Partway through the powered descent to the lunar surface, a guidance computer alarm appeared on the display. The alarm code was 1202.

Nobody outside of Mission Control’s technical staff knew what 1202 meant. The television commentary went quiet for a moment. Armstrong asked Mission Control what it was. And then Mission Control’s Steve Bales — working with a young man named Jack Garman, who had memorized all the alarm codes — said “Go. We’re go on that alarm.”

The landing continued.

The 1202 alarm means the guidance computer’s executive program is overloaded and is dropping lower-priority tasks to keep the highest-priority calculations running. The computer is saying, in effect: I have more work than I can do, so I’m choosing which work to do. The computer can keep flying the spacecraft in this state; it’s shedding tasks that aren’t needed for guidance, like certain display updates. The guidance itself continues correctly.

Jack Garman was 24 years old. He had written the procedure for this alarm and had been drilled on it. When Bales turned and asked him, Garman said “go” and the landing continued.

If Garman had said abort, the mission would have ended. Armstrong and Aldrin would have fired the ascent engine and left the surface — never having touched it. And the reason would have been a guidance computer alarm that wasn’t actually catastrophic.

The alarm happened three times during the descent. Three times, Mission Control said “go.” Three times, the landing continued.

This is what I mean when I say Mission Control is the other half of every spaceflight. The crew does their job. But the crew can’t evaluate their own guidance computer alarms while they’re in the middle of the most complex twelve minutes in spaceflight history. Jack Garman sat at his console and kept the mission alive.

I didn’t know any of this in real time. I knew there was a problem and then I knew they kept going and then I watched the rest of the descent. Only afterward, reading the debriefings, did I understand what 1202 meant and who made the call.

Twenty-four years old.