Three minutes before Apollo 11's lunar module touched down on the moon on July 20, 1969, a yellow warning light flashed the code 1202 on its control panel: the guidance computer was processing more than it could handle in real time. It did not fail, because Margaret Hamilton, the MIT engineer who led the team that wrote its software, had designed the program to shed its lowest-priority tasks and keep landing. Hamilton, who later coined the term "software engineering" to describe the field she helped invent, died September 30, 2026, at a hospital in Cambridge, Massachusetts, at 90, according to WBUR.

Hamilton was born August 17, 1936, in Paoli, Indiana, and grew up largely in Michigan's Upper Peninsula, according to WBUR. She enrolled at the University of Michigan to study mathematics in 1955, then completed her degree at Earlham College in Richmond, Indiana, graduating in 1958 with a major in mathematics and a minor in philosophy. The following year she moved to the Boston area and took a programming job at MIT, writing weather-prediction software for the meteorologist Edward Lorenz, MIT News reported.

In the early 1960s she moved to MIT Lincoln Laboratory to write code for the SAGE air-defense radar system, and in 1965 she was hired as the first programmer on NASA's new Apollo program at MIT's Instrumentation Laboratory. By 1968 she had been named assistant director in charge of the Command and Service Module software team, overseeing a division that eventually grew to more than 400 engineers. At a time when the discipline barely had a name, Hamilton began calling her own work "software engineering," arguing it deserved the same professional standing given to the hardware engineers she worked beside.

Her caution about software failure was rooted in direct experience. One evening in the mid-1960s, her four-year-old daughter Lauren was playing in the lab and keyed in the command module simulator's prelaunch program, P01, in the middle of a live simulation, crashing it. Hamilton asked NASA to add a safeguard against the same mistake happening during an actual flight; engineers judged it too implausible for a trained astronaut to repeat and declined, though they let her note the risk in the mission documentation, MIT Technology Review reported. During Apollo 8 in December 1968, astronaut Jim Lovell keyed in P01 at the wrong moment and the navigation data vanished mid-flight; Hamilton's team used her documented fix to help restore it. "I remember saying to a couple of guys there, 'It's the Lauren bug!'" she recalled.

That same insistence on rigor was what kept the 1202 alarm from becoming a catastrophe seven months later. Hamilton's team had built the Apollo Guidance Computer's software with asynchronous priority scheduling, so that if the system was overloaded it would restart and protect only the functions needed to land, rather than freeze or crash outright. When a stray radar switch flooded the computer with extra data minutes before Neil Armstrong and Buzz Aldrin reached the surface, that design let mission control clear the crew to continue. "Looking back, we were the luckiest people in the world; there was no choice but to be pioneers," Hamilton said of the small team that had built the software largely from scratch.

After Apollo, Hamilton kept working on the problem of making large software systems resistant to their own errors. In 1976 she founded Higher Order Software, built around her idea that systems should be designed to prevent faults before they occurred rather than debugged afterward, and in 1986 she founded Hamilton Technologies, Inc., in Cambridge, where she developed the Universal Systems Language, a formal method for specifying software before it was written. For much of this period, outside the engineers who used her tools, she worked largely out of public view.

Recognition caught up with her late. NASA gave her its Exceptional Space Act Award in 2003, and on November 22, 2016, President Barack Obama presented her with the Presidential Medal of Freedom, citing software architecture that "led to giant leaps for humankind, writing the code that helped America set foot on the moon." The Computer History Museum named her a 2017 Fellow "for her leadership and work on software for DOD and NASA's Apollo space missions," according to the Computer History Museum, and in October 2018 the Universitat Politecnica de Catalunya in Barcelona awarded her an honorary doctorate. She was named to the National Aviation Hall of Fame's 2022 induction class, and in 2017 a Lego minifigure modeled on her, standing beside a miniature stack of her team's code listings, was released as part of the company's Women of NASA set. MIT professor Olivier de Weck said Hamilton's career had been "dedicated to preventing errors and what she called 'handling the unknown,'" and that she had "established a practice of software engineering based on problem-solving and systems engineering that we all benefit from."

Hamilton was survived by her daughter, Lauren Hamilton; son-in-law Richard Selesnick; two grandsons; four great-grandchildren; and her siblings John, David and Kathryn, MIT News reported. "She gave me, from the get-go, this notion that I could do anything, that there were absolutely no gender barriers," Lauren Hamilton said of her mother.

What endures from Hamilton's career is unusually countable. The stack of hand-annotated Apollo Guidance Computer source code listings she once posed beside, taller than she was, survives today as an artifact of a discipline that barely had a name when she started writing it. The term she insisted on using for her own job, software engineering, is now how millions of people describe theirs. And the priority-scheduling logic she built so that a spacecraft's computer could fail gracefully under pressure remains a foundational case study in how software should be designed to survive the unexpected.