NASA landed the first humans on the Moon 51 years ago today, on July 20, 1969. The monumental mission carried out by astronauts Neil Armstrong, Edwin “Buzz” Aldrin and Michael Collins brought together the brightest mathematicians, spaceflight engineers and scientists the world had to offer. Aerospace engineer Jack Clemons was one of the hundreds of thousands of people whose work allowed NASA to pull off the first Moon landing.
Mr Clemons, who worked on the Apollo and Space Shuttle programme for NASA, worked for a company called TRW Systems in Houston, Texas, during the Apollo era.
Mr Clemons specialised in high-speed aerodynamics and was assigned to work on the Apollo Command Modules for atmospheric reentry.
During this time, he was involved in Apollo 8 in 1968 through to Apollo 17 in 1972.
Speaking to BBC Sky at night Magazine in 2019, Mr Clemons shed some light on the technological limitations of the time that NASA overcame to put a man on the Moon.
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When Apollo 11 flew to the Moon, the guidance computers used on the mission were thousands of times weaker than the average smartphone people carry today.
Mr Clemons said: “On Apollo we had a fairly primitive guidance computer, relatively speaking, and we were using slide rules and making sure if something happened we would know what to do.”
But according to David Grossman of Popular Mechanics, the computers were still at least a decade ahead of anything commercially available on the market.
The missions were also monitored by multimillion-dollar machines back on Earth, such as the IBM 360 Model 75s mainframe computers.
Ultimately, Mr Clemons argued there was a lot of manual work, training and calculating that ensured the success of the missions.
With those we could simulate most of the mission
Jack Clemons, NASA aerospace engineer
He said: “Technology was old compared to today, or even the Shuttle, but at the time it was the best we had.
“We made up the difference with people: a lot of training, a lot of procedures.
“We did have some fairly advanced computers on the ground: IBM mainframes that IBM built and supplied for Mission Control.
“With those we could simulate most of the mission so that during the actual mission you had these computers crunching along, telling us how it was going and what needed doing.”
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But the Apollo 11 Moon landing was not smooth sailing from launch to splashdown, as the mission’s guidance computer threw up an error during the lunar descent stage.
As Apollo 11’s Lunar Module Eagle was descending towards the Moon with Armstrong and Aldrin inside – Collins stayed in lunar orbit aboard the Command Module Columbia – a 1202 program alert rung out, followed by 1201 error.
The programme errors were later described as “checklist errors” as they were caused by the guidance computer being overloaded with tasks.
Space historian Robert Pearlman told LiveScience: “What was happening was that too many commands were being loaded into the computer and it was running out of memory.
“It was a warning that it did not have the ability to calculate everything that was needed to be calculated.
“But that was OK, because the computer was designed to drop processes as needed, and it had rankings as to what was most critical.”
After working out why the computer was going haywire, NASA’s Mission Control decided to push forward with the landing.
In the final stages of the descent and the Lunar Module running on fumes, Armstrong took over manual control and guided the spacecraft to a flat patch on the Moon.
On July 20, 1969, the Lunar Module touched down on the Moon after which Armstrong famously said: “Houston, Tranquility Base here. The Eagle has landed.”
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