"Computer Got Loaded"

About This Document

Citation: Hamilton, Margaret H. “Computer Got Loaded.” Letter to the editor, Datamation, 17(5): 13, March 1, 1971.

Source: Archived magazine scan from bitsavers.org. Original copyright Cahners Publishing Company. Hamilton’s letter appears on page 13 of the magazine (PDF page 15), in the Letters section.

This is Hamilton’s earliest known published account of the Apollo 11 program alarm incident. Written just 20 months after the July 1969 landing, the letter is a direct response to Daniel D. McCracken’s January 1971 article “…but the Ambivalence Lingers On,” in which McCracken cited the Apollo 11 computer as an example of “a computer foulup.” Hamilton’s rebuttal established the narrative framework through which the incident has been understood ever since.

View source document — Datamation, March 1971 (PDF)

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Computer Got Loaded

Margaret H. Hamilton, Director of Apollo Flight Computer Programming, MIT Draper Laboratory, 1971

The Datamation exchange between McCracken and Hamilton is a pivotal moment in the public understanding of software reliability. To appreciate the letter fully, one must understand both halves of the call-and-response.

The Provocation: McCracken’s January 1971 Article

Daniel D. McCracken — a prominent computing consultant, author of ten books on programming including the widely used A Guide to COBOL Programming, and a Datamation contributing editor — published “…but the Ambivalence Lingers On” in the January 1, 1971 issue. The article surveyed public attitudes toward computers: billing errors, election prediction failures, welfare check mistakes. In a litany of examples, McCracken included: “The Apollo 11 computer almost forced an abort of the moon landing, although nothing was actually wrong.”

McCracken’s characterization contained two claims that Hamilton would directly contest. First, that the computer “almost forced an abort” — Hamilton would argue it prevented one. Second, that “nothing was actually wrong” — Hamilton would identify a specific checklist error as the root cause. McCracken was not a fringe voice; he was a mainstream authority. His casual framing of the incident could have become the permanent public record.

Hamilton’s Response

Two months later, Hamilton wrote approximately 300 words that reshaped the narrative. Her account, concise and technically precise, established the following:

Root cause: The rendezvous radar switch was placed in the wrong position due to an error in the checklist manual. This caused the radar to send erroneous signals to the computer.

Overload characterization: The computer was asked to perform all normal landing functions while simultaneously processing “an extra load of spurious data which used up 15% of its time.”

Error detection: The software recognized it was being asked to perform more tasks than it should, and sent an alarm to the astronaut.

Priority-based recovery: The alarm communicated: “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.”

Recovery action: The software eliminated lower-priority tasks and re-established the more important ones.

Counterfactual: “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.”

The Key Reframing

Hamilton’s central correction: “The computer, rather than almost forcing an abort, prevented an abort.” In a single sentence, she transformed the computer from villain to hero — from a system that nearly failed to a system that performed exactly as designed under unexpected conditions.

Significance

The letter is remarkable for several reasons beyond its historical importance.

Precision in error attribution. Hamilton is careful to distinguish between the computer, the software, the checklist, and the operators. This precision — determining where in the system an error originated versus where it manifested — became the basis for her later interface error taxonomy that would culminate in the 2018 “What the Errors Tell Us” paper.

Anthropomorphization as argument. Hamilton writes that “the software in it was smart enough to recognize” the overload condition. This is deliberate. She is making the case that software can embody engineering judgment, not merely execute instructions. In 1971, this was a radical claim.

Authority by title. Hamilton signs as “Director of Apollo Flight Computer Programming, MIT Draper Laboratory” — a title that itself helped establish software development as a leadership-level engineering discipline.

Context of publication. The letter appears mid-page in Datamation’s Letters section, between a letter about science fiction and one about venture capital. No special prominence was given to what is now recognized as a historically significant document. The cover of this issue, by coincidence, features Roy Lichtenstein’s “Temple of Apollo” painting — though the cover article is about university computing, not spaceflight.

Modern Connection

Hamilton’s letter demonstrates that the person who defines the narrative of a system failure determines public understanding for decades. McCracken’s “foulup” framing could have become permanent. Hamilton’s correction — that the software worked as designed under unexpected conditions — established the framework through which the Apollo 11 computer incident is now universally understood. This dynamic recurs constantly in modern incident post-mortems: the first narrative to gain traction often becomes the accepted one, whether or not it is technically accurate.

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References

• McCracken, Daniel D. “…but the Ambivalence Lingers On.” Datamation, 17(1): 28—30, January 1, 1971.
• Hamilton, Margaret H. “Computer Got Loaded.” Letter to the editor, Datamation, 17(5): 13, March 1, 1971.

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Related Documents

• What the Errors Tell Us (2018) — Hamilton’s mature error taxonomy. The intuitions expressed in this short 1971 letter find their theoretical formalization 47 years later. The 2018 paper cites this letter as reference 3.
• The Apollo On-Board Flight Software (2019) — Hamilton’s retrospective, telling the same Apollo 11 story 48 years after this letter, with the benefit of a lifetime’s reflection on error categorization and preventive systems.
• USL: Lessons Learned from Apollo (2008) — Retells the Apollo 11 incident with full technical detail and places it within the USL theoretical framework. The 37-year gap between 1971 and 2008 shows how Hamilton’s understanding of the incident deepened over time.
• Higher Order Software (1976) — The formal axioms paper. The graceful degradation and priority-based task shedding described in the 1971 letter are formalized here as properties of the control system.
• HOS Conference Papers (1974, 1978) — The 1974 paper explicitly references the “Computer Got Loaded” letter and the Apollo error analysis as empirical foundations for the axioms.
• Apollo DSKY Simulator — Run the software Hamilton describes in this letter. The simulator executes Luminary099 in your browser — the same priority-based Executive that shed lower-priority tasks and kept the landing guidance running during the 1202 alarms.