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A July Day on the Moon

On July 20, 1969 workers called in sick and children stayed home from school. Crowds gathered around televisions in department store windows to watch the Apollo 11 moon landing. Digital Apollo: Human and Machine in Spaceflight by David Mindell examines the design and execution of each of the six Apollo moon landings, drawing on transcripts and data telemetry from the flights, astronaut interviews, and NASA’s extensive archives. In honor of the anniversary of the first moon landing, the following is an excerpt from Digital Apollo that describes the high tension of that fateful day.

On a July day in 1969, after a silent trip around the far side of the moon, the two Apollo spacecraft reappeared out of the shadows and reestablished contact with earth. The command and service module (CSM) (sometimes simply ‘‘command module’’) was now the mother ship, the capsule and its supplies that would carry the astronauts home. The CSM continued to orbit the moon, with astronaut Michael Collins alone in the capsule. ‘‘Listen, babe,’’ Collins reported to ground controllers at NASA in Houston, ‘‘everything’s going just swimmingly. Beautiful.’’ His two colleagues Neil Armstrong and Edwin ‘‘Buzz’’ Aldrin had just separated the other spacecraft, the fragile, spidery lunar module (LM, pronounced ‘‘lem’’), nicknamed Eagle, from the command module. This odd, aluminum balloon, packed with instruments and a few engines, would carry the two men down to the lunar surface.

A rocket engine fired to slow the LM, causing it to fall out of orbit. Once on its way down, the spacecraft would either execute a dangerous abort or soon hit the moon. Whether the impact was a landing or a crash depended on the next ten minutes—the longest continuous series of critical events in the entire mission.

In the LM, weight was such a premium that seats had been eliminated altogether. The astronauts stood up, stabilized by tensioned cables connected to the floor. The spacecraft was an enclosing home, complementing the astronauts’ bodies. It supplied their food, exchanged their gases, and collected their wastes. The human occupants, in turn, controlled flows of the spacecraft’s numerous fluids, drinking some for hydration and carefully igniting others for propulsion. An inertial navigation system— accelerometers wedded to precisely spinning gyroscopes—measured the vehicle’s motions. A radar reached an invisible beam down to sense the first approach of the moon’s surface, like a blind man’s stick tapping for a curb.

Tying the whole thing together was an embedded digital computer, made out of exotic devices called ‘‘integrated circuits’’—silicon chips, running a set of esoteric programs. In the middle of the instrument panel, amid familiar dials and switches, stood the computer interface, a numeric keypad glowing with segmented digits. Throughout the mission the astronauts punched in numbers, ran programs, and read the displays. Much of the landing was under direct control of these programs. Neil Armstrong, when he did fly, did not command the spacecraft directly, but rather used two control sticks to command the computer, whose programs fired the thrusters to move the LM. Every move was checked and mediated by software, written by a group of young programmers half a world away.

Nor was the LM alone with its computer, for the command module had an identical computer of its own; both were linked to Houston’s control center. Down in Houston, numerous experts monitored systems, offered advice, and controlled some parts of the flight (they even had a remote computer keypad for entering commands directly into the computer in either of the two spacecraft). Communication between the three nodes was calm, matter-of-fact conversation, the precise technical banter of professionals, with an audience of millions.

As the LM began to descend the ground controllers focused their attention. Mission Control locked its doors.

Suddenly, the LM lost contact with Houston. The main antenna that carried data and voice communications to NASA’s control center in Houston was having problems. It had to point directly at the earth to work, but other parts of the spacecraft blocked the path, so a computer-controlled feedback loop was commanding the antenna to ‘‘hunt’’ around to seek a new orientation. Aldrin intervened, turned off the automatic control, and adjusted the antenna by hand. The imperfect communications now required Aldrin’s attention to keep on track. Frustrated flight controllers in Houston strained through the noise to hear the astronauts, struggling to piece together a continuous story from intermittent bursts of data. ‘‘This is just like a simulation,’’ one controller observed on the intercom. Indeed, the performance had been rehearsed, countless times in countless variations, in computer-controlled virtual simulations on the ground.

