Landing on the Moon

Part 1: Apollo 11 - First humans on the moon

▲ ELI5
Most people know that humans landed on the moon. Most people do not know the details. If you know that achieving orbit is more about velocity than altitude then this story is for you. If you've never heard words like periapsis, delta-v, pitch, and sink rate, then you're about to learn. If you think the moon landings are fake then the following is probably not for you.

On July 20, 1969, Apollo 11 Mission Commander Neil Armstrong and Lunar Module Pilot Edwin "Buzz" Aldrin became the first humans to land on the moon. This feat had never before been accomplished and, with a preposterous number of things that could go horribly wrong, nobody was sure if it could be accomplished. All across the Earth, anyone with access to a TV or radio was tuned in to this historic event.

The plan was for Neil Armstrong and Buzz Aldrin to ride the Lunar Module (LM), call sign Eagle, down to the moon's surface while Command Module Pilot Michael Collins stayed behind in lunar orbit. After spending a couple hours walking around on the moon's surface, taking pictures, collecting rocks, and generally going where no one had gone before, the astronauts would leave behind the descent stage on the surface of the moon and ignite the ascent stage to return to the command module and eventually, hopefully, return safely to Earth. But first they had to successfully land on the moon and that would not be easy.

While orbiting around the far side of the moon and out of contact with Earth, with a Mission Elapsed Time (MET) of 4 days, 4 hours and 12 minutes[1], the lunar module separated from the command module.

The Eagle has wings reported Armstrong after re-establishing contact with mission control in Houston.[2]

Roger came the upbeat but professional reply from Capsule Communicator (CapCom) Charlie Duke.

It took about an hour for the two spacecraft to drift apart far enough that the LM could safely start the descent. They also had to wait for exactly the right moment so they would land at the pre-selected spot on the moon. When the correct moment arrived, the LM's computer engaged the Descent Propulsion System (DPS, pronounced "dips") by opening pintle injectors which allowed the nitrogen tetroxide (N2O4) oxidizer to mix with aerozine 50 fuel. These hypergolic propellants ignited upon contact, starting the 16.5 ton, 23 feet tall, 31 feet wide LM on its descent to the lunar surface.

The Descent Orbit Insertion (DOI) burn lasted only 28 seconds. That's all it took to initiate the Hohmann transfer from the 60-nautical-mile circular parking orbit down to an elliptical orbit with a perilune (the lowest point of orbit, or periapsis, around the moon) of 50,000 feet. It was a short burn but exact timing was critical. If the orbit insertion burn was too short, perilune would be too high and there wouldn't be enough fuel to land. On the other hand, an overburn of only three seconds too long would result in an impacting trajectory. Either way, a mistake would be bad.

Houston, Eagle. How do you read?

Five by, Eagle. We're standing by for your burn report. Over.

Roger. The burn was on time.

Aldrin then proceeded to report all the new numbers to the engineers back at mission control could. They double checked everything then confirmed that the descent was indeed going as planned.

It took nearly an hour to reach perilune. Then, with no atmosphere to slow down the LM, it was time for Powered Descent Initiation (PDI). They started by firing the small RCS thrusters for 7.5 seconds. This ullage burn applied just enough force to push the propellant to the bottom of the tanks, or more accurately, push any bubbles to the top of the tank. Instead of fuel pumps, helium was used to pressurize the tank bladders. The ullage burn made sure there weren't any bubbles in the way so the descent engine would start like it's supposed to. It would take about 12 minutes to slow down and land. Since they only had 12 minutes of fuel, it was important to get everything correct on the first try.

At MET 04:06:33:05, the descent engine was lit and taken to full throttle. From this point on, the astronauts no longer felt weightless. At maximum throttle they felt approximately 0.6g (60% Earth's gravity). That's nowhere near the 4.5g they felt leaving Earth on the Saturn V and not even enough to strain against but it was definitely enough to rattle their tiny spaceship and make it obvious that the engine was firing. To conserve weight there were no seats so the astronauts stood at the control panel and windows, holding on while monitoring the descent.

