How Margaret Hamilton Programmed the Apollo 11 Moon Landing

Margaret Hamilton

One of the founding “fathers” of the software industry was in fact a mother. She coined the term “software engineering” and wrote the code that took mankind to the moon.

On July 20, 1969, Neil Armstrong and Buzz Aldrin were just three minutes away from touching down on the moon when an alarm sounded and the error code “1202” appeared on the lunar module’s computer screen.

Within moments, mission control had to make a call: to abort or not.

Thanks to Margaret Hamilton, the computer programmer who led development of NASA’s flight software, it was quickly determined that the error did not involve a crucial function. The issue involved a faulty radar switch, which was not a threat to landing the module safely on the moon’s surface, nor to taking off to reconnect with the command module then circling in lunar orbit.

As a result, the mission continued, the Eagle landed, a zero-gravity flag was planted, and mankind proceeded with both small steps and giant leaps.

“[Hamilton] symbolizes that generation of unsung women who helped send mankind into space,” said President Barak Obama, who awarded her the Presidential Medal of Freedom in 2016. “Her software architecture echoes in countless technologies today.”

Margaret Hamilton
Margaret Hamilton earned the Presidential Medal of Freedom for her work on Apollo 11. Photo courtesy of MIT Museum.

In a field dominated by men, Hamilton was a trailblazer in the field of software engineering — incidentally, a term she coined. She not only played a significant role in making space travel possible, she also forged a decades-long career as a programmer and even founded her own company, Hamilton Technologies, Inc. (HTI).

The Path to Apollo

After graduating from Earlham College in 1958 with a degree in math, Hamilton and her husband moved to Boston, where he attended law school at Harvard and she got a computer programming job at the Massachusetts Institute of Technology (MIT). She intended it to be a short-term gig, just until her husband graduated and she could pursue an advanced degree herself.

At the time, the field of computing was still relatively new. Computers were large, often taking up entire rooms, if not an entire warehouse, and the work of programming was still developing.

“When I first got into it, nobody knew what it was that we were doing,” Hamilton told Wired. “It was like the Wild West. There was no course in it. They didn’t teach it.”

Under the guidance of Dr. Edward N. Lorenz, the mathematician who coined the term “butterfly effect” and was a pioneer in chaos theory, Hamilton learned a couple of programming languages and began the process of designing and building software.

One of her early assignments was working on SAGE, an early missile defense computer system.

“SAGE was one of the first jumping off points where I became interested in the subject of software reliability,” said Hamilton in another interview. “When the computer crashed during the execution of your program, there was no hiding. Lights would be flashing, bells would be ringing and everyone, the developers and computer operators, would come running to find out whose program was doing something bad to the system.”

By the time NASA selected MIT to design spacecraft guidance and navigation systems, Hamilton was hooked.

She led the in-flight software group, which included designing an alarm system to alert the flight team if the computer’s processor became overloaded while prioritizing tasks to support critical functions and shut down those deemed less crucial.

This was the system that came into play in 1969 when the Eagle was looking for a place to roost.

Cognizant that lives hung in the balance, Hamilton insisted on rigorous testing to ensure the success of Apollo software.

“There was no second chance,” she said. “We all knew that.”

As a woman in tech, Hamilton was in the minority throughout her career.

However, she wasn’t the first woman in the computer field. Consider Grace Hopper, the women of ENIAC and the code crackers of Bletchley Park, all pioneers in computer programming during WWII.

Nor was she the lone woman at NASA, as has been recently brought to light in the book and subsequent film Hidden Figures, which depicts the real-life contributions of African-American mathematicians who played a significant role during the early years of the U.S. space program.

Yet, many of these stories have only recently been shared in the public sphere.

Dr. Renee Horton, lead metallic/weld engineer for NASA’s Space Launch System, said she wasn’t aware of Hamilton’s story until after 2003, when NASA presented Hamilton with a special award for her innovations in Apollo software development.

NASA engineer Dr. Renee Horton
NASA engineer Dr. Renee Horton. Photo by Alexander Le’jo. MUA is @Tvglamm

“It gave me such pride learning about such an awesome woman who helped the Apollo program be successful,” said Horton, who praised Hamilton for her “powerful mind.”

“From the time I was a child, I wanted to work for NASA and make an impact,” said Horton. “History made it seem like only white men were the reason this happened, when in actuality it was someone who looked like me, not just in skin tone, but in gender as well. It was so exciting to learn the story of how these women persisted to do the work, no matter what obstacles were presented.”

Hamilton herself said that it helped her to focus on the work instead of the representation — or lack thereof.

“I was so involved in what we were doing, technically, that I was oblivious to the fact that I was outnumbered by men,” she said.

Honoring Hamilton

In 2003, Hamilton was awarded a NASA Exceptional Space Act Award for her scientific and technical contributions to the Apollo missions.

“I was surprised to discover she was never formally recognized for her groundbreaking work,” said Dr. Paul Curto, the NASA technologist who nominated her for the award. “Her concepts of asynchronous software, priority scheduling, end-to-end testing, and man-in-the-loop decision capability, such as priority displays, became the foundation for ultra-reliable software design.”

Further, Hamilton’s work served as a launch pad for all that has followed, including Skylab, the Space Shuttle, and other fly-by-wire aircraft.

Meanwhile, her company, HTI, founded in 1986, continues to provide software solutions to a variety of industries. HTI has spent decades perfecting its Universal Systems Language (USL), which was derived from lessons learned from the Apollo onboard flight software effort.

As Obama said at the Presidential Medal of Freedom ceremony in November, “[Hamilton’s] example speaks of the American spirit of discovery that exists in every little girl and little boy who know that somehow to look beyond the heavens is to look deep within ourselves and to figure out just what is possible.”

Archival images courtesy of MIT Museum.

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