Its July 20, 1969, Apollo 11 mission was close to touchdown on the moon. Neil Amstrong, Buzz Aldrin, and Michel Collins checking last-minute preparation for landing. Over their onboard computer, something started flashing. It was Error 1201 followed by error 1202. Human beings first landing on the moon is jeopardized by two computer errors
Apollo 11 was the first moon lander to carry humans to the moon. During the height of the cold war, both the USA and USSR wanted to put their men on the moon. History has picturized the famous speech of John F Kennedy, stating. “We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard.” With funding from the USA government, NASA was able to plan the Apollo mission as fast as it can.
Apollo 11 was one of the first space vessels with Onboard computers much as we have in our modern cars. The onboard computer is in charge of navigation and crucial components of the space vessel. The smartphone from which you might be reading this article is about 120 million times more powerful than the Apollo Guidance Computer. The Apollo 11 computer had a six-slot logic module in which the code was woven in. 1,45,000 lines of code were written, and it created one of the famous computer science photos of all time. It was Margret Hamilton from MIT who was in charge of the command module and lunar module standing next to the code she wrote.
Margaret Hamilton’s Apollo code
Apollo 11 system overview
Apollo guidance computer Source Wiki
The advantage of these computers is that they have a magnetic read-only memory. The code is written and plugged into the main CPU, and it runs instructions in manner of order as instructed by the user. The mission was divided into the Base station, Lunar module, and Orbit module. The orbit module handles communication between the Base station in the earth and the Lunar module in Moon.
The central microcontroller was in charge of executing all the commands as per instructions. It was also parallelly getting data from the various sensors and actuators as explained in the below diagram
https://github.com/chrislgarry/Apollo-11
http://svtsim.com/moonjs/agc.html
https://history.nasa.gov/afj/ap11fj/a11-documents.html
Source: NASA Flight Manual
On the fine day of landing just a few minutes before the lunar module is going to touch down, there were two errors in their on-board computer. It was error 1202 and 1201. This was caught vividly in the technical transcript between Apollo crew and base station on earth
Apollo 11 onboard transcript
What caused the problem
The lunar module guidance computer has various sensors. Its purpose is to maintain constant communication with the base unit. There were two communication lines. One of the astronauts accidentally switched on the secondary communication link. This flooded the computer with a large quantity of data, which it couldn’t process as it was causing a memory overflow. The error 1202 and 1201 means the memory is full, and the computer kept restarting.
Quickly judging the situation, the base station on earth informed the crew not to abort the mission. Neil Armstrong was asked to take over manual control of the lunar module. Neil Armstrong maneuvered the module manually to the moon.
From Technical documents NASA
The timely response from base station saved the mission if not error 1202 and 1201 would have been the worst errors in human history