Margaret Hamilton, whose pioneering software played a critical role in the first moon landing, hated it when computers crashed without explanation. In an era when computers and people rarely interacted directly, she made sure the flight software she developed for NASA's Apollo onboard computers alerted astronauts the instant something went wrong.

She found her insistence on such openness between man and machine put to an unusually public test in 1969, when Apollo 11 astronauts descended to the lunar surface for the first moon landing. Computer alarms grabbed the attention of astronauts Neil Armstrong and Buzz Aldrin just as they prepared to touch down. Hamilton, who had programmed the warnings, felt her heart racing. "Neil's blood pressure was 150. I felt mine was 200," she later recalled.

Even so, she knew the alarms signaled that the computer, as designed, was dumping unimportant tasks in order to keep flying safely. In the tension of the moment, flight directors at the National Aeronautics and Space Administration's Mission Control in Houston also understood that her alarms signaled the computer was still functioning reliably. Reassured, they gave the astronauts the "Go" to press ahead to a successful landing.

Hamilton died on Sept. 30, the Massachusetts Institute of Technology announced Wednesday. She was 90 years old.

In the annals of aerospace computing, her innovative alerts are a testament to the robust Apollo software crafted under her supervision at the Instrumentation Laboratory at the Massachusetts Institute of Technology. The code -- now in the collection of the Smithsonian Air and Space Museum -- helped lay the foundations of modern computing. And Hamilton became an icon to a generation of women in computer science.

Recognizing her pioneering work, NASA in 2003 gave her a special award for her role in the Apollo software development. In 2016, President Barack Obama awarded her the Presidential Medal of Freedom, saying that "her 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."

She was born Aug. 17, 1936, in Paoli, Ind., and grew up in the copper-mining country of upstate Michigan. She earned a bachelor's degree in mathematics with a minor in philosophy from Earlham College in 1958.

When she began working on computers in 1959, programming was more art than engineering. She learned the rudiments of coding by running calculations for MIT meteorologist Edward Lorenz, who discovered chaos theory.

She honed her skills by working on a $27 billion national air-defense system. Whenever a programming mistake crashed the 250-ton military computer, she recalled, alarms shrieked throughout the building; yet the computer had no way to track errors. She purchased a Polaroid camera to take instant "debugging portraits" of the errant code before it vanished from the computer display.

At the time, programmers weren't normally given direct access to mainframe computers. "We would send the computer operators off to play Ping-Pong, so we could run the computer ourselves," she recalled. "It gave us such a feeling of power."

In 1965, she started work on Apollo flight computer code at the Instrumentation Laboratory, now known as the Charles Stark Draper Laboratory. She first took responsibility for the spacecraft's system software. By Apollo 11, she had broad responsibility for managing the command module and lunar-lander software. She later became director of the laboratory's Software Engineering Division.

While primitive by today's standards, the Apollo Guidance Computer in the 1960s was the most advanced general-purpose portable computer in the world. It was the first to have a digital interactive display that allowed astronauts and the computer to communicate with each other and exchange data. She created alarm codes to ensure the computer always informed the crew of its problems.

"What if there is an emergency and there is no way to tell the astronauts that they are in trouble? I figured out a way to let them know in the software that there is an emergency, and let mission control know, too," she said.

In fact, the Apollo guidance software she helped manage was so reliable that no software bugs were found on any crewed Apollo missions. It was adapted for use in Skylab, the Space Shuttle, and the first digital fly-by-wire systems in aircraft.

From 1976 through 1984, Hamilton was the CEO of a company she co-founded called Higher Order Software. In 1986, she became founder and CEO of Hamilton Technologies in Cambridge, Mass.

In 2017, Lego introduced a Margaret Hamilton action figure, as part of its Women of NASA collection. Her daughter assembled it for her. In 2018, Blue Origin, the space venture founded by Amazon.com billionaire Jeff Bezos, flew her Lego figure into orbit and back. "It meant a lot to me," she said.

Her survivors include her daughter, Lauren, two grandsons and four great-grandchildren.