Trouble on the Way to the Moon 13 April 1970

You are a flight controller in Mission Control, Houston.

About 56 hours after launch and roughly 200,000 miles from Earth, the crew of Apollo 13 report a bang. The service module is losing oxygen and electrical power. The Moon landing is off; the question is how to bring the three astronauts home.

Known information

Both oxygen tanks in the service module appear to have failed, and the command module will soon need to be powered down to save its batteries for re-entry.
The attached lunar module has its own engine, oxygen and power, but was designed to support two people for about 45 hours.
A direct abort would fire the service module’s main engine to turn the spacecraft around quickly, before it reaches the Moon.
Nobody knows whether the explosion damaged that main engine or the structure around it.
Swinging around the Moon would take longer, stretching supplies of power, water and oxygen.

Reveal what happened (spoilers: decide first)

Fact

What actually happened

Mission Control chose the route around the Moon, using the lunar module as a lifeboat. (Loop around the Moon)

About five hours after the explosion, a 35-second burn of the lunar module’s descent engine put Apollo 13 back on a free-return path. After swinging around the Moon, a second burn sped up the return. The lunar module supported the three astronauts for around 90 hours, and the crew splashed down safely in the Pacific on 17 April 1970.

Important

Possible consequences · Analysis and speculation, not fact

If the main engine had worked, a direct abort might have shortened the trip considerably; if it had been damaged, the attempt could have been disastrous. Engineers at the time could not tell which, which is why the risk was judged unacceptable.

History does not reveal what would definitely have happened under the other choice. Many factors could have changed the outcome.

Context · Why it was decided this way

Controllers were reluctant to fire the service module’s main engine because the extent of the damage was unknown; if it failed or the structure gave way, the crew could have been lost. The slower route relied on engines and systems that were known to be working.

Getting home still required improvisation, including building adapters so that square carbon dioxide filters from the command module would fit the lunar module’s round sockets. NASA has described the mission as a “successful failure”.