The fascinating part of the historic Apollo 11 landing on the moon (50 years ago today) is the handling of the 1202 and 1201 alarms during the decent of the Eagle to the lunar surface.
Some Apollo-nerd stuff that you only get here: The Eagle's computer had three programs to run from the time it undocked to landing. The first was P63: which controlled Eagle from undocking while it was still in orbit to powered-descent. P63 controlled the attitude (pitch and yaw) of Eagle and ran the burn which took Eagle out of orbit and into a controlled descent to the lunar surface. PDI (powered-descent initiation) occurred about 500 kilometers east of the landing site and 12 minutes to landing. The first go-no-go from Houston after the go-no-go for undocking was for PDI. When the Eagle is three minutes from landing and 7K/M from the site, the computer ran P64. The P64 program pitched the Eagle forward and gave Armstrong a view of the lunar surface so he could check for landmarks. At this point the computer is telling the astronauts and Houston where it intends to land. If you listen to the raw landing tapes, the guidance officer in Houston is indicating that they are headed for a spot longer down-range than anticipated.
When the Eagle is 600 meters from the landing site, Armstrong activates P66 in which he and the computer share the throttle while Armstrong alone controls the descent.
Somewhere along the way a switch was flipped powering on the rendezvous radar that was supposed to be off. The rendezvous radar began feeding more information to the computer than it was designed to handle at exactly the wrong time- during the dangerous descent phase. How Armstrong and NASA handled this critical error is the stuff legends are made of.
As PDI begins, Houston immediately loses signal and data from Eagle, prompting a call to Collins in Colombia to tell Aldrin to re-aim an antenna. Meanwhile Neil Armstrong has several issues to contend with, including no communication with the men who are supposed to guide him.
As the Eagle descended Armstrong began to realize that the guidance computer was taking him farther down range then it was supposed to and was putting the Eagle into a crater. The Eagle has two guidance systems: PGNS (pings) is the main system, and AGS is the back-up. Both Houston and Aldrin are monitoring both systems and comparing their data to see if it matches as the Eagle descends towards history.
As P63 turns the Eagle around so that now it's Engine is facing the lunar surface, the landing radar and other radar (that's supposed to be off) both lock on to the lunar surface. The computer is overloaded with data, and now come the words that almost ended the mission: "1202...1202 alarm." Eagle is 33,500 feet above the surface of the moon.
In the simulator Armstrong and Aldrin had practiced with several different scenarios, including the loss of various functions of the Eagle, wrong indicators, loss of communication, and the like. But in the thousands of hours of training, they had never encountered a 1202 alarm.
Here is what happened on the ground in Houston and their success showed why NASA was able to achieve the extraordinary moon landing:
Gene Kranz was the flight director, later responsible for the saying "failure is not an option". Kranz was the one who made the final decision to allow the Eagle to land.
Jack Garman was an engineer and part of the team working on the computers and the landing guidance system of the Eagle. At a meeting several weeks before the landing, Kranz told Garman to write down every possible alarm and the response to the alarm.
Steve Bales was the guidance officer who was one of the men responsible to answer Kranz during various "Go-No-Go" calls when Houston had to tell Armstrong if he could continue to land.
Astronaut Charlie Duke was "cap-com" the man responsible for speaking directly to Aldrin and Armstrong.
Several seconds after the 1202 alarm, Armstrong having heard nothing from Houston, asked "give us a reading on that 1202 alarm." When you listen to Armstrong, there is some uncertainty in his voice. He is about 30,000 feet above the surface of the moon and less than seven and half minutes from landing. He is looking for a place to land, watching his fuel, working on the high-gain antenna issue, and now an alarm that he does not recognize is going off which may cause the mission to be aborted.
Meanwhile in Houston, when the alarm went off, Kranz was looking towards his guidance officer Bales or anyone else who knew what the alarm was. Nobody knew. There were blank stares all around as Krantz's landing team statrted scrambling though massive three-ring binders looking for what a 1202 alarm was. Eventually Krantz asked Bales and Bales called over to a back room where there were dozen of engineers One of them- Jack Garman - knew what the alarm meant.