During this braking phase the descent engine was at full throttle (FTP) and used the bulk of its propellant. They had to go from 5560 feet per second (3800 miles per hour) all the way down to zero. That's a lot of delta-V (change in velocity). For maximum deceleration, the computer kept the engine pointed forward with the thrust vector straight through the LM's center of mass. There's no air turbulence when descending to the moon but the sloshing fuel did cause some concern. The RCS was firing constantly to compensate and keep the LM on course.

The descent had started with the windows facing the moon but after five minutes of full throttle descent, it was time to perform the "face-up" maneuver. This would rotate the windows up, temporarily obscuring the astronauts' view but allowing the landing radar to be engaged. Even though the astronauts could no longer see the moon, Aldrin did report a good view of the Earth.

Photo taken by Mike Collins in the command module.

That's when the Master Alarm lit up. This is never good news. It could be minor, like a slow leak or maybe a bad sensor, or it could be a major problem, like "The spaceship is on fire! Abort now!" A quick glance at the guidance computer showed Program Alarm 1202. Unfortunately, neither astronaut knew what that meant and neither did mission control. While the engineers back on Earth scrambled to figure out whether or not the alarm was critical enough to call for an abort, the astronauts kept on flying and hoping the problem wasn't too serious. The good news is that the guidance computer was still keeping them on course, or so it seemed.

By modern standards, the Apollo Guidance Computer (AGC) was barely a computer. It had 36K of hard-wired core-rope memory (loops of copper wire around tiny magnets) and only 2k of RAM. There were no hard drives, no memory cards, no Internet. It was less powerful than a modern $5 Arduino chip and far, far slower. Even more amazing, it had been programmed by two recent college graduates. The more experienced engineers had been assigned to the Command Module software that was responsible for getting to the moon and back again. That left the young guys to work on the landing guidance software.

When the Program Alarm appeared, it was the job of a 26-year-old mission control guidance officer to make a "Go" or "No go" decision. That particular engineer hadn't programmed the computer, all he knew is that the manual said the 1202 alarm was a "Go" unless it occurred too often or the trajectory deviated. The trajectory wasn't perfect but it was still within limits and this was the first time that alarm had occurred. Ever.

The 26-year-old looked over to the senior engineers behind him but they had never seen this error either. During all the pre-flight testing, this 1202 error had never occurred once. So, without knowing what was causing the alarm or whether the problem was serious enough to warrant an abort, the 26-year-old made a judgement call and gave the "Go" answer to Flight Director Gene Krantz. He relayed the "Go" to CapCom Charlie Duke who passed it up to the astronauts. It had been 30 seconds since the Master Alarm light first lit up and, as you can imagine, the astronauts were feeling a bit concerned.

The navigation computer unexpectedly displayed a 1202 program alarm.

, Armstrong: Give us a reading on the 1202 Program Alarm.

, CapCom: Roger. We got - We're GO on that alarm.

Nobody knew what was causing the alarm but the decision was made to press on anyways. Forty seconds after the first alarm, it happened again. According to the manual, if this error kept happening, it would require an abort. Luckily, the full-throttle part of the burn was almost over.

, Aldrin: Wow! Throttle down--

, Armstrong: Throttle down on time!

, CapCom: Roger. We copy throttle down--

, Aldrin: You can feel it in here when it throttles down. Better than the simulator.

The excitement was deceptive. Staying calm and professional over the radio is important, especially when the entire world is listening. This is the only place in the formal transcript where they used exclamation marks. Whenever the astronauts showed any emotion, you know they were really feeling it.

Everyone was relieved because the descent engine used ablative cooling which means it could only operate at maximum throttle for a limited time. If the computer was acting up, it might have failed to shut down the engine on time which would destroy the engine. That would be bad. The engine was still running but by pulling power back below 60%, it was no longer tearing itself apart.

