Error 1202
How Jack Garman and Steve Bales decided, in a few dozen seconds, that Apollo 11 could continue its landing.
I’m reading A Man on the Moon by Andrew Chaikin. One story that really stayed with me is about a few dozen seconds before the first Moon landing. We know how it ended. The people in Houston were still deciding whether the landing could continue.
July 20, 1969. The lunar module Eagle is descending towards the surface. Touchdown is about seven minutes away. An alarm sounds in the cabin, and the computer displays code 1202. Armstrong reports it to Earth. A few seconds later, he asks what it means. Eagle keeps descending.
In Mission Control, 26-year-old Steve Bales is responsible for guidance. He has to recommend whether to continue or abort the landing. A whole team supports him, but right now he particularly needs someone in the back room: onboard computer specialist Jack Garman.
Garman understands the code: the computer is overloaded and has started its recovery process. He doesn’t yet know what caused the overload. But he knows the condition for continuing: the alarms must not be continuous, and the computer must have enough time to recover.
There is a reason for his confidence. During a simulation before the flight, the team had encountered a similar alarm. Bales had called for an abort. The debrief revealed that it had been unnecessary: the computer could cope.
After that simulation, flight director Gene Kranz insisted that the team work through the alarm codes and the response to each one. Garman wrote a cheat sheet by hand and placed it under the clear cover on his console. This time, the Moon was real, and so was the spacecraft carrying two people.
Garman tells Bales they can continue as long as the computer recovers. Bales checks the telemetry: navigation data and the spacecraft’s behaviour remain normal. That gives him the basis to accept the recommendation and tell Kranz: “Go.” Charlie Duke, who handles communications with the crew, relays the clearance to Eagle.
About 27 seconds pass between Armstrong’s first report of the alarm and Duke’s reply.
Then the alarm sounds again. Later, another code appears: 1201. Garman recognises the same class of overload. The decision to continue has to be confirmed again, watching the computer recover and checking that the spacecraft remains under control. There will be five such alarms during the descent.
After touchdown, Bales says over the internal voice loop: “Hey Jack, thank God we had that meeting!” Their preparation has just passed a test they could never run again.
The cause would be established later: extra signals from the rendezvous radar system were taking time away from an already busy computer. Its software could clear the accumulated tasks and resume the critical navigation and control calculations. That gave the people on Earth a reason to keep clearing the landing.
Margaret Hamilton deserves a mention here. She led the team developing Apollo’s onboard software at MIT. Reliability and recovery from errors were part of that work. In the famous photograph, she stands beside printouts of code produced by the team. Hamilton died on September 30, 2026, aged 90.
What strikes me is that Garman and Bales made their decision before they knew the cause of the fault. They had an alarm code, data from the spacecraft, and the experience of a failed simulation. They had enough time to establish that the system was coping and tell the people above the Moon: you can continue.