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The inside story of Apollo 11's nail-biting descent to the surface of the moon

NASA legends look back on Apollo 11

Watch the CBS News prime-time special "Man on the Moon," celebrating the 50th anniversary of Apollo 11, Tuesday at 10/9c.


Less than two weeks before Apollo 11's historic launch to the moon 50 years ago, flight controllers and stand-in astronauts carried out a final simulation of the planned descent to the lunar surface. Though few knew it at the time, that exercise ended up playing a key role in the history-making moments the world would soon follow on live TV.

Three minutes after the computers running the simulation began the imaginary rocket firing that kicked off the descent, 26-year-old Steve Bales, the guidance officer in mission control, suddenly saw a computer alarm code — 1201 — flash on his monitor. He had never seen a code like that during earlier training runs and he had no idea what it meant.

Neither did the two astronauts standing in for commander Neil Armstrong and lunar module pilot Buzz Aldrin.

Quickly referring to a software manual for the lander's guidance computer, Bales saw the 1201 alarm indicated "executive overflow" and "no vacant areas." Clearly, something was overloading the computer, preventing it from completing all its planned computations in a given cycle. But what wasn't getting done? And was it mission critical?

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Guidance officer Steve Bales at his console in mission control shortly before the launch of Apollo 11. NASA

In his book "Failure Is Not An Option," Apollo 11 descent flight director Gene Kranz described the critical moments that followed. After more alarms popped up in quick succession, Bales called Jack Garman, a 24-year-old lunar module software expert in a nearby support room.

"Jack, what the hell is going on with those program alarms? Do you see anything wrong?" Kranz recalls Bales asking. Garman replied, "It's a bailout alarm. The computer is busier than hell for some reason, it has run out of time to get all the work done."

Meanwhile, the lunar module was continuing its imaginary descent, and its systems seemed to be operating normally. But something was clearly wrong, and there were no flight rules defining a procedure to correct the problem. After still more alarms, Bales, seated on the right end of a string of consoles known as "The Trench," reached a decision.

"Flight, Guidance," he called over his headset to Kranz. "I've got a bunch of computer alarms. Abort the landing ... ABORT!"

Astronaut Charlie Duke, the capsule communicator, or CAPCOM, responsible for relaying comments and instructions to the crew, turned to Kranz and asked, "We going to call an abort, Flight?" Kranz sharply replied, "Abort, CAPCOM, abort." Duke relayed the instructions and the crew executed an abort, carrying out the steps needed to jettison the descent stage and return to orbit.

The simulated abort came just 11 days before Apollo was scheduled to take off for real.

Kranz was not happy. He wanted to end the final simulation with a successful landing, an upbeat note leading into NASA's ultimate flight. "Dammit, we should have finished our training with a landing on the surface," he wrote.

NASA almost didn't film the first moon landing

But as it turned out, the aborted landing was a godsend. Richard Koos, the simulation supervisor who added the computer alarms to the final test run, said the alarms in question were not mission critical.

In fact, the lunar module was operating smoothly and only lower-priority items were failing to execute from time to time, triggering the alarms. Continuing with the descent was a much safer option than a low-altitude abort.

Bales, now 76 and long retired from NASA, recalls the simulation like it was yesterday.

"After the sim, we all unplugged, Gene said come up to console," Bales said in a recent interview with CBS News from his home near Philadelphia. "He said, I want you to go and add these (computer alarms) to your rules. We didn't have any rules about them. I said Gene, I've got a gazillion things to do, I listed off two or three things, and he said I don't care what you say, get it done."

Bales called Garman and asked him to consult with the MIT engineers who developed the lunar module's software and come up with a "cheat sheet" defining what needed to be done in response to various alarm codes.

"The MIT guys will probably tell you he couldn't program it, but who cares?" Bales said. "I didn't want somebody who could program it, I wanted somebody like Jack, who was one of the smartest guys I ever met, looking at what somebody else had done and tell you how it should work."

A week later, Garman presented a list of codes and responses, written by hand on a single sheet of paper. Bales taped a copy to his flight console in the mission control center. He did not think it would ever be needed.

"There were not that many that we could do anything about," he said. "Some of them, if they came up the computer was gonna stop. You're done. But there were a number — 1210, 1201, 1202, maybe one or two others — you had to make a choice. So I said OK, and off we went."

Tensions high as Apollo 11 gets underway

Apollo 11 finally got underway at 9:32 a.m. EDT on July 16, 1969.

Strapped into the Apollo command module Columbia atop their huge Saturn 5 rocket, mission commander Neil Armstrong, lunar module pilot Buzz Aldrin and command module pilot Mike Collins shot smoothly into Earth orbit.

Two hours and 50 minutes later, after checking out the spacecraft's myriad systems, flight controllers at NASA's Manned Spacecraft Center (now the Johnson Space Center) in Houston cleared the crew to restart the third stage engine and head for the moon.

Three days later, at 1:21 p.m. on Saturday, July 19, the astronauts braked into orbit around the moon. The next day, Armstrong and Aldrin bid Collins farewell, entered the lunar lander Eagle and undocked during the crew's 13th lunar orbit.

The moon landing at 50: Neil Armstrong in his own words

Coming into work that Saturday morning, Bales immediately noticed a change in the atmosphere of the mission control center. "You could cut that tension with a knife," he said.

Making one more loop around the backside of the moon, Armstrong and Aldrin began the descent toward a target altitude of 50,000 feet above the lunar surface. At that point, the crew would fire the lunar module's main engine — powered descent initiation, or PDI — to begin the 12-minute plunge to the targeted landing zone in the Sea of Tranquility.

During that final pass behind the moon, with the lander out of contact with mission control for about a half hour, flight controllers took a final break. Kranz gave an impromptu, now legendary speech on an unrecorded audio channel only accessible to his White Team flight controllers.