The astronauts stood in a high state of tension. Their attention was a scarce resource, and any increase in the ‘‘workload’’ could cause them to lose control of the situation. Indeed, for Armstrong the faulty communications detracted from the intense focus of his task.

Uncertainty and ambiguity: on one hand, the astronauts were in control, piloting an autonomous machine far from home; on the other hand, they were part of a network of communications channels, human experts, and control centers. Intermittent communications caused Aldrin to oscillate across this borderline: ‘‘You didn’t know where you were—whether you were on your own, or whether you were still under the close supervision of ground control. And that sort of reality is rarely simulated in training.’’ The engineers who designed the system (including the astronauts themselves) did not anticipate how electrical noise could interfere with this critical control loop, half a million miles long. Still, in the scheme of things, these problems were minor, easily handled by the conservative design of the LM and the calm professionalism of the astronauts.

At 50,000 feet above the moon’s surface, the LM rocket engine fired again, the powered descent initiation (or PDI) that would bring the vehicle to the surface. ‘‘Throttle up. Looks good!’’ Aldrin radioed. They were going down. The computer was in control. Then, as the spacecraft passed 35,000 feet above the moon, an unexpected light flashed on the computer display.

‘‘Program alarm,’’ Armstrong called out, with noticeable concern.

The computer was having a problem, calling the astronauts’ attention. Like its users, the machine had a limited amount of workload and the processor was overloaded with data. This problem might not be serious, but at this moment even a benign distraction could cause trouble. The computer restarted itself, Aldrin punched some keys to inquire about the problem, and it indicated alarm code 1202.

‘‘Give us a reading on the 1202 Program Alarm,’’ Armstrong urgently asked Houston. With the push of a button, the astronaut could abort the landing, an action practiced countless times in simulation. Yet he held off. Armstrong later explained himself as a mechanism: ‘‘In simulations we have a large number of failures and we are usually spring-loaded to the abort position. And in this case in the real flight, we are springloaded to the land position.’’

Houston checked out the problem. Young engineers recognized it from a recent simulation, and conferred with their support teams in the back room. They quickly found the cause. The computer was overloading and restarting but not shutting down. It was ignoring low-priority tasks, but these were not critical for the mission. ‘‘We’re go on that alarm,’’ the ground controller replied, meaning the LM could proceed. For his role in clearing the landing, engineer Steven Bales later accepted a presidential award on behalf of the flight control team.

Armstrong surveyed the computer display; it had frozen. He checked the LM’s systems. The vehicle seemed to be responding to his commands, meaning the computer was still running. So he continued. But these checks focused his attention inside the cockpit for critical moments when he should have been looking out the window for a landing site.

Now 2,000 feet above the surface, he again looked out the window. There he saw a potential disaster—a large crater stood where the landing area should have been, boulders surrounding its rim. In that moment Armstrong quit being a computer operator and became a pilot. He seized control of the spacecraft from the computer and flew the LM past the crater. The move took precious additional seconds, and ground controllers became concerned the LM would run out of fuel. Armstrong knew the limits, however, and guided the vehicle down. When the computer sensed the LM was a few feet off the moon Armstrong hit a button and shut off the engine. The LM fell the last few feet with a gentle thud. Aldrin called out the descent systems’ shutdown sequence: ‘‘Mode control: both auto. Descent engine command override: off. 413 is in.’’

Relieved, Armstrong then chimed in with his definitive line of technical poetry: ‘‘Houston, Tranquility Base here. The Eagle has landed.’’

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The MIT PressLog is the official blog of MIT Press. Founded in 2005, the Log chronicles news about MIT Press authors and books. The MIT PressLog also serves as forum for our authors to discuss issues related to their books and scholarship. Views expressed by guest contributors to the blog do not necessarily represent those of MIT Press.