon without enough supplies to survive until a rescue mission could arrive from Earth.