07/21/2012 09:34 am ET Updated Sep 20, 2012

Apollo 11 Flight Director Gene Krantz Explains How To Park On The Moon

Forty-three years ago, Apollo 11's "Eagle" Lunar Module was well on its way to landing on the Moon.

Near the end of the powered descent, Neil Armstrong judged that to land safely, he and crewmate Buzz Aldrin would have to overshoot their targeted area because of a dangerous crater. That, of course, meant having to burn more fuel.

Armstrong took the controls manually and set course to land beyond the obstacle. But within minutes, a series of unexpected alarms flashed at Mission Control in Houston -- a "1201" and a "1202." Theoretically, either was enough to abort the historic landing and send Eagle back up to Michael Collins orbiting the Moon in the Command Module.

In 1969, the computers that guided Apollo 11 had less computing power than today's advanced cell phones. The "1201" and "1202" alarms were just the machine's way of saying, "I'll get to that, I'm overloaded at the moment with more important things." Luckily, the young mission control guys had run every possible simulation before the flight -- including that one, albeit late in the game -- and knew it was just a software problem.

Had they not known, Charles Duke -- at the time a Mission Control specialist and later an Apollo 16 moonwalker -- never would have been able to say, in his iconic southern drawl: "Tranquility, we copy you on the ground. You got a bunch of guys about to turn blue. We're breathing again. Thanks a lot!"

I still get chills when I hear Duke say that on the original Apollo recordings. I'm pretty sure much of the world still does, too.

This past spring as research for my eBook The Right Stuff: Interviews with Icons of the 1960s, I was able to discuss the tense landing with the famous former Apollo Flight Director Gene Krantz.

Stout and brawny and still looking like a marine drill instructor at 78, Krantz set the record straight for me.

Jim Clash: Talk about the warnings as Eagle was powering down to the lunar surface.

Gene Krantz: The "1201" and "1202" were two program alarms we learned about in our final day of training for the Apollo 11 team deployed to the Cape [Kennedy]. We made a wrong call in simulation, and didn't understand the nature and significance of the alarms. So we sent the team back to work overnight to come up to speed with implications of the alarms on the mission and the LM [Lunar Module] guidance computer during the process of going down to the Moon. The amazing thing was this was our final training run. Normally final training is a graduation ceremony, but the instructors didn't treat it that way. We thought we weren't quite ready. When the real mission came along, we had come up to speed on what the program alarms meant. And, right in the battle of getting the crew down to the surface, we saw the alarms. We knew exactly what to do, continued the mission and landed on the Moon with less than 17 seconds of fuel remaining.

JC: In the Apollo movies, everyone looks to one young guy for a signal whether to abort or continue when those alarms come on.

GK: The average age of my entire team at Mission Control was 26. That was a young man by the name of Steve Bales. He knew exactly what to say. We were all glued to our own consoles because we had a lot of other problems at the same time. We knew we were going to be landing long, we had trajectory changes, we were waiting to get our landing radar in. We also had a minor electrical problem - communications weren't all up to snuff - so we had our hands full.

JC: How close to aborting was Eagle if Bales hadn't known the alarms were benign?

GK: That's one thing I always go back and offer my thoughts about. I don't think we would have [aborted]. It's one thing in training, when you're very conservative. When you get to mission time, and you know the entire program rests on it, you tend to be a little less conservative. I think we would have pressed on. How far and how long, I don't know.

JC: At launch, what odds did you give Apollo 11 of success at landing on the Moon?

GK: Basically every mission controller believes that whatever objective has been set, he'll succeed. So, as far as I was concerned, it was a sure thing.