While the tiny, fragile Apollo lunar lander descended rapidly to the Moon’s surface, its guidance computer disturbed the crew with several unexpected alarms.
The Apollo Lunar Module, the Eagle, was at an altitude of 33,500 feet from the Moon’s surface, which by then could be seen in the very bottom of the window used by Buzz Aldrin, the lunar module pilot. The transcript of the first Moon landing records how commander Neil Armstrong ‘with the slightest touch of urgency’ said “Program Alarm.”
Armstrong to mission control, in Houston: “It’s a 1202.”
Armstrong onboard, to Buzz: “What is it?”
Armstrong would later reflect that landing was by far his biggest concern, saying “the unknowns were rampant,” and “there were just a thousand things to worry about.”
Armstrong to Houston: “Give us a reading on the 1202 Program Alarm.”
Charlie Duke, the CapCom [Capsule Communicator] for the lunar landing: “Roger. We got you… (‘with some urgency in his voice’) We’re go on that alarm.”
The crew had not seen the 1202 alarm during simulations, which was triggered by data overflow in the computer. In Houston, Steve Bales, an expert in lunar module guidance systems, determined that the landing would not be put in jeopardy. It was safe for the Eagle to continue the descent.
Aldrin focused on the computer, instrument displays, calling off the numbers for altitude, speed and so on, while Armstrong took over manual control. However, the Eagle was not where it was supposed to be, based on a grid in Armstrong’s window, scribed with vertical and horizontal scales, and cues from the computer.
The Eagle was coming in ‘long’ or downrange, overshooting the predicted landing zone. Geologist Eugene Shoemaker estimated that they were several kilometres downrange.
Just after Aldrin says “700 feet” Armstrong replies: “Pretty rocky area.”
They were confronted with a crater field and boulders measuring two or three metres in size, so Armstrong levelled off at about 400 feet [122 meters] to find a better spot to land.
Duke recalled: “I started getting a little nervous, and they weren’t telling us what was wrong. It was just that they were flying this strange trajectory.”
With a rapidly diminishing fuel supply, they would soon reach the 60-second mark when they would have to abort the mission.
“We heard the call of 60 seconds, and a low-level light came on. That, I’m sure, caused concern in the control centre,” Aldrin recalled. “They probably normally expected us to land with about two minutes of fuel left. And here we were, still a hundred feet [30 m] above the surface, at 60 seconds.”
Just after Armstrong asks “Okay, how’s the fuel?” and Aldrin replies “Eight percent”, Amstrong declares “Okay. Here’s a…looks like a good area here.”
In the final seconds of the white-knuckle descent, the four-legged lunar module made it to the dusty surface.
Aldrin: “Contact Light.” That meant at least one of the probes hanging from three of the craft’s footpads had touched the surface – they had landed on a site they would call Tranquility Base.
“Houston, Tranquility Base here,” said Armstrong. “The Eagle has landed.”
Mission control erupted in cheers and claps as the tension lifted, and Duke told the crew, “You got a bunch of guys about to turn blue, we’re breathing again.” Post-flight analysis indicated that they had more fuel than they thought. The low fuel fear had been triggered by fuel sloshing in the lander’s tanks.
With 600 million people watching on television, Armstrong climbed down the ladder and declared: “That’s one small step for [a] man, one giant leap for mankind.”
Aldrin joined him soon after, and they explored what Aldrin calls the ‘magnificent desolation’ of the Moon for two and a half hours, collecting samples and taking photographs.
They left behind an American flag, a patch honouring the Apollo 1 crew, who had perished in a fire, a laser experiment that is still in use to track the Moon’s orbit, and a plaque on one of the legs of the Eagle lander: ‘We came in peace for all mankind.’
Celebrate the 50th anniversary of the Moon landing at our Summer of Space festival and explore the history of human space exploration and look at where future endeavours might take us.