During Apollo 11’s lunar descent on 20 July 1969, the guidance computer displayed 1201 and 1202 program alarms. The crew and ground controllers had to decide whether landing could continue.
What the alarms meant
NASA’s account of the program alarms explains that the Executive lacked resources for additional jobs: 1202 concerned core sets, while 1201 concerned VAC areas. Restart protection re-established essential tasks rather than letting the overload end guidance.
Spurious rendezvous-radar-related interrupts contributed to the overload. A radar-interface and reference-signal problem is a more accurate description than a mechanically “stuck radar.”
Software and people worked together
Margaret Hamilton led the Instrumentation Laboratory’s onboard flight-software work. The scheduling and restart system was developed and tested by a team; it should not be attributed as a single program written entirely by one person.
NASA’s account of Jack Garman’s role describes the NASA engineer’s advice during the descent. Guidance officer Steve Bales relied on that expertise in assessing whether the alarms allowed the mission to continue. Garman was not an MIT engineer.
A recovery mechanism, not the disappearance of the problem
The alarms signalled that the computer needed recovery. Critical guidance resumed, and the ground response judged that the mission could proceed. That is different from claiming the radar’s unwanted interrupts were simply removed forever by priority scheduling.
The lesson is broader than a lone-hero account. A system needs a way to recover, people need to understand what its alarms mean, and the recovery must have been tested before the moment it is needed.
The successful landing makes the engineering memorable. Preserving the distinctions between the radar issue, the software’s response and the controllers’ decision makes the history useful.
Correction, 3 October 2026: The earlier article called the radar stuck, attributed the entire scheduling system to Hamilton alone, misidentified NASA’s Jack Garman as an MIT engineer, and misdescribed the alarm resource codes and timing. The account now distinguishes the interface issue, team-developed recovery and ground response.
