At Raytheon facilities during the 1960s Apollo program, female workers referred to as Little Old Ladies threaded copper wires through tiny metallic magnetic cores to create flight memory. Each magnetic core stored 64 bits of information. Passing a wire directly through a magnetic core registered as a binary 1, while wrapping the wire around the core signified a binary 0. Led by Margaret Hamilton, who directed onboard flight software development for the Apollo computers and coined the term software engineering, this physical manufacturing process turned digital instructions into concrete hardware for lunar missions.
Woven Code in the Space Age
The flight software for the Apollo spacecraft was written in a specialized computer language called Basic, operating under an interpreter and an executive system designed by Hal Laning. Hamilton and her programming team authored the instructions by punching holes into cards, which were then processed on a Honeywell mainframe computer to simulate software execution. Once the flight routines were thoroughly tested in simulation and verified as solid, the software specifications were sent to Raytheon. There, the instructions were physically woven into magnetic core rope memory.
The Frozen Weeks and Indestructible Memory
Transforming code into woven memory imposed rigorous constraints on software development. The physical weaving of core rope memory at Raytheon took several weeks to complete, during which all code modifications were completely frozen. Despite the delay, this physical medium offered exceptional resilience. Because the program logic was hardwired into the wire patterns, the memory was physically indestructible. During the launch of Apollo 12, when lightning struck the spacecraft directly, the core rope memory survived the high-voltage disturbance intact without losing or corrupting any flight instructions.
Anticipating the Human Element
While testing programs in flight simulators, Hamilton observed that human operator mistakes could trigger system crashes. Recognizing that astronauts might accidentally select incorrect routines during flight, she requested permission to add protective error-checking code. NASA officials rejected her proposal as excessive, maintaining that intensive astronaut training would prevent operational errors. Prevented from incorporating defensive logic into the software build, Hamilton added explicit warning annotations to the program documentation to alert crew members and mission controllers to the risks of improper sequencing.
Vindication in Lunar Orbit
Hamilton's safety concerns were confirmed during the Apollo 8 mission. An unexpected human operational sequence occurred in flight, triggering a mistake that completely wiped out all stored navigational data from the computer memory. Engineers on the ground were forced to reconstruct and transmit fresh guidance parameters to restore trajectory tracking. The incident on Apollo 8 demonstrated that system architecture had to account for human operational error, establishing software engineering as a critical design discipline essential for complex computing systems.
AI-generated editorial illustration; not a documentary photograph.
Source: History of software
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