Meet the man who helped double-check the sums to keep Apollo 11 safe

In the months leading up to the 50th anniversary of the Apollo 11 mission, we will be sharing the stories of the people who made the moon landing possible as part of our Airlock space newsletter. (Check out our last story, “The man behind the Apollo boot print.”) This week: Dennis Sager.

In space, your calculations have to be right. That means you probably want someone to check your work. The bulk of the computation for the Apollo missions was carried out by the Real-Time Computer Complex (RTCC), a room of computers developed by IBM. But relying entirely on one set of machines for such a critical series of missions was not enough for NASA.

So the agency also employed a whole backroom of expert mathematicians who used some good old-fashioned punch cards to double-check the main machines’ computations. The facility operated during the Gemini and early Apollo missions. Dennis Sager was one of the youngest in that backroom of Building 30 in Houston, Texas, also known as the Real-Time Auxiliary Computing Facility (RTACF).

As well as marking the machines’ homework, the group’s job was to help the missions prepare for the unexpected—and adapt when circumstances changed. While the RTCC had to lock in its code and trajectories before launch, the RTACF was agile. “We were able to make changes on a real-time basis on the flight,” says Sager. “We could do things that weren’t even thought of beforehand.”

His room was tasked with things like figuring out how to guide the mission if a hurricane hit the Gulf of Mexico, and calculating trajectories several orbits ahead of the spacecraft’s current position in case of an abort or error.

That meant they were thrown quite a few curveballs. One came in the form of a surprise Russian spacecraft. During the Apollo 11 mission, the Americans weren’t the only nation headed toward the moon. The Russians had launched an uncrewed satellite, Luna 15, that was designed to snatch away a US first.

“They intended it to land on the moon, grab some…