This goes along with the general discussion about the state of technology available to NASA in the 1960s. But since computer capability has compounded many fold since Apollo, it is sometimes treated separately.
As with the general level of technology, conspiracists often try to compare the availability and sophistication of consumer computing equipment with that available to NASA. Computer companies of the 1950s and 1960s had to produce general purpose computers at a cost that would attract business and scientific customers. NASA had to solve only one problem -- guidance -- and could easily afford to have a custom system designed and built for them using cutting edge components and techniques.
We could today, if we wanted, produce very fuel-efficient automobiles that would go for hundreds of thousands of miles without any regular service or mechanical breakdown. Unfortunately that car would cost well over a million dollars a unit, and would therefore be out of reach of most consumers. And so automobile companies produce vehicles more tailored to the economy of their intended customer. As a result the level of technology lags behind what would be achievable if money were no object.
The question to ask is not what kinds of computers were available in IBM's color brochures, but what kind of computer was available to NASA with its essentially bottomless pockets.
The Apollo guidance computer had the computer power equivalent only to today's kitchen appliances, far less than what would be required to go to the moon.
It always amuses us to hear this from people who sit at multi-gigahertz computers and can't imagine that anything less was ever remotely usable for anything. This is a good example of a mental technology trap. People believe that because we use a particular technology to solve a particular problem today, that problem wasn't solvable before the technology was available.
As a matter of fact, John Glenn flew his spacecraft to earth orbit without any onboard computer whatsoever. Yet the trajectory was precisely controlled, and his capsule could have operated completely automatically if necessary. (In fact, the original design called for it to be completely automated, but the astronauts demanded the ability to pilot the capsule.)
So far no conspiracist has yet been able to accurately enumerate what computational tasks were required for going to the moon. It's one thing to say that a computer in the 1960s would be no match for a computer today. But it's another thing entirely to say that the computer built in the 1960s wasn't up to the task for which it had been designed. The conspiracists claim the latter, but provide evidence only for the former. To make the case that the guidance computer was not adequate to its task, one must first describe the task. Then one must show the specific deficiency of the computer with respect to that task.
Just to run a moon landing simulation requires dozens of megabytes. It would require more to accomplish the actual task.
This is the typical computer-illiterate attempt to compare the guidance computer to its task. Not being able to speak intelligently about the problem of guidance in space travel, the conspiracists select a problem they believe is similar (a lunar lander arcade game) whose requirements they believe they know.
There are of course a number of things wrong with this argument. First, moon landing simulations do not inherently require lots of computer resources. They do on today's personal computers, but only in the sense that any task on today's personal computers requires lots of resources. That's because those computers have heavyweight, general-purpose operating systems and are expected to provide lots of bells and whistles.
Some of the first programs on the small minicomputers of the 1960s and 1970s were rudimentary one-axis lunar lander games, including one for the DEC PDP-8 (Fig. 7), a computer with similar capabilities as the Apollo guidance computer. Of course they lacked the fancy three-dimensional graphics and realistic sound effects (Fig. 1), but they captured the essence of the physical behavior. See below for a description of the difference between a special-purpose computer and a general-purpose computer.
The notion that the real thing would be more involved than a simulation is intuitively wrong. The simulation not only has to embody the behavior of the simulated object, but it also has to programmatically create the environment -- the external effects like gravity. The Apollo guidance computer didn't have to create the lunar environment as part of the program; it was in the lunar environment.
In a flashy lander simulation, throwing a switch means performing a mouse gesture over its icon on the screen. The lander simulation must contain program code to create the icon, animate it, interpret the mouse motion, and translate that into a change in the operating state of the program. In the real guidance computer the guidance program does none of that; the pilot flips a switch and the corresponding computer "bit" is set or cleared in the computer's memory by the switch electronics
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