In Their Own Words: Margaret Hamilton's 'Computer Got Loaded'
Letter to the Editor of Datamation Magazine from Margaret Hamilton, March 1st 1971. Hamilton worked in computer science and software engineering for the Apollo Program and the MIT Instrumentation Lab / Draper.
I'd like to clear up a gross misconception that one of your contributors has concerning the behavior of the onboard computer during the Apollo 11 landing. Evidence of the misunderstanding can be found in the Jan. 1, 1971 issue, in the article, "...but the Ambivalence Lingers," by Daniel D. McKracken. In this article, he gives the following as an example of "a computer foul up": "The Apollo 11 computer almost caused an abort of the Apollo 11 moon landing, although nothing was actually wrong." There definitely was something wrong, although it was not the computer or its software.
Due to an error in the checklist manual, the rendezvous radar switch was placed in the wrong position. This caused it to send erroneous signals to the computer. The result was that the computer was being asked to perform all of its normal functions for landing while receiving an extra load of spurious data which used up 15% of its time. The computer (or rather the software in it) was smart enough to recognize that it was being asked to perform more tasks than it should be performing. It then sent out an alarm, which meant to the astronaut, "I'm overloaded with more tasks than I should be doing at this time and I'm going to keep only the more important tasks; i.e., the ones needed for landing ..."
To blame the computer for the Apollo 11 problem is like blaming the person who spots a fire and calls the fire department. Actually, the computer was programmed to do more than recognize error conditions. A complete set of recovery programs was incorporated into the software. The software's action, in this case, was to eliminate lower priority tasks and re-establish the more important ones. The computer, rather than almost forcing an abort, prevented an abort. If the computer hadn't recognized this problem and taken recovery action, I doubt if Apollo 11 would have been the successful moon landing it was.
Margaret Hamilton, Director of Apollo Flight Computer Programming, MIT Draper Laboratory, Cambridge, Massachusetts. March 1, 1971
Datamation was a computer magazine that was published in print form in the United States between 1957 and 1998, and has since continued publication on the web. Today, Datamation is published as an online magazine.