In 1961, the average rocket-borne computer ran on average for 15 hours before an electronics failure crashed it. That dismal performance record didn’t matter much to the military, whose suborbital missiles required only minutes of computer on-time. But a manned moon shot required that computers run 1500 hours between failures.
As David Mindell points out in Digital Apollo: Human and Machine in Spaceflight , NASA’s project managers not only met that 1500-hour goal but greatly overshot it. When Neil Armstrong and his compatriots strode on the lunar surface between 1969 and 1972, the total mean time between failures of the onboard computers turned out to be closer to 50 000 hours.
The story is more than a historical footnote. Computer systems reliability was perhaps the biggest spin-off the space program ever had. The entire world has benefited from the achievement.
Yet getting there was half the fun. The most compelling part of the book is its careful examination of the ways computers occasionally misbehaved. All six moon landings, for instance, took place with the mission commander manually overriding an overwhelmed computer—which carried just 4 kilobytes of RAM and clocked in at 1 megahertz.
Digital Apollo is certain to interest readers of a technical bent, especially those curious to delve into the gritty details of some of the first portable computers. Mindell joyfully plumbs the deep history of Apollo’s decade-long clash between the MIT eggheads who built the computers and the thrill-jockey military test pilots who used them. Even I, who last programmed a computer in the early 1990s, found myself swept up in the account of the Apollo 11 mission. The entire world watched as the Eagle ’s computer blurted out strings of alarms to its pilots, who simply ignored the warnings and landed the thing anyway. Neil Armstrong’s cool control, leaving just 40 seconds of fuel in the tank, deserves shared credit with all the hardware and software Mindell cites as having saved Apollo 11 from disaster.
Unfortunately, the book devotes just one paragraph to the most extraordinary test the project’s computers ever faced: the Apollo 13 mission, which after a midflight oxygen tank rupture became a storied quest to rescue three men from spacebound oblivion. By the time Mindell gets that far in his narrative, he seems to want to concentrate only on moon landings—even though he leads off his book with a poetic Antoine de Saintâ''Exupéry quote about man versus machine. Mindell ought to have known that inflexible schedules and programs shouldn’t always be allowed to carry the day.