Thanks to the radio telemetry streaming from Apollo 11’s Lunar Module Eagle on July 20, 1969, the engineers at Mission Control could tell, second by second, just how bad the astronauts’ predicament was getting as they prepared to make the first landing on the moon. Or could they? Three thousand feet over the lunar surface, the onboard computer guiding the landing appeared to be failing, but the cryptic alarms that flashed on the monitors in Houston—1202, 1201—were understood by only a few of the ground controllers. If NASA could not resolve the problems, Neil Armstrong and Buzz Aldrin might have to abort their landing. The LM’s gauge showed that it had barely enough fuel to land, and no margin for lengthy debate.
The alarms signaled an overload of Eagle’s main computer, an all-purpose machine that was running the LM’s navigation system and controlling its descent to the lunar surface. Contrary to popular belief, the LM couldn’t be landed “manually”—the finesse required to control Eagle’s thrusters without computer assistance was beyond the capability of any astronaut who had tried it in the simulator. Rather, as David Mindell writes in Digital Apollo: Human and Machine in Spaceflight, the astronauts were supposed to work with the LM’s Apollo Guidance Computer, using their control sticks to nudge the computer’s calculations.
But on Apollo 11, something went wrong. The guidance computer wasn’t completely user-friendly: It reported system conditions using codes the astronauts couldn’t readily decipher. Overwhelmed by radar data, the computer started dropping unnecessary programs, launching its own version of the Microsoft Windows Task Manager. Neither the astronauts nor the computer ever lost their cool, though, and thanks to NASA guidance officer Steve Bales, and Jack Garman, a young computer specialist with an alarm code crib sheet, NASA didn’t abort the landing. (Garman later received an award for his quick response.)
So what happened? Crossed wires. During the design of the guidance computer at MIT’s Instrumentation laboratory, Buzz Aldrin wanted the computer to be able to simultaneously handle radar data from the lunar surface and the Apollo Command and Service Modules in lunar orbit, just in case Apollo 11 needed to abort the landing and rendezvous with the CSM. Despite discussions, the added capability was never built into the system, to Aldrin’s surprise on July 20th. (Simulations had never fully tested the feature and failed to reveal its absence.)
In all other respects, though, the system worked fine. Later accounts described an overloaded computer and Armstrong landing the craft by hand. In fact, though, the guidance computer had performed as it was supposed to perform. As the craft approached the lunar surface, Armstrong used his controller to instruct the computer to change the landing site to a smoother section of terrain, and the guidance computer kept humming right along until the contact light went on.
An unsung hero of the decision not to abort the landing is Richard Koos, a NASA simulation supervisor who, on the afternoon of July 5, 11 days before the launch of Apollo 11, put the team of controllers including Bales, Garman, and capcom astronaut Charlie Duke, through a simulation that intentionally triggered a 1201 alarm. The astronauts involved in the simulation were Dave Scott and Jim Irwin, the backup crew for Apollo 12 and the prime crew for Apollo 15. Unable to figure out what the 1201 was, Bales aborted that simulated landing. He and Flight Director Gene Kranz were dressed down for it by Koos, who put the team through four more hours of training the next day specifically on program alarms. When the 1202 and 1201 alarms occurred during the actual landing, Garman, Bales, and even Duke recognized them immediately.
The efforts of astronauts and engineers on the ground had made the landing a success. There would be other scares during the Apollo landings, but none so harrowing: in spaceflight, a computer “crash” had taken on a whole new meaning.
Hersch, an HSS/NASA Fellow in the History of Space Science at the University of Pennsylvania, is writing a labor history of American astronauts. He’ll be blogging regularly about the Apollo anniversary during the month of July.