In July 1969, weeks before Apollo 11 launched, a NASA engineer named Jack Garman handwrote a list of every alarm code the Apollo Guidance Computer could throw and placed it under the plexiglass on his console — and when Margaret Hamilton's software triggered a 1202 alarm during the lunar descent with roughly 1,800 metres to go, that scrap of paper kept Mission Control from aborting the landing.

In July 1969, weeks before Apollo 11 launched, a NASA engineer named Jack Garman handwrote a list of every alarm code the Apollo Guidance Computer could throw and placed it under the plexiglass on his console — and when Margaret Hamilton's software triggered a 1202 alarm during the lunar descent with roughly 1,800 metres to go, that scrap of paper kept Mission Control from aborting the landing.

The scrap of paper was handwritten in ballpoint, folded once, and placed under the plexiglass on a console in the Staff Support Room in Houston. It listed every program alarm the Apollo Guidance Computer could throw during a lunar descent, and next to each one, in Jack Garman’s shorthand, a single instruction: land, or abort. On July 20, 1969, at roughly 1,800 metres above the Sea of Tranquility, the number 1202 lit up on Buzz Aldrin’s display. …