Flying 69 miles above the far side of the moon, out of contact with Earth and a world-wide audience holding its collective breath, Apollo 11 commander Neil Armstrong and Buzz Aldrin fired their lunar module's main engine for 30 seconds to begin the first piloted descent to the moon's surface.
It was July 20, 1969 - 45 years ago Sunday - and in a little more than one hour, one way or the other, Armstrong and Aldrin would either be safely on the surface or making an emergency abort back to orbit where crewmate Mike Collins was standing by in the Apollo 11 command module.
Those were the two most positive scenarios. There were other, grimmer, possibilities ranging from a failure that could leave the lunar module stranded on the surface, unable to take off, to any number of potentially catastrophic malfunctions that could trigger a crash landing or some other equally deadly outcome.
NASA managers, engineers and the astronauts were confident the hardware would work and that the crew was well trained and ready to handle virtually any contingency.
But no one knew for sure and as the drama unfolded a quarter of a million miles away, a huge global audience followed along via radio and television as flight controllers at the Johnson Space Center in Houston monitored telemetry and managed humanity's initial visit to another world.
The rocket firing on the dark side of the moon lowered the far side of the lunar lander's orbit to a point 50,000 feet above the surface, 300 miles from the planned landing site. Flying ahead of the lander, the Apollo 11 command module - Columbia - rounded the limb of the moon and Collins reassured flight controllers the "burn" went well.
"Listen, babe, everything's going just swimmingly. Beautiful," he radioed.
As the lander, dubbed Eagle, flew back into contact with Houston, legendary Flight Director Gene Kranz and his team of controllers had a scant 13 minutes or so to evaluate telemetry and assess the health of the spacecraft before clearing the crew to fire the engine again, at the low point of the new orbit, to begin the final landing phase.
As controllers looked over the data, two problems quickly became apparent: intermittent communications and a subtle down-range navigation error that likely would move the landing site a few thousand feet away from the planned touchdown point. That likely would force Armstrong to take over manual control for the final moments of the descent.
But that was not considered a major problem as long as it did not get worse. To improve communications, Armstrong and Aldrin were asked to change the orientation of the lander, rolling about its long axis to improve an antenna's line of sight to Earth.
After concluding that an unrelated electrical issue was the result of instrumentation and not an actual problem, Kranz cleared the crew to begin powered descent.
"Eagle, Houston. If you read, you're go for powered descent," astronaut Charles Duke, serving as capsule communicator, or CAPCOM, radioed from mission control.
Eagle's landing engine restarted as the spacecraft sailed through the low point of its lunar orbit, slowing the ship's forward velocity and beginning the final five-mile plunge to the surface.
Guidance and navigation officer - GUIDO - Steve Bales quickly noticed the lander's flight computer believed the spacecraft was descending 20 feet per second - about 14 mph - faster than it actually was, a manifestation of the down-range navigation error.
"Unfortunately, we had abort limits set at 35 feet per second," Bales, then 26 years old, said in a later interview. "If that deviation would have grown to about 35 feet per second, we'd have had to stop the landing."
A heavy responsibility for a 26-year-old.
"You'd have had to be an idiot not to understand that this was the time we were going to try to land on the moon," he said in an interview years later. "The good thing about it, the one good thing that sort of insulated us from it was Gene Kranz was very good about taking charge, he was a leader. I think everybody on that team felt that no matter what happened Gene was going to stand behind us. And that was a great help."
Even so, "I was afraid from the moment I saw that 20-foot-per-second error until we actually landed on the moon. I was just scared to death, mortified. I was really glad I could talk. I was that scared, all the time."
The powered descent was proceeding smoothly when Armstrong and Aldrin saw the numbers "12 02" appear on the display of their guidance computer, indicating an alarm.
"Twelve-oh-two, twelve-oh-two," called Aldrin.
"Give us a reading on the twelve-oh-two program alarm," Armstrong radioed, his voice tense as he monitored his cockpit displays.
As the call came in from the crew, "Bales had half-risen from his seat, pulled up by a surge of adrenaline," Charles Murray and Catherine Bly Cox wrote in "Apollo: The Race to the Moon." Bales recalled the moment with great clarity, saying the program alarm "meant that in a two-second cycle something hadn't gotten done, it had not had time to complete all the computations it was supposed to complete."
"The computer had been programmed to do a number of tasks: navigation, guidance, crew display, and finish all its tasks and then two seconds later start the same task all over again," he said. "It was also programmed that if it ran out of time to do them, to do the least important ones, put them at the bottom ... and then provide this alarm. But it wouldn't tell you how many it hadn't done."
With two lives on the line, and possibly the fate of the Apollo program on his shoulders, Bales had to make a quick decision. He had to tell Kranz, and thus the crew, whether they could safely proceed with the landing or whether they should abort and return to the safety of orbit.
In a stroke of profound good fortune, the flight control team had run into such program alarms during a simulated landing a few weeks before Apollo 11's launch.