Apollo 11's computer's were rudimentary. The landing radars started giving the computer more data then it could handle. When this occurred the computer had a line of programing to tell it to prioritize its work and to trigger a 1202 alarm to let Houston and the Eagle know what it was doing. Essentially the computer was rebooting without shutting down. If the computer had shut down, Kranz would have ordered an abort.
Garman reasoned that as long as the alarm didn't continually repeat, which would mean the computer was in a non-recoverable loop, that they were "go" on the alarm. Garman told Bales. Bales told Kranz. Kranz told Duke and Duke told Armstrong.
There was another 1202 alarm and then a 1201 alarm at 27,000 feet above the surface. Aldrin tells Houston about why he thinks the alarm is occurring. Meanwhile Garman quickly told Bales that the 1201 alarm was the same type of alarm as the 1202 and that they were "go" on that.
Armstrong never doubted what Duke was telling him. Kranz had faith in Bales and Bales knew Garman knew the landing computer software better than anyone.
At about 9 minutes into the landing, and 5200 feet about the surface the computer switches to P64 and the program pitches Eagle over so that the attitude of the Eagle is more upright and it begins to descend in the same attitude that it will have upon landing. Armstrong is now looking at the surface so he can find a place to land. Kranz quickly runs through a "go-no-go" for landing and Retro (the controller monitoring the engines) FIDO (flight dynamics) ECOM (electrical, environmental and consumables) Guidance and the flight surgeon all give Kranz and enthusiastic "GO!" for landing which Charlie Duke relays to Aldrin and Armstrong. At 3,000 feet they get another 1201 alarm, but they are quickly told they are "go on that alarm" and the descent continues. In another minute, at 1300 feet they get another 1202 alarm, but they are still go to land.
At 600 feet the P64 program is steering the Eagle toward a sea of boulders and craters. Armstrong decides to switch to the P66 program and manually take control of Eagle. He began to use the thrusters to navigate the Eagle past a large crater and then looked for a relatively flat area to set his craft down on, all the while monitoring an ever dwindling fuel supply and a host of other issues. They are 300 feet from the surface and Aldrin tells Armstrong that he is "pegged at horizontal velocity" meaning they are going forward at the top speed on the indicator. At 250 feet Armstrong is now slowing the forward velocity. He sees a landmark he recognizes: "Little West Crater" and he pilots Eagle just past it where he sees a relatively smooth surface to land. The forward velocity has slowed from 50 feet/second to 19 feet/second. At 175 feet they have 94 seconds to land or they will get a "fuel-bingo" call from Houston and will have to abort.
The Eagle had one chance to land on the moon. If the landing was aborted, they activated the ascent engine, flew back to Michael Collins in Colombia, and headed back to earth having failed in their attempt to land. There were no second chances on this flight.
At 100 feet they have 75 seconds to land. Armstrong is demonstrating why he was chosen for this mission- he is at the top of his game and pulling off the greatest landing in the history of aviation- and one of the most difficult and dangerous to boot. At 75 feet their velocity has slowed to six feet forward/second. At 60 feet Charlie Duke calls out "sixty seconds": they have a minute of fuel left to land. At 20 feet Charlie Duke calls out "thirty seconds". There is almost no fuel left to land.
With seventeen seconds of fuel remaining Aldrin calls out "contact light": a 1.5 meter probe below Eagle has touched the surface.
The first words spoken by a human being on another celestial body belonged to Buzz Aldrin: "Contact light. Ok. Engine stop. ACA out of descent. Mode control both auto descent engine command override off. Engine arm off. 413 is in."
Hardly memorable or historic words, but before Armstrong tells the world that from Tranquility Base "The Eagle has landed" Aldrin had a checklist he needed to run through to make sure the descent engines were shut down and the abort-ascent engine couldn't be accidentally triggered.
Charlie Duke responds "we copy you down Eagle".
Armstrong says "The Eagle has landed" and Duke responds that there were a bunch of guys about to turn blue but were breathing again. This is in response to the fact that with less than 30 seconds of fuel, the Eagle had not landed and everyone in mission control was holding their breath.
A half a million people worked on some part of the Apollo program. But on July 20, 1969, it was Kranz, Steve Bales, and Jack Garman who gave the go ahead to Armstrong and Aldrin to continue the landing in the face of 1202 and 1201 alarms.
Brave and historic actions indeed.