The next stage of the powered descent was the approach phase (P-64). If all went well, they would land on the surface of the moon in about five minutes. It wasn't going to be an easy five minutes though. Things would start happening faster and faster with no room for mistakes.

The computer controlled the throttle and gimbal all the way through the braking and approach phases but the astronauts would take over soon. They had noticed that they were slightly off course. The target landing site, officially known as Site 2 (Lat 0.6°N, Long 23.5°E[11]), was a 10-mile long ellipse in the Sea of Tranquility. The problem was, the astronauts saw that they were further down range than they were supposed to be. They were already halfway across the landing site instead of at the eastern edge of it as planned. Apparently the guidance computer had shut down the throttle a little too soon so they were still going a little too fast. Maybe that 1202 alarm wasn't harmless after all.

, CapCom: Eagle, Houston. You're GO for landing. Over.

, Aldrin: Roger. Understand. GO for landing. 3000 feet. PROGRAM ALARM.

, CapCom: Copy.

, Aldrin: 1201

, Armstrong: 1201

, CapCom: Roger. 1201 alarm. We're GO. Same type. We're GO.

This was a different alarm but someone back at mission control in Houston must have decided that, because it was similar to the previous alarm, it was not critical enough to warrant an abort. In the middle of his control panel, Neil Armstrong had a giant "Abort" button. Pressing it would end the mission but not pressing it might end his life. With so many unexplained alarms, the astronauts probably couldn't help but glance at that abort button once or twice. The engineers on the ground had better know what they were doing.

Armstrong had no way to know that the engineers on the ground still did not have a clear answer. They made the judgement call to ignore the alarm even though they didn't know what was causing it. They suspected it was because the computer was running too slow but they didn't know exactly why.

Both the 1201 and 1202 alarms meant that the guidance computer was running out of memory. That meant some of the lower priority tasks couldn't execute so the computer would quickly reboot and start over again. The high priority tasks, like steering the ship, always happened first. The question was, what low priority tasks were being skipped? With no time to investigate, the guys on the ground had made the decision that it didn't matter, they would press on anyways. One senior engineer recalls that he was too scared to say the word "GO." All he could do is give a thumbs up.

Armstrong and Aldrin didn't have time to dwell on the subject. The controls felt like they were operating correctly so they kept going. They were now passing 1000 feet and ready to take manual control for the final landing. With most of the forward velocity nullified, they were slowly rotating the LM forward so they could see the ground and land upright.

Recreation photo from the book Digital Apollo

With too much of his attention focused inside the cockpit and the computer alarms, Armstrong hadn't been watching the landing site closely enough. Now, when he looked out the window, he saw that they were landing just short of a large rocky crater with very large rocks covering a high percentage of the surface. [Neil Armstrong, "Apollo 11 Technical Crew Debriefing"; NASA, July 31, 1969]

At , with an altitude of 650 feet and only three minutes of fuel remaining, Armstrong took the autopilot out of AUTO. Instead of trying to stop before the crater, he was going to try flying past it. The trick would be slowing the descent enough that they could glide past the crater but not so much that they ran out of fuel.

, Aldrin: Okay, you're pegged on horizontal velocity.

, Aldrin: 300 feet, down 3 ½, 47 forward. Slow it up.

With a ground speed of 47 feet per second (fps), they were beyond the 20 fps limit of the indicator. Instead of slowing down, Armstrong had "tipped it over like a helicopter" to avoid the crater. Once past the crater, he would tip the LM back again to reduce their ground speed and continue the descent.

, Aldrin: Altitude-velocity light.

The warning lights indicated that the landing radar had lost track of the surface. Not knowing where the ground was might seem like a good reason to panic but is a great example of the difference that training makes. The astronauts were extremely well trained and professional pilots. They knew this might happen. Instead of panicking and saying things with lots of exclamation marks, they stayed calm and got the job done. Armstrong kept flying while Aldrin kept an eye on the computer and called out the elevation, descent rate and ground speed. CapCom kept their mouth shut unless they had something really important to say, like telling the astronauts that they were running out of fuel.

