Margaret Hamilton died on 30 September in a hospital in Cambridge, Massachusetts. She was 90. Her daughter, Lauren, gave the details to the Associated Press, and the cause was not disclosed. MIT, where she worked from 1959 until the mid-1970s, announced her death in an obituary on 7 October.
The stories of great expeditions usually follow an old order. The commanders come first, then the men in the vehicle, and somewhere far behind come the people who built the machine and wrote its instructions. Hamilton started out in that last rank. She spent much of her life making the world see it.
A pencil in the corridor
She was born in 1936 in Paoli, Indiana. Her family moved to Michigan, where she finished high school in 1954. She studied mathematics at the University of Michigan and then at Earlham College, graduating in 1958 with a minor in philosophy. That minor, Jennifer Ouellette writes in Ars Technica, was a nod to a father who was a poet and a grandfather who was a headmaster. The same year she married James Cox Hamilton. The plan was the usual one for the time: she would work until he finished law school, and then he would support her through a doctorate in mathematics.
In 1959 they went to Boston, where he studied law at Harvard. She took a job with Edward Lorenz, the MIT meteorologist who wanted to model the weather with differential equations. Neither of them knew how to program, so they taught themselves. She programmed his LGP-30 and also, according to Lindsay Clark’s obituary in The Register, DEC’s PDP-1. In 1961 Lorenz saw that tiny differences in starting values could produce very different weather, a sensitivity that became a hallmark of chaos theory. Hamilton had programmed the machine he was watching. Once, rather than go through the clunky debugging process, she rolled the paper tape down the hallway and fixed the binary by hand. “I’d poke holes for ones, and I’d cover up with Scotch tape the others,” she recalled. Before she left she trained her successor, Ellen Fetter, a recent Mount Holyoke graduate, and Lorenz credited both women in his papers.
Her next job was on SAGE, the Air Force air-defence system. Her first task was a program that nobody had managed to get running, whose author had written his notes in Latin and Greek. She got it working, and it printed its answers in Latin and Greek too. This was when she first became interested in whether software could be trusted.
The rope mother
In 1965 graduate school was within reach again. Then her husband spotted a newspaper advertisement: MIT’s Instrumentation Laboratory needed people to write guidance software for Apollo. She was the first programmer it hired, and the first woman. In those days a program was something you could hold. A copper wire threaded through a core meant one, and a wire that went around it meant zero. New England textile workers wove these wires into long ropes, and because the ropes were her responsibility she was called the “rope mother”.
As the newcomer, she was handed the abort routine, the job nobody thought would matter. “Everybody said that was never going to happen, [so they said] ‘Oh well, good. We’ll give this one to Margaret because she’s a beginner,’” she told the Computer History Museum. Then an uncrewed flight did abort. “It went to [run] this program I had written, which I named Forget It. All of a sudden, I became an overnight expert.”
On nights and weekends she brought her four-year-old, Lauren, to the lab. Lauren liked to play astronaut in the simulator. One day, pressing buttons at random, she brought up P01, a pre-launch program, in the middle of a mission, and the system crashed. Hamilton asked to add a safeguard against that mistake. She was overruled, because trained astronauts were not expected to make it. On Apollo 8, Jim Lovell made it, and the navigational data vanished. Hamilton was called in to recover it and called it the “Lauren bug”. After that the software was changed.
One might reflect that institutions trust the people they have trained, while a child trusts nothing and so finds the flaw.
The alarm over Tranquility
By 1968 she was assistant director in charge of the Command and Service Module team. MIT says she went on to lead development of the onboard flight software, overseeing more than 400 people. She had built a failsafe into the display routines for the moment the computer was asked to do more than it could handle in real time.
That moment came in July 1969, as Eagle descended toward the Sea of Tranquility. The computer began raising alarms, among them a 1202, which signalled “executive overflows”: it could not finish all of its tasks. MIT attributes the overload to a fault in a hardware switch. The Register traces it to unwanted inputs from the rendezvous radar. Mission Control considered an abort. Computer engineer Jack Garman told guidance officer Steve Bales that the alarms did not call for one. “We’re go on that alarm,” Bales confirmed. The software was dropping the work that could wait and keeping the work the landing needed. Hamilton described it in a 1971 letter:
Actually, the computer was programmed to do more than recognize error conditions. A complete set of recovery programs was incorporated into the software. The software’s action, in this case, was to eliminate lower priority tasks and re-establish the more important ones … If the computer hadn’t recognized this problem and taken recovery action, I doubt if Apollo 11 would have been the successful Moon landing it was.
She worked on six lunar landing missions, from 1969 to 1972. That generation is now passing. Days before her death was announced, rews reported the death of Milt Windler, a flight director who helped bring Apollo 13 home.
A name for the work
Back then, writing code was not taken as seriously as other kinds of engineering. So she began calling herself a “software engineer”, and the term is now standard. A well-known photograph shows her standing beside a stack of her team’s Apollo program listings that is almost as tall as she is.
After Apollo, the Instrumentation Lab became the independent Draper Laboratory. She stayed there until 1976 and then founded Higher Order Software, built on error prevention and fault tolerance. In 1985 she left to found Hamilton Technologies. Its Universal Systems Language and her Development Before the Fact methodology were both meant to stop errors before anyone made them.
Public honours came late. On 22 November 2016, in the East Room of the White House, Barack Obama gave her the Presidential Medal of Freedom. She was one of 21 recipients that day, and Grace Hopper was honoured posthumously in the same ceremony. “Our astronauts didn’t have much time, but thankfully they had Margaret Hamilton,” Obama said. The White House credited her with concepts such as asynchronous software, priority scheduling, priority displays and human-in-the-loop decision capability. A year later LEGO made her a minifigure in its “Women of NASA” set, next to Mae Jemison and Sally Ride. The Apollo code was posted on GitHub in 2015. In 2019 Google angled more than 100,000 mirrors at its solar farm in the Mojave Desert to make a portrait of her in reflected moonlight.
“To say Margaret Hamilton was a pioneer—to say she was ahead of her time—would be a dramatic understatement,” said Olivier de Weck, interim head of MIT’s Department of Aeronautics and Astronautics.
She is survived by her daughter, Lauren Hamilton, the child who once crashed the simulator. She also leaves her son-in-law, Richard Selesnick, two grandsons, four great-grandchildren, and her siblings John, David and Kathryn.