Landing the first two astronauts on the Moon in July of 1969 ranks as one of the great engineering achievements in human history.

Four thousand IBM employees, most of them from the company’s Federal Systems Division, built the computers and wrote many of the complex software programs that launched the Apollo missions and guided them safely to Earth.

IBM engineers and technicians at the George Marshall Space Flight Center in Huntsville, Alabama, built the guiding instrument unit embedded in the giant Saturn rockets. At Cape Kennedy, now Cape Canaveral, in Florida, they performed final system tests and helped launch the 3000-ton rocket with its 40-ton payload. And at NASA’s Manned Spacecraft Center (now called the Johnson Space Center) in Houston, Texas, they sat at consoles beside the NASA flight directors, making the minute-by-minute analyses needed to navigate the spacecraft from Earth orbit to lunar orbit and back.

More IBM employees at the Goddard Space Flight Center near Washington, DC, developed the worldwide network of relay stations and ships to track and communicate with the spacecraft. Employees at IBM in Owego, NY, and other locations invented and built the miniaturized integrated circuitry used to shrink the equivalent of an IBM ® System/360 mainframe down from the size of a refrigerator to that of a suitcase—and made it rugged enough to blast into space.

Gene Kranz was the flight director on duty on July 20, 1969, when astronauts Neil Armstrong and Buzz Aldrin separated Apollo’s lunar module from the command/service module and began their successful descent to the surface of the moon. “The systems information that we used to make the go, no-go decisions was developed by IBM, and the ultimate go, no-go decision [that day] was provided to me by computers operated by IBM engineers within NASA’s Mission Control Center. Without IBM and the systems they provided, we would not have landed on the Moon.”

IBM acquired the skills and invented the tools needed for space flight over a 30-year span stretching back to the 1940s. The US Navy used an early IBM electromechanical calculator to compute the ballistic trajectories of artillery shells. In the late 1950s, the US Naval Research Laboratory employed an IBM 650 computer to solve the orbital math needed to launch small satellites. During this time, IBM built SAGE (Semi-Automatic Ground Environment) [Read more about this Icon of Progress], an early warning radar defense system, and began developing small, heavy-duty computers for bombers and US Strategic Air Command’s intercontinental ballistic missile (ICBM) Titan rockets.

By the time NASA began putting astronauts into Earth orbit with its Mercury and Gemini programs, IBM was there with a 60-pound space guidance computer. It used innovative three-dimensional, multilayer etched circuit boards to interconnect components, saving miles of wire and pounds of weight.

When the call came from Dr. Wernher Von Braun’s Saturn rocket development team, in search of a guidance system, IBM engineers took one of their Titan rocket computers and modified it for space launch conditions. They then put it in the back of a station wagon and drove it from Owego, New York, to the Marshall Space Center in Huntsville, Alabama, plugged it in, and ran it for a year without a glitch. NASA liked what they saw and invited IBM to bid on the critical guidance system for the Saturn.

NASA wanted a system with a guaranteed mean-time-between-failure of 25,000 hours, and a component density—number of transistors, circuits, semiconductors—of 45,000 per cubic foot. IBM came back with a design for a mean-time-between-failure of 40,000 hours and a component density of 250,000 per cubic foot. IBM won the contract and built 27 Saturn instrument units. Each unit became a 3-foot-high section of the 360-foot-long rocket, sitting on top of the third stage.

The IUs, as they were called, did their duty. On one of the early Saturn I flights, when one of the rocket’s eight engines failed, threatening the mission, the IU compensated for the change in thrust by adjusting the other seven engines, saving the flight. On Apollo 12, the Saturn V rocket was struck twice by lightning, temporarily knocking out communications with Mission Control and the flight instruments in the astronauts’ command/service module. The IU, however, kept on working and kept the rocket on course.

Much less visible was the IBM army of programmers and systems analysts who pioneered the bridge between the theory of celestial mechanics and the prosaic numbers needed to launch the expedition, see it gently land on a moving target 240,000 miles away, and bring it back to within a few miles of a recovery ship in the Pacific Ocean.

IBM programmers at the Marshall Space Center and Cape Kennedy helped build the IUs and write the programs that launched the Saturn rockets into translunar trajectory. Programs written by the team at Goddard established a worldwide tracking network of 17 stations and four ships that followed and communicated with the spacecraft.

And at Mission Control in Houston, a large group of engineers and programmers worked with five IBM System/360 Model 75 computers in NASA’s Real Time Computer Complex. Every bit of data on the mission’s velocity, flight path angle, and time and position of impact was monitored and calculated constantly. The re-entry trajectory for Apollo 11—which had to hit a narrow six degree line of flight to safely return to earth—was calculated and recalculated some 400 times during the mission.

Apollo 11, with Armstrong, Aldrin and command module pilot Michael Collins aboard, landed in the Pacific Ocean on July 24, 15 miles from its recovery ship. Before the program ended in 1975, Apollo made six lunar landings, and twelve astronauts walked on the Moon. It is the only time in history that humans have visited another celestial body.

The very day Apollo 11 splashed down in the Pacific, Bob Evans, who was president of the IBM Federal Systems Division, wrote a letter to his employees: “Certainly Armstrong, Aldrin and Collins, as well as all of the other astronauts, must be recognized as courageous and skillful men of the highest order. But we must not forget that the men who walked on the Moon were members of a far larger team. … I take enormous pride in knowing that 4000 members of the team were IBM men and women of the Federal Systems Division.”