By now, more than 50 years after the Apollo 13 mission, most of us know the details of that story. Several of the participants, including mission commander Jim Lovell, have written memoirs of their experiences. And there was the 1995 Hollywood film Apollo 13, starring Tom Hanks and directed by Ron Howard, which defied a lot of expectations by being one of the most popular that year. Space historians have pointed out some of the liberties the film took with what actually happened, but it did convey the drama and tense situations that confronted Mission Control and the crew.

As we remember of the Apollo 13 mission, I’d like to mention a detail that is less well-known: the Inertial Measurement Unit. It’s a small detail, but one that was essential to ensuring the safe return of the astronauts after an explosion damaged the service module on the way to the Moon.　

As Mission Control realized the seriousness of the explosion that rendered the service module inoperative, they directed the crew to shut down nearly all systems in the command module and move to the lunar module, using it as a lifeboat. The reason behind that directive was that the limited battery power on the CM had to be held in reserve for guiding the module during the brief time of reentry and return to Earth. Among the systems shut down was the command module’s Inertial Measurement Unit (IMU), a critical device needed for the mission’s guidance, navigation, and control. An IMU is based around a set of sensitive gyroscopes and accelerometers, whose accuracy depends on low-friction bearings and keeping the unit at a constant temperature. But no one was certain that the Apollo IMU, soaking in the cold after several days of inactivity, could be restarted. Engineers at the MIT Instrumentation Laboratory, where the IMU was designed, took a spare unit from their Lab and tried testing it under cold conditions. Their results were encouraging but by no means enough to assure a successful restart.

Inertial Measurement Units were not unique to Apollo – nearly all launch vehicles and spacecraft, piloted or not, have them. During their development in the early days of space exploration, IMU designers were split into two camps: One camp argued that the only way to minimize friction and drift of the gyroscopes was to suspend the rotors on a film of gas. The other argued that traditional ball bearings, if carefully manufactured, would work just as well. The design of the IMU for the Minuteman ICBM, for example, initially used ball bearings, but for the production rockets they were replaced by gas bearings. The decision to switch to gas was controversial, at least among those involved in the classified work being done on Minuteman. There were sound arguments for both designs.

The IMU on the Saturn V rocket used gas bearings. A tank of pressurized nitrogen fed the gas into the gyroscopes. The IMU in the command module used ball bearings. When the time came to restart the unit shortly before reentry, the device worked perfectly. The crew landed safely very close to the intended target. Had the IMU used gas bearings, it is likely that the gyros might have lost the gas and thus have been rendered unable to be restarted.

Were the astronauts just lucky? Or was their safe return the result of careful attention to detail by NASA engineers? There was luck involved in many aspects of the mission. In this case, I would say it was a combination of both.