John Aaron was born in Wellington, Texas and grew up in rural Western Oklahoma near Vinson, one of the youngest of a family of seven children. His mother was a minister, and his father was a cattle rancher. After spending a year attending Bethany Nazarene College, he transferred to Southwestern Oklahoma State University, from which he graduated during 1964 with a Bachelor of Science degree in Physics. Although he had intended to teach mathematics and science after graduating from college, he applied for a job with NASA on the recommendation of a friend.
When he arrived at NASA, Aaron was trained as an EECOM, a flight controller with specific responsibility for the electrical, environmental and communications systems on board the spacecraft. By January 19, 1965, when the unmanned Gemini 2 was launched, he was already working in Mission Control.
When Apollo 12 launched on November 14, 1969, Aaron was on shift. Thirty-six seconds after liftoff, the spacecraft was struck by lightning, causing a power surge. Instruments began to malfunction and telemetry data became garbled. The flight director, Gerry Griffin, expected that he would have to abort the mission. However, Aaron realized that he had previously seen this odd pattern of telemetry.
A year before the flight, Aaron had been observing a test at Kennedy Space Center when he had noticed some unusual telemetry readings. On his own initiative, he traced this anomaly back to the obscure Signal Conditioning Electronics (SCE) system, and became one of the few flight controllers who was familiar with the system and its operations. For the case that first drew his attention to the system, normal readings could be restored by putting the SCE on its auxiliary setting, which meant that it would operate even with low-voltage conditions.
Aaron surmised that this setting would also return the Apollo 12 telemetry to normal. When he made the recommendation to the Flight Director, "Flight, try SCE to Aux", most of his mission control colleagues had no idea what he was talking about. Both the flight director and the Capcom asked him to repeat the recommendation. Pete Conrad's response to the order was, "What the hell is that?" Fortunately Alan Bean was familiar with the location of the SCE switch inside the capsule, and flipped it to auxiliary. Telemetry was immediately restored, allowing the mission to continue. This earned Aaron the lasting respect of his colleagues, who declared that he was a "steely-eyed missile man".
Aaron was off duty when the Apollo 13 explosion occurred, but was quickly called to Mission Control to assist in the rescue and recovery effort. Flight Director Gene Kranz put Aaron in charge of the Lunar Module's power supply. He was allowed to veto the ideas of other engineers, particularly when they affected the power usage of the modules. He was in charge of rationing the spacecraft's power during the return flight and devised an innovative power up sequence that allowed the Command Module to re-enter safely while operating on limited battery power.
Contrary to existing procedures, he ordered the instrumentation system, which included telemetry, visibility, and the transmitters for communications, to be turned on last, just before reentry, rather than first. The call was a calculated risk. Without the instrumentation system, the crew and controllers would not know for certain if the cold startup had been successful until the last possible moment before reentry. However without the change, the capsule would have exhausted its battery supply before splashdown. The procedure was a success, and the crew was recovered safely.
After the Apollo program ended, Aaron remained at NASA. He worked on the Skylab program, and was involved with the development of the Space Shuttle software. Starting during 1984, he worked on the abortive Space Station Freedom project; he became manager of Johnson Space Center's space station projects office during 1989. Four years later, however, he was forced to resign from the job after Texas Senator Robert Krueger blamed him for $500 million of overspending on the station project.
Aaron became a manager in Johnson Space Center's Engineering Directorate during 1993, and stayed in the directorate until he retired from NASA during 2000.
Aaron is a former mayor of Meadowlakes, Texas.
A character based on John Aaron was portrayed by actor Loren Dean in the 1995 movie Apollo 13. Aaron was also played by John Travis in the 1998 mini-series From the Earth to the Moon. He was interviewed in the PBS documentary Apollo 13: To the Edge and Back, and in two History Channel documentaries about Mission Control, Failure Is Not an Option and Beyond the Moon: Failure Is Not an Option 2.
The 2015 science fiction film The Martian contains a reference to the "steely-eyed missile man" title bestowed on Aaron, as does the Legends of Tomorrow second season episode "Moonshot."