On the morning of July 20, 1969, Neil Armstrong and I floated up through the access tunnel that linked Apollo 11’s command module to its lunar module, the spacecraft in which Neil and I would descend to the lunar surface.

Three days earlier, I had entered the lunar module to check things out and prepare what would be Neil’s and my home away from home for approximately 24 hours. The “LM,” though a technological wonder, was the epitome of bare-bones construction. Because it had to be as light as possible, it was far from luxurious inside. There were no seats or sleeping couches. Neil and I would sleep in makeshift hammocks hung from the walls, and we would fly the lunar lander while standing up, wearing our 21-layer pressurized suits and helmets. Two small triangular windows provided our only sight of the surface.

Crucial to the success of the landing was the module’s guidance computer. Like its twin on the command module, this machine had a 19-button keyboard, a 2.048 MHz clock processor, and about 74 kilobytes of memory. In short, many modern mobile phones have more computing power than Apollo 11 did. But those two computers enabled us to measure our velocity changes to a hundredth of a foot per second, determine course corrections, and make minute maneuvers for our descent to the moon. On the day that Neil, our crew mate Michael Collins, and I launched, we were probably 60 percent certain that we would succeed in landing on the moon and 95 percent sure that we would make it home alive. We depended on those computers to work astoundingly well.

The three of us were on the far side of the moon, during our 13th orbit, when Mike announced that we were ready to commence undocking. Until this point, our linked pair of spacecraft had simply been known as Apollo 11. Now, as we sealed off the hatches to become two separate entities, the command module would take on the name picked by Mike, the Columbia, and the lunar module became known to Mission Control and the world as the Eagle.

Mike wasted no time when Houston ordered the undocking. As though he were backing a truck out of a parking space, he pulled the Columbia away from the Eagle, releasing us with a thump.

“Okay, Eagle,” he said. “You guys take care.”

“See you later,” Neil replied, just as casually.

More than two hours later, Neil and I were flying about eight miles above the lunar surface when the voice of astronaut Charlie Duke, communicating from Mission Control, parted the static: “Eagle, Houston,” Charlie said, in his friendly Texas drawl. “If you’re ready, you’re go for powered descent.”

Neil nodded. Inside my helmet, I grinned. In 11 minutes we were going to set the Eagle down for a landing unlike any other.

Oddly, when Neil threw the switch to ignite the descent burn, we could barely feel any sensation caused by the orange plume pouring from our engine into the black space below us. Had we not seen the change on the instrument panel in front of us, we might not have even known that the engine was whisking us downward. But downward we were going, and rapidly, too.

Five minutes into our powered descent, everything was looking good. Suddenly, though, an alarm flashed on the screen in front of us.

“Program alarm!” Neil said instantly. Even with our transmissions traveling at the speed of light, there was a three-second delay in our communications with Earth, meaning that Charlie couldn’t respond immediately.

“It’s a twelve-oh-two.” Neil added. “What is it?” We had never seen a 1202 alarm in our simulations, and in the middle of our crucial landing maneuver, we weren’t about to take out the thick navigation dictionary we had brought along. To Houston, Neil said, “Give us a reading on the twelve-oh-two program alarm.”

“Twelve-oh-two,” I repeated, as the data screen in front of me went blank. We were now at 33,000 feet, not a time to have our landing data disappear. Neil and I exchanged tense looks. Something was causing our guidance computer to have difficulty handling the gigantic array of information coming into it from the landing radar.

Neil and I weren’t thinking about aborting; we didn’t want to get this close and have to turn back. On the other hand, the alarm was ominous. Even if we succeeded in landing without the aid of the computer, the malfunction could prevent us from blasting off the moon and making our rendezvous with Mike the next day. The demands on the computer then would be even greater.

While we grappled silently with these possibilities, we continued descending toward the moon, the large red abort button looming large in front of us. If either Neil or I hit the button, the Eagle would instantly blast back up toward Columbia, and America’s attempt to land on the moon would be dubbed a failure.

“Roger,” Charlie’s voice broke through the static into our headsets. “We’ve got you ... we’re go on that alarm.” Even from 250,000 miles away, I could hear the stress in Charlie’s voice. Yet for some reason the experts at Mission Control judged the computer problem an “acceptable risk,” whatever that meant. There was no time for discussion; we could only trust that Mission Control had our best interests at heart. Of the hundreds and hundreds of people who had helped get us here, nobody wanted to abort the mission. Yet at the same time we knew that Mission Control would not jeopardize our lives unnecessarily. Two nights before we launched, NASA’s top administrator, Tom Paine, had eaten dinner with Neil, Mike, and me in the crew quarters. “If you have to abort,” he said, “I’ll see that you fly the next moon landing flight. Just don’t get killed.”

