But these just-so sayings obscure the real power of the Apollo computer. Of course, any contemporary device has vastly more raw computational ability than the early machine, but the Apollo computer was remarkably capable, reliable, and up to the task it was given. You could not actually guide a spaceship to the moon with a smart doorbell.

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To understand how significant the Apollo system was, and why its tiny amount of raw processing power is irrelevant, you only have to listen to the OG computer programmer and volunteer NASA historian Frank O’Brien, who has spent his life lovingly detailing the functions of the Apollo Guidance Computer. O’Brien’s father was a pilot, so Frank became a military brat. He was interested in computers from an early age, and when one of his dad’s old friends rose up the ranks at NASA, he came into possession of the technical manuals that governed the operation of the computer.

“At 13 years old, I get a box on Christmas, around two feet on a side, weighed a million pounds,” O’Brien told me. “I open it up, and it had all the technical manuals on Apollo. You had tons and tons of kids looking at Playboys; I was reading about guidance computers.”

Since then, he’s spent countless hours learning precisely how these machines worked. Even as a teenager, he could fly NASA’s Apollo simulator. As an adult, after earning a computer-science degree and working a long stint as a corporate programmer, he wrote the book The Apollo Guidance Computer, an ode to the machine.

The Apollo Guidance Computer in the command module had two main jobs. First, it computed the necessary course to the moon, calibrated by astronomical measurements that the astronauts made in flight, with a sextant not unlike that used by oceanic navigators. They’d line up the moon, Earth, or the sun in one sight, and fix the location of a star with the other. The computer would precisely measure those angles and recalculate its position. Second, it controlled the many physical components of the spacecraft. The AGC could communicate with 150 different devices within the spacecraft—an enormously complicated task. “It has dozens of thrusters and all kinds of interfaces and a guidance platform and the sextant,” O’Brien said. “You start adding up all this stuff and go, Holy cannoli. This is really capable.”

Conceptually, the MIT Instrumentation Laboratory, which designed the system, built it atop the work they’d done for the Polaris guided-missile system, made to launch nuclear weapons from American submarines. The Apollo computer's hardware, as Mindell has noted, was fairly well understood “in the world of military avionics.”

Building it dominated the project at first—the lab heavily underestimated the complexity of the software-engineering task. For years afterward, deep into the 1970s, programmers were still using punch cards to code. But the necessity of having Apollo astronauts and NASA engineers “in the loop,” making decisions, required a different kind of software. There had to be an interface. Multiple operations had to run at the same time.