R.I.P. Margaret Hamilton, whose code saved the Apollo 11 Moon landing

Hamilton would often bring her 4-year-old daughter, Lauren, to the lab on nights and weekends. The child loved pretending to be an astronaut in the simulators, and one day all her random button-pressing caused the system to inexplicably crash. Apparently Lauren had typed “P01,” a pre-launch sequence, in the middle of the mission. Hamilton asked NASA if she could add a safeguard to prevent this from happening on an actual mission, but the higher-ups didn’t take it seriously. Then astronaut Jim Lovell actually made that error during the Apollo 8 manned mission to orbit the Moon, causing the navigational data to vanish. Hamilton was called in to retrieve the data, jokingly referring to this as the “Lauren bug.”

By 1968 Hamilton was assistant director in charge of the Command and Service Module team. Hamilton had the idea to incorporate a failsafe into the system software’s “priority display” interface routines, should the onboard guidance computer become overloaded with more tasks than it could handle in real time.

That’s exactly what happened on the historic Apollo 11 mission in 1969. As the Eagle lunar module approached the Moon, Buzz Aldrin entered the codes to display altitude and other relevant data on the screen. The onboard computer started flashing alarms, forcing Mission Control to consider aborting the landing. Hamilton’s failsafe meant that the computer was prioritizing essential tasks and cancelling nonessential ones, and the alarm was to let astronauts know that this was happening. Thanks to Hamilton’s foresight, Eagle still safely landed on the Sea of Tranquility.

As Hamilton herself later recalled in a 1971 letter:

The computer (or rather the software in it) was smart enough to recognize that it was being asked to perform more tasks than it should be performing. It then sent out an alarm, which meant to the astronaut, “I’m overloaded with more tasks than I should be doing at this time and I’m going to keep only the more important tasks”; i.e., the ones needed for landing … Actually, the computer was programmed to do more than recognize error conditions. A complete set of recovery programs was incorporated into the software. The software’s action, in this case, was to eliminate lower priority tasks and re-establish the more important ones … If the computer hadn’t recognized this problem and taken recovery action, I doubt if Apollo 11 would have been the successful Moon landing it was.

The Apollo software she helped develop was added to GitHub in 2015. In 2019, Google marked the 50th anniversary of the Moon landing—and Hamilton’s crucial contribution—by training over 100,000 mirrors at its Mojave Desert solar power farm to create a massive portrait of Hamilton in reflected moonlight.