Just as I was getting over my concern about the first alarm, another 1202 alarm appeared on the display. I felt a shot of adrenaline surge through my system.

At Mission Control, 26-year-old Steve Bales was the expert in the LM guidance systems. When the alarms started flashing in the Eagle, they showed up on Steve’s computer as well. Even if Steve didn’t know the source of the problem, he did know that the computer was programmed to ignore the data causing the overload while it did the more important computations necessary for landing. Or so he hoped.

In any case, he had little time to think when mission flight director Gene Kranz called out to him by the acronym Steve held as guidance officer: “GUIDO? Are you happy?”

Eyes glued to his computer screen, Steve called back, “Go!”

At about 1,000 feet, Neil began a visual search, looking for a good spot to land. My gaze, meanwhile, was glued to the panel in front of me. With the dropouts in radar information, it was vital that Neil receive accurate altimeter readings. Moreover, our fuel level was becoming a concern.

Neil was not happy with what he saw as we headed to our designated landing site.

“Seven-fifty, coming down at 23,” I said, letting Neil know that we were a mere 750 feet above the surface and descending at 23 feet per second.

“Okay,” Neil said. “Pretty rocky area …”

“Six hundred, down at 19.”

Neil had made up his mind. “I’m going to …” He didn’t have to finish his statement. I knew that Neil was taking over manual control of the Eagle. Good thing, too, since our computer was leading us into a landing field littered with large boulders. Neil made a split-second decision to fly long, to go farther than we had planned, to search for a safe landing area.

“Okay, 400 feet,” I let him know, “down at nine.” Then, for the first time, I added, “Fifty-eight forward.” We were now skimming over the moon’s surface at 58 feet per second, about 40 miles an hour.

“No problem,” Neil responded, but I could tell by the tone of his voice that he still wasn’t satisfied with the terrain. I started to be concerned about our fuel. It would be problematic to get this close and “run out of gas.”

“Three hundred,” I called.

“Okay, how’s the fuel?” Neil asked without taking his eyes from the surface.

“Eight percent,” I responded.

“Okay, here’s a ... looks like a good area here.” Then: “Gonna be right over that crater.”

“Two hundred feet, four and a half down,” I said.

I looked at our fuel gauge. We had about 94 seconds of fuel remaining, and Neil was still searching for a spot to bring us down. Once we got down to what we called the “bingo” fuel call, we would have to land within 20 seconds or abort. If we were at 50 feet when we hit the bingo mark, and were coming down in a good spot, we could still land. But if we still had 70 to 100 feet to go, it would be too risky to land; we’d come down too hard. Not wanting to say anything that might disrupt Neil’s focus, I pretty much used my body English, as best I could in a spacesuit, as if to say, Neil, get this on the ground!

“Sixty seconds,” Charlie warned. Our ascent engine fuel tanks were filled to capacity, but that fuel did us no good, since the descent engine tanks were completely separate. We had 60 seconds’ worth of fuel left in the descent tanks to either land or abort.

“Sixty feet, down two and a half.” Neil had slowed our descent. “Forty feet … Picking up some dust.”

We were moving over the lunar surface like a helicopter coming in for a landing, but we were now in what we sometimes referred to as the “dead zone.” If we ran out of fuel at this altitude, we would crash onto the moon before our ascent engine could push us back into space.

“Thirty seconds,” Charlie said, the nervousness evident in his voice. Neil slowed the Eagle even more, searching … searching …

Then I saw it—the shadow of one of the three footpads that had touched the surface. Although our engine was still running and the Eagle was hovering, a probe had touched the surface.

“Contact light,” I said.

Neil and I looked at each other with a stolen glance of relief and immense satisfaction. The LM settled gently, and we stopped moving. “Shutdown,” I heard Neil say.

It was 4:17 p.m. (EDT). We had less than 20 seconds’ worth of fuel remaining, but we were on the moon.

“We copy you down, Eagle,” Charlie Duke said.

I had already turned my focus to completing a flight checklist, but I paused now, and for the first time glanced out my window. The moon’s pockmarked terrain, which now for the first time in its existence hosted human beings, stretched out as far as I could see.

At that moment, Neil did something that really surprised me. “Houston,” he said calmly. “Tranquility Base here. The Eagle has landed.”

“We made it!” I whispered, almost as if I didn’t want to seem amazed.

Reprinted from Magnificent Desolation: The Long Journey Home From the Moon ©2009 by Buzz Aldrin. Published by Harmony Books, a division of Random House, Inc.