NOEL KING, HOST:
Neil Armstrong took his first small step for man on the moon 50 years ago tomorrow. And when he did it, he walked on into the history books. Andrew Chaikin is with me now. He wrote the book "A Man On The Moon: The Voyages Of The Apollo Astronauts." Good morning.
ANDREW CHAIKIN: Good morning.
KING: So how did the landing play out?
CHAIKIN: Well, landing on the moon is basically a controlled fall out of orbit.
KING: Wow.
CHAIKIN: And you only have enough fuel for one try.
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CHARLIE DUKE: Eagle, Houston. You are go to continue powered descent. You are go to continue powered descent.
CHAIKIN: The onboard computer, which was absolutely primitive by today's standards, had had about 37,000 words of memory, although those were very...
KING: (Laughter).
CHAIKIN: ...Well-chosen words. But the computer was critical. It was absolutely the third crew member in that lander. And everything pretty much went OK until about five minutes into the final descent to the moon...
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BUZZ ALDRIN: Got the Earth right out our front window.
CHAIKIN: ...When, all of a sudden...
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NEIL ARMSTRONG: Program alarm.
ALDRIN: It's a 1202.
ARMSTRONG: 1202.
CHAIKIN: ...Something was wrong with the computer.
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ARMSTRONG: Give us a reading on the 1202 program alarm.
CHAIKIN: Some of the people who were really caught up short by these alarms were the people who had actually written the software for the computer, and they were at MIT in Cambridge, Mass. And one of the main authors of that software was a young computer engineer named Don Eyles.
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DON EYLES: We looked at each other with sort of blank faces. Something was going on in the computer that we didn't understand. If it had been up to me, I would have called an abort.
KING: That is absolutely terrifying. So why did they not abort?
CHAIKIN: Well, one of the heroes here is the flight director, Gene Kranz. Couple of weeks before the mission, they had done a simulation in which computer alarms had come up, and they had not been ready for them. Kranz said to his computer guys, I want you guys to be prepared for every conceivable computer alarm that might come up. So when these alarms came up, there was one guy in a back room. He was only 24 years old. His name was Jack Garman.
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JACK GARMAN: It's executive overflow. If it does not occur again, we're fine. It's continuous that makes it no-go. If it reoccurs, we're fine.
KING: He's so calm. We're fine. We're fine.
CHAIKIN: It's absolutely amazing to hear him. (Laughter) It's just one of these stunning examples of the amazing performance of these young people who were given awesome responsibility during these missions.
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DUKE: You're looking great to us, Eagle.
CHAIKIN: So they're still about 3,500 feet above the moon when Gene Kranz pulls his flight controllers and then finally gives them the go-ahead to continue the landing.
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GENE KRANZ: OK. All flight controllers, go, no-go for landing. Retro.
UNIDENTIFIED PERSON #1: Go.
KRANZ: FDO.
UNIDENTIFIED PERSON #2: Go.
KRANZ: Guidance.
UNIDENTIFIED PERSON #3: Go.
KRANZ: Control.
UNIDENTIFIED PERSON #4: Go.
KRANZ: Telcom.
UNIDENTIFIED PERSON #5: Go.
KRANZ: GNC.
UNIDENTIFIED PERSON #6: Go.
KRANZ: EECOM.
UNIDENTIFIED PERSON #7: Go.
KRANZ: Surgeon.
UNIDENTIFIED PERSON #8: Go.
KRANZ: Capcom, we're go for landing.
DUKE: Eagle, Houston, you are go for landing, over.
ALDRIN: Roger, understand, go for landing.
CHAIKIN: And then after that, there were more alarms on the computer.
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ARMSTRONG: Program alarm.
ALDRIN: 1201.
ARMSTRONG: 1201.
DUKE: Roger, 1201 alarm. We're a go. Same type. We're a go.
EYLES: We had four more of those alarms before we actually got to the surface. And at one point, the display went blank for a period of about 10 seconds. Armstrong laughed about it afterwards.
KING: I guess if you're going to have a - if you're going to have to go through that, have a sense of humor, right?
CHAIKIN: Oh, you know, this is exactly what Armstrong had signed up for. But by the time that Armstrong actually got to look out the window and see where the computer was taking them, he saw they were headed for a crater the size of a football stadium. And it was completely surrounded by boulders, which were themselves the size of small cars.
KING: Wow.
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ARMSTRONG: Pretty rocky area.
CHAIKIN: So he took over semi-manual control to fly past the crater and head on and look for a safer spot.
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ALDRIN: Altitude-velocity, light.
CHAIKIN: Down in Mission Control, nobody knew about the big crater in the boulders. For Gene Kranz and his controllers, this was getting very, very tense.
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KRANZ: During all of our training exercises, we had normally landed by this time. And from this time on, all we do is give him readouts on fuel remaining.
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DUKE: Sixty seconds.
CHAIKIN: Armstrong is trying to bring the lander straight down the last hundred feet. And what's happening now is that the lander's engine is blowing dust away in all directions. And it makes it very hard for him to judge his motion over the surface. Armstrong knew that his fuel supply was dwindling, and Kranz knows he and his guys really are not running the show anymore. It's going to be up to Neil Armstrong whether to land or not.
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KRANZ: Now, this is when the - it started to get sort of dicey in the Mission Control Center because we're in to the point where literally many of us have stopped breathing at that time.
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ALDRIN: Forward, forward, 40 feet down, 2 1/2.
CHAIKIN: Thirty feet is the altitude above the surface. Two and a half down means that they're descending at a rate of 2 1/2 feet per second.
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ALDRIN: Kicking up some dust, 30 feet, 2 1/2 down, faint shadow. Four forward.
CHAIKIN: Four forward is the velocity over the surface, the forward velocity of four feet per second.
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ALDRIN: Four forward - drifting to the right a little, down a half.
DUKE: Thirty seconds.
CHAIKIN: Thirty seconds means they have 30 seconds left before they either have to abort or go ahead and land with the fuel they have remaining.
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ALDRIN: Down a half. Contact light. OK. Engine stop. ACA - out of detent.
ARMSTRONG: Out of detent.
ALDRIN: Mode control - both auto. Descent engine command override - off. Engine arm - off. Four hundred thirteen is in.
DUKE: We copy you down, Eagle.
ARMSTRONG: Houston, Tranquility base here. The Eagle has landed.
DUKE: Roger, Tranquility. We copy you on the ground. You got a bunch guys about to turn blue. We're breathing again. Thanks a lot.
KING: God, it gives you goose bumps, doesn't it?
CHAIKIN: Every time. And, you know, I feel like it's taken me my entire adult life to understand what an amazing achievement it was.
KING: Andrew Chaikin - author of the book "A Man On The Moon: The Voyages Of The Apollo Astronauts." Andrew, thanks so much.
CHAIKIN: Thank you.
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KING: And just quickly, something we want to clarify - on Monday, we aired a story from Susan Stamberg about an exhibit centered on the sculptor Augusta Savage. We said Wendy Ikemoto curated the show. Ikemoto is associate curator of American art at the New York Historical Society Museum and Library, and she coordinated the "Augusta Savage: Renaissance Woman" show in New York. But it was originally curated by Jeffreen Hayes for the Cummer Museum of Arts and Gardens in Jacksonville, Fla. Transcript provided by NPR, Copyright NPR.