, Aldrin: Okay. 75 feet. There's looking good. Down a half. 6 forward.

, CapCom: 60 seconds.


, Aldrin: Light's on.

, Aldrin: Down 2 ½. Forward. Forward. Good.

, Aldrin: 40 feet, down 2 ½. Picking up some dust.

, Aldrin: 30 feet, 2 ½ down. Faint shadow.

, Aldrin: 4 forward. 4 forward. Drifting to the right a little. Okay. Down a half.

, CapCom: 30 seconds.

, Aldrin: Drifting forward just a little bit; that's good.

, Aldrin: Yes.

, Aldrin: Okay.


Hanging off the landing pads are sensors, 5.6 feet long, that feel for the ground. One of those sensors had just felt something. They were close but not quite on the ground yet. Procedure is to stop the engine immediately in case the nozzle gets blocked by the ground. With the moon's low gravity, the LM could simply drop the remaining few feet. At this point, they only had about 15 seconds of fuel remaining anyways.

, CapCom: We copy you down, Eagle.

, Armstrong: Houston, Tranquility Base here.


, CapCom: Roger, Tranquility. We copy you on the ground. You got a bunch of guys about to turn blue. We're breathing again. Thanks a lot.

The astronauts took the next ten minutes to shut everything down and make sure nothing was broken. It was important to immediately prepare for an emergency ascent, just in case they had to abort. After all, nobody had ever done this before, there was no telling what might go wrong. Fortunately, with all the systems checking out, they got the go ahead to stay on the ground for awhile.

, Armstrong: Hey, Houston, that may have seemed like a very long final phase. The AUTO targeting was taking us right into a football field size - football field sized crater, with a large number of big boulders and rocks for about ... one or two crater diameters around it, and it required a ... in P66 and flying manually over the rock field to find a reasonably good area.

, CapCom: Roger. We copy. It was beautiful from here, Tranquility. Over.

, Aldrin: We'll get to the details of what's around here, but it looks like a collection of just about every variety of shape, angularity, granularity, about every variety of rock you could find. The colors - Well, it varies pretty much depending on how you're looking relative to the zerophase point. There doesn't appear to be too much of a general color at all. However, it looks as though some of the rocks and boulders, of which there are quite a few in the near area, it looks as though they're going to have some interesting colors to them. Over.

, CapCom: Roger. Copy. Sounds good to us, Tranquility. We'll let you press on through the simulated countdown, and we'll talk to you later. Over.


, Armstrong: Roger.

, Aldrin: Okay. This one sixth g is just like the airplane.

, CapCom: Roger. Tranquility. Be advised there's lots of smiling faces in this room and all over the world. Over.

, Armstrong: Well, there are two of them up here.

, CapCom: Roger. That was a beautiful job, you guys.

, Collins: And don't forget one in the command module.

, CapCom: Roger.

, CapCom: Tranquility, Houston. We have you pitched up about 4 ½ degrees. Over.

, Armstrong: That's confirmed by our local observation.

, CapCom: Roger.

, Collins: And thanks for putting me on relay, Houston. I was missing all the action.

, CapCom: Roger. We'll enable MSFN relay.

, Collins: I just got it, I think.

, CapCom: Roger, Columbia. This is Houston. Say something. They ought to be able to hear you. Over.

, Collins: Roger, Tranquility Base. It sure sounded great from up here. You guys did a fantastic job.

, Armstrong: Thank you. Just keep that orbiting base ready for us up there now.

, Collins: Will do.

It was another 3 ½ hours before they started preparing for their first EVA and another three hours after that before they opened the hatch and stepped outside with the now famous words:

That's one small step for man... one giant leap for mankind