This week in rocket history, Apollo 11. Apollo 11 launched from Pad 39A at Kennedy Space Center on July 16, 1969, at 13:32 UTC. About three days later on July 20, Neil Armstrong and Buzz Aldrin became the first humans to land on the lunar surface after leaving Michael Collins in the Command Module Columbia in lunar orbit.

The landing was almost aborted early in the descent after the 1201 and 1202 master program alarms flashed on the computer display several times. Neither crewmember had any idea what the alarm meant, but two flight controllers in Mission Control remembered the alarm from a simulator run and knew it wasn’t an issue, clearing Neil and Buzz to continue to the surface.

So that 1202 master alarm: what was it and why was it okay to ignore it? Basically, the alarm was the computer screaming at the commander that it had been given too much to do at one time, which caused it to become overloaded.  In order to correct the issue, the pilot needed to turn off some functions. The overload was caused by running the descent program and the rendezvous radar at the same time, which had been started by Buzz to save time in the event of an abort.

The 1201 alarm is probably something you may have experienced yourself. It signaled that the overload was from a program loaded into RAM. (RAM at the time was only four kilobytes, so it didn’t take much to cause an overflow.)

The alarms weren’t the end of the excitement during this very eventful landing. Most of it was done on autopilot, but Neil took over manual control after he saw the landing site the computer was headed to was filled with massive boulders. He came within what he thought was about 25 seconds of depleting the propellant in the descent stage when he should have had 120 seconds worth of propellant in reserve. In actuality, the propellant was sloshing around in the tank causing false low propellant warnings when there was actually more propellant than indicated.

One other tense incident occurred when the astronauts were preparing to depart the lunar surface. At some point during their time on the surface, one of the astronauts accidentally broke off the switch to the breaker that controlled the ascent stage engine ignition circuit. If they couldn’t flip the switch they wouldn’t be able to go back into lunar orbit to meet up with Collins and return to Earth in Columbia. Luckily, Aldrin was able to shove the cap of a felt tip pen into the circuit breaker, completing the circuit and sending them on their way into history as the first humans to land on the Moon and return.

Overall, Armstrong and Aldrin spent just over two and a half hours out of a total of 21 hours and 36 minutes on the Moon exploring the surface, collecting samples, and setting up experiments.

The lunar module Eagle rejoined with Collins in the Command Module, and the three of them returned home. Splashdown in the Pacific Ocean southwest of Hawaii took place on July 24, 1969, at 16:50 UTC, thus concluding humanity’s first crewed mission to the surface of the Moon.

More Information

Apollo 11 Mission Overview (NASA)

Trying to Rest (NASA Transcript)