Here are the final four and a half minutes of the lunar descent. Unlike the recent descent of Curiosity to the Martian surface, this descent was not punctuated with rounds of cheering as each milestone was successfully passed. There are several reasons for this dramatically different response.
First, the Curiosity descent was reported based on remote sensing.
Second, the entire operation was running autonomously.
Third, there were no humans on board.
Fortunately for the Apollo Mission, humans were on board and Neil Armstrong was at the controls. To understand exactly what happened during the final critical four minutes of descent, we have to go to the radio transmissions. We begin about a mile above the lunar surface. Neil Armstrong takes manual control of the lunar module because of a computer problem and the last minute need to change landing sites.
00:08 [Mission Control] Altitude 5200 feet.
00:10 [Armstrong] Manual Attitude Control is good.
Armstrong had switched to manual control, in part, because the originally planned landing field turned out to be strewn with rocks "the size of Volkswagens." That meant all the calculations for how much fuel would be used on a computer-controlled descent were now moot.
00:18 [Mission Control] Houston. You are go for landing. Over
This is the green light.
00:28 [Armstrong] Roger that. Go for landing, we have an alarm.
This is the moment everything went off the rails. There had been earlier warnings, but Mission Control -- specifically flight controller, Steve Bales, overruled those.
00:30 [Armstrong] 1201
00:32 [Mission Control] 1201, Roger 1201 alarm.
00:36 [Mission Control] We're go. Hang tight. We're go.
The "1201 alarm" was a signal triggered by the Apollo Guidance Computer. The Apollo flight computer was the first integrated circuit computer ever built. It was state of the art in 1969.
Computers have come a long way in the last 40+ years. The digital watch you wear on your wrist has more computing power than the Lunar Module had for its navigation. The Apollo flight computer had about 4K read/write memory and 32K ROM. That's barely enough space to send a text message and small file attachment. The smartphone you read this with has almost as much computing power as all of Mission Control and much faster processors. That's all they had to work with.
Here's why all that is more than just an interesting fact. The 1201 alarm meant the main navigational computer for landing (the Primary Guidance, Navigation and Control System (PGNCS)was not working because it ran out of memory. That meant they were not able to calculate the difference in altitude measured by their radar and what was being calculated by Mission Control. When you are hurtling towards the lunar surface, figuring out the right answer in real time is kind of important. Screw it up and you will do more than land on the Moon, you will create a new crater.
Fortunately, there was a safety feature built in to the system. The computers were designed to reboot when something like that happened. It did. During that time, Armstrong was flying blind. The computer came back on line and this time they reported a new error.
01:07 [Mission Control] Roger, 1202. We copy it.
This is not a better error. That means the computer is trying to do too many things at once and is postponing some of them. What the computer was trying to do was calculate the distance to the surface from the radar, coordinate communications back to mission control via audio and video, AND keep contact with the command module circling above. This meant the computer was working at about 85% of capacity. Unfortunately, a bug in the radar software kept jerking it back and forth between trying to contact the ground and trying to contact the command module. That "jitter" added a little more than 10% to the load and kept crashing things.
At that point -- Neil Armstrong was manually descending towards the lunar surface with no computer back up available to assist in landing. The only redundant system left, the Abort Guidance System was only useful in the event of a mission abort. They could use it to get the Lunar Module safely back to the Command Module, but not to land.
For the next three minutes the only voice you hear on the tape is Armstrong's. Mission Control is silent. This is a dramatic difference between what you saw if you watched the Martian landing of Curiosity, where everyone was whooping and hollering as each milestone of the landing was met.
There's a reason Mission Control is silent. That's because Neil Armstrong, and Neil Armstrong alone is in sole control of the Lunar Module as it descends. Nothing they can say or do will help him. Everyone is holding their breath. A lot of people were busy praying.
Armstrong's monologue continues uninterrupted until he is about 100 feet above the surface. Then you hear this cryptic message from Mission Control:
03:16 [Mission Control] Sixty seconds.
Armstrong continues with his monologue. Then you hear this cryptic message from Mission Control:
03:46 [Mission Control] Thirty seconds.
Six seconds later you hear this from the Lunar Module.
03:52 [Aldrin?] Contact light engaged.
03:55 [Aldrin?] Ok. Engine Stopped.
04:10 [Mission Control] We copy you down Eagle.
And then came the immortal words everyone had been waiting to hear:
04:12 [Armstrong] Houston, uh [long pause] Tranquility Base here, the Eagle has landed.
04:18 [Mission Control] Roger Tranquility. We copy you on the ground. You got a lot of guys about to turn blue here. We're breathing again.
The world erupted with glee. We had put a man on the Moon! Here's what we learned years later about that fateful moment. Those cryptic messages? That was Mission Control telling Neil Armstrong how many seconds of fuel he had left.
That's right. When push came to shove and the guidance systems failed, Neil Armstrong took the bull by the horns and brought that bird down with less than 30 seconds of fuel left in the tank and changed history.
Neil Armstrong's contribution was more than expert flying. He made real what we had dreamt of for centuries. Because of his achievement I grew up in a time when we believed anything was possible. "We put a man on the Moon, why can't we....." Now, we have lost that. We need to get it back. I can think of no better way to honor Armstrong than to rekindle that spirit.
We need to dream again.
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