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Twelve-Oh-Two

True story · 3 min read

On 20 July 1969, with the lunar module Eagle a few minutes from touchdown on the Moon, a yellow warning light came on in the cabin and the computer displayed a code: 1202.

Neil Armstrong and Buzz Aldrin did not know what the code meant. Armstrong asked Mission Control to tell them. Nearly four hundred thousand kilometers of space lay between the crew and a room in Houston, with a delay of a second or so each way, and in that room a twenty-six-year-old named Steve Bales had a few seconds to decide whether the landing should go on.

Bales was the guidance officer. In an earlier training simulation, a 1202 alarm had sent him calling for an abort that turned out to be unnecessary. Afterwards, his younger colleague Jack Garman had written out every alarm the software could raise, with what each one meant, on a sheet he kept beside him. Now Garman looked down the list. If the alarm showed up only now and then, he said, they could keep going.

That made sense once you knew how the computer worked.

The Apollo guidance computer was tiny by any modern measure, with far less memory than a calculator app. It handled its work by priority. Every job, from steering the engine to updating the display, was put in a queue ranked by importance. When it had more work than time, it was designed to drop the least important jobs, restart itself, and carry on with the vital ones. The alarm was an executive overflow: more jobs than the computer had time for. It was a sign the computer was managing its load.

Why overloaded? A radar meant for rendezvous in orbit was switched on in the position the checklist called for, in case they had to abort. Nobody had foreseen that its electrical signals, out of step with the computer, would generate a stream of useless work. It took up about a seventh of the machine's time.

So the real question for Bales was this: is the computer still doing the important jobs, and will it keep doing them? He said "Go." The alarms came back, 1202 again and then 1201, and each time the room tensed and Bales, with Garman's help, said Go again.

Meanwhile Armstrong had a second problem. Looking out the window, he could see that the autopilot was steering them toward a crater ringed with boulders, much too rough to land in. He took manual control, flew the lander level over the rocks, and went searching for flat ground. Searching takes fuel. When the controller back on Earth counted down the time left, it was sixty seconds, then thirty, before Armstrong would have to land or abort.

Through a haze of dust blown up by the engine, a blue light came on: the probes under the lander's legs had touched the surface. "Houston, Tranquility Base here. The Eagle has landed."

Afterwards, the praise went to what the people on the ground had prepared. Bales and Garman had a list. The programmers had designed the computer to fail gracefully. The astronauts had trained for emergencies in a hundred simulated forms.

Words in this story: guidance, overflow, priority, simulation, rendezvous