Bolted into the console of the Lunar Module when it touched down on the Sea of Tranquility in July 1969, was the Apollo Guidance Computer (AGC).
The combined two parts were about the size of a briefcase and weighed 32kg.
Today we'd consider its specifications laughable and it would easily be outperformed by a smart digital watch.
But at the time it was the most advanced miniature computer ever built and NASA considered it a crucial component. They put enormous resources into making sure it was reliable.
Given the primitive technology of the day, building the AGC was a daunting problem. Computers were gigantic machines which filled an entire room and used huge amounts of power.
They were so complicated that only top-level mathematicians could operate them.
The first commercially available integrated circuits only became available in 1961 and, prior to that time, computers used vacuum tubes that were hot, unreliable and bulky.
The job of building the AGC was given to MIT because they had been developing inertial guidance systems for the US Navy's Polaris Missile Computer. Even with that experience, they still didn't have a working version by 1966.
Given that US president John F. Kennedy's promise had been to land a man on the moon by the end of the decade, things were getting tight.
As we know now, of course, they did.
Once they had a working design, they took 2000 "man years" to build, which is not surprising since the AGC was built using 30,000 components mounted on two metal trays (one for memory and one for logic circuits).
A curious feature was the "rope memory", which consisted of tiny iron rings with wires running through them.
Depending on whether the wire ran through the centre or the outside, they would store the binary digit (a 'bit') of 1 or 0.
Even with automation, constructing these circuits was a slow and painstaking job.
The AGC worked perfectly on Apollo 8, but then on Apollo 11 it faced a major test.
As the Lunar Module descended towards its landing, the computer was flooded with data from the rendezvous radar that had accidentally been left on.
Instead of crashing, the AGC flashed up an overload error before switching off every program other than the primary navigation task.
In all, the AGC was used in nine moon flights, six lunar landings, three Skylab missions and the 1975 Apollo-Soyuz Test Mission.
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