In the simulation, the unexpected and little-understood alarms prompted Bales to call an abort. But Kranz ordered a detailed post-test analysis and the team ultimately concluded the descent could have safely continued as long as the alarms were not continuous.
"I would guess we were about five or six minutes from the touchdown," Bales said. "We got a 12 02 and going through our mind is, uh oh, what's the rule?"
Jack Garman, a 24-year-old computer whiz and a member of the guidance support team, told Bales, "It's executive overflow. If it does not occur again, we're fine."
Bales saw that same rule written out on a cue card he had posted nearby. After taking a few seconds to make sure the downrange navigation error had not gotten worse, Bales told Kranz the descent could proceed.
"We've got ... we're go on that alarm," Duke radioed Armstrong and Aldrin.
"It was 19 seconds since he had first told Kranz to stand by," Murray and Cox wrote. "Steve Bales, 26 years old, with some advice from a 24-year-old, given 19 seconds to think it over, told Gene Kranz, (senior NASA managers), President Nixon and the world - and, not incidentally, Buzz Aldrin and Neil Armstrong - 'ignore the computer and trust me.'"
Kranz wanted to make sure.
"We're go on that alarm?" he asked.
"If ... if it doesn't recur, we'll be go," Bales replied. In the back room, Garman clarified the matter, reminding Bales they could safely continue the descent as long as the alarms were not continuous.
As Eagle descended through 4,000 feet, Kranz polled his flight controllers one more time to make sure everyone was on the same page.
"All flight controllers, coming up on go-no go for landing," Kranz said. "GUIDO, you happy?" Kranz asked.
Bales told Kranz the crew was "go" to continue the descent. Twenty seconds later, as Eagle's altitude dropped below 2,000 feet, another program alarm popped up.
"Twelve alarm; twelve-oh-one," Armstrong radioed.
"GUIDO?" Kranz asked Bales.
"Go!" Bales replied.
Another 12 02 alarm flashed at an altitude of about 700 feet. The crew ignored it.
"All these alarms had kept us from studying our landing zone," Aldrin wrote in "Men From Earth." "The computer ... was taking us to a boulder field surrounding a 40-foot-wide crater."
Armstrong took over manual control and hovered, looking for a better landing area. Back in mission control, a red light flashed on: Eagle was down to the last 5 percent of its landing fuel and if it was not on the surface within about 90 seconds, the touchdown would have to be aborted.
Armstrong, displaying the same cool he brought to the cockpit of NASA's X-15 rocketplane, would not be hurried. With just 30 seconds of fuel remaining, Eagle still was not on the surface.
The tension in mission control was almost unbearable. No one spoke. For some reason, Duke told the flight control team, "OK, let's everybody quiet down," Kranz recalled. "Well, that room was about as still as a tomb between the 30- and 15-second time frame because it was now a horse race of getting on the surface and shutting down versus running out of propellant."
The lander was not equipped with a fuel gauge and flight controllers could only make a rough estimate as to how much propellant was left based on throttle settings. If Armstrong ran out of fuel this close to the surface, there might not be enough time to fire up the lander's ascent engine to prevent a crash.
"We picked up the countdown at 15 seconds propellant remaining," Kranz recalled. "We were somewhere between seven and 12 seconds to the best of our estimate, that the crew advised lunar contact."
"Contact light," Aldrin radioed as 5.5-foot-long probes attached to the lander's footpads touched the surface. "OK, engine stop. ACA out of detent. Modes control both auto, descent engine command override off. Engine arm off. Four-thirteen is in."
"We copy you down, Eagle," Duke radioed, sounding uncertain.
"Houston, Tranquillity Base here," Armstrong replied a few seconds later. "The Eagle has landed."
"Roger, Tranquility, we copy you on the ground," Duke said. "You've got a bunch of guys about to turn blue. We're breathing again. Thanks a lot."
Armstrong and Aldrin would spend 21 hours and 36 minutes on the moon, including two hours, 31 minutes and 40 seconds walking about its surface.
"That's one small step for (a) man, one giant leap for mankind," Armstrong famously said as he stepped off the lander's footpad and onto the moon.
President Richard Nixon, in a long-distance telephone call to the moonwalkers, captured the spirit of the mission when he said:
"For one priceless moment in the whole history of man, all the people on this Earth are truly one; one in their pride in what you have done, and one in our prayers that you will return safely to Earth."
Armstrong and Aldrin successfully blasted off and rejoined Collins aboard the Columbia command module, bringing about 48 pounds of lunar rocks and soil with them. They splashed down in the Pacific Ocean four days later, on July 24, to close out the historic mission. They never flew in space again.
Kranz, who later gained fame as a central character in the movie "Apollo 13," summed up the feelings of many in the space program during an address at an astronaut reunion last year:
"We lived as explorers and charted America's path in space. We knew about high risk and the grief for our friends who gave their lives to the effort.
"In the words of Teddy Roosevelt, 'We knew the triumph of high achievement, and when we failed, at least we failed while daring greatly. Our place will never be with those cold and timid souls who neither knew victory or defeat.'"