"I remember this so clearly," Bales recalled. "He said however we walk out of this room, whatever happens, we're walking out as a team. I'm taking responsibility. However we get out of here, we're going out as a team, we're not pointing fingers at anybody. And that was an incredible plus. The leader says I'm with you, and it's my responsibility whatever you guys do. That was incredible. ... I remember that clearer than the landing."

The Eagle swung back into contact with Earth and during the next 13 minutes — the time it would take the lander to reach 50,000 feet where powered descent could begin — the flight control team had to verify the spacecraft's trajectory, velocity and general health. And right away, there were problems.

"So we come around the hill, and we don't know anything's going wrong at all, neither does the crew," Bales said. "And then the comm [communication] is so bad, if you listen to that comm you'd wonder are we going to be able to go on or not. Gene is having to figure out is he even going to allow us to go on. That was his call ... and boy, it was dicey."

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Astronaut Charles Duke, left, Jim Lovell and Fred Haise listen intently to mission control audio loops during Apollo 11's descent to the moon. NASA

As it turns out, a shield that was added near the nozzle of a maneuvering thruster was partially blocking the radio signal. Don Puddy, a flight controller monitoring communications, asked the crew to turn the lander slightly "and comm came back," Bales said. "So Gene says, you're go for powered descent, the crew keys in the program, everything's looking great."

But voice communications and telemetry were still intermittent, "and just before we turned that engine on everything drops out and we have no filter (radar) data, we have no telemetry data, we have no voice data, we just have a bunch of noise," Bales said.

"I'm sitting there praying"

Two minutes later, telemetry resumed and Bales quickly saw a guidance issue. The lander was descending toward the moon 20 feet per second faster than it should have been. That mismatch could have been due to an external force that had acted on the lander that was not taken into account; it could have meant trouble with the inertial guidance system; or it could have been a problem with the spacecraft's accelerometers.

The powered descent phase of flight was designed so the crew could carry out an emergency abort at any point during the trip down to the surface — but only if the descent rate was within 35 feet per second of the planned velocity. Eagle was more than halfway to that limit.

"So I'm sitting there praying," Bales said. "I said oh, this thing's got to hold (steady). If it holds, we're OK. If you listen to the tapes, somewhere I say, Gene, that's downrange, we're going to make it, I think. You're not supposed to say 'I think,' but I said it anyway. He knew that we were two thirds of the way to the abort limit. But it held constant, it held and held and held."

Bales began to relax a bit. Aboard Eagle, Armstrong and Aldrin, who had been facing the surface to track landmarks, yawed the lander to help its radar lock onto the ground.

"And we said this is great, because you could see that the radar said hey ... we're off about 3,000 feet in altitude and we're off exactly what we saw, 20 feet per second in altitude rate," Bales said. Using the radar data, the computer could correct for the higher-than-expected rate of descent. And it did just that.

The lander would still miss its planned touchdown target by about four miles, "but it was going to correct for altitude rate so they wouldn't be getting big problems when they got down lower. It was going to fix the altitude and altitude rate, it could not fix the downrange."

In other words, so far, so good.

Around this time — just over seven minutes to touchdown — Aldrin entered commands for the guidance computer to calculate the difference between the radar readings and the computer's predicted altitude. That was necessary to give the crew situational awareness in case of problems that prevented communications with mission control.

Aldrin entered the command "and wham, he says we just got an alarm, 1202," Bales said. It takes telemetry a few seconds to reach Earth, get processed and displayed in mission control, "I think it's about a three- to four-second delay before we even see it."

Bales and Garman immediately thought of the July 5 landing simulation. So did Kranz and Duke. Armstrong and Aldrin did not participate in the simulation and apparently were never briefed on the program alarms or what could cause them.

Welcome to July 1969: Inside NASA's restored Mission Control

The 1202 alarm "tells us the computer is behind in its work," Kranz wrote. "If the alarms continue, the guidance, navigation and crew display updates will become unreliable. If the alarms are sustained, the computer could grind to a halt, possibly aborting the mission. Each alarm must be accounted for. They have the capacity to create doubt and distraction, two of a pilot's deadliest enemies."

A few seconds went by. Suddenly, Armstrong comes on the audio loop and ratchets up the tension, saying "give us a reading on that 1202 program alarm." His tone of voice left no doubt he expected a quick response.

"I am frantically ... trying to remember where my little cue card is and Jack, thank God, Jack is saying Steve, Steve, we're GO unless it keeps reoccurring. ... I said Jack, I trust you. And I say, Flight, we're GO on the alarm. Now, it had been 20 seconds, maybe? Maybe longer. It was an eternity in the control room."

It was more like 31 seconds, and in the world of flight operations, that's as close to eternity as any controller would ever want to get. Charlie Duke knows that as well as Bales.

"When Neil says 'give us a reading,' Charlie knew he'd best do something quick," Bales said. "And so the only time in the descent, the only time ever I heard him do so, I said we're GO and he didn't even wait for Gene. If you listen to the loops, he's telling the crew before Gene even tells him it's OK. And that's usually a no-no.

"But if he hadn't done that, I think we wouldn't have made it, I think the crew would have bailed out. I really do. We'll never know. But Charlie saved the mission by doing that. I think he saved it right then and there."

But it was Bales' responsibility. The fate of the moon landing was on his shoulders in that moment, not Kranz, Duke or even Garman, who Bales always credits for the guidance and expertise that ultimately won the day.

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Jack Garman, an expert on the Apollo lunar lander's guidance computer, wrote up a cheat sheet listing alarm codes after a pre-launch simulation led to a landing abort. Steve Bales had this copy taped to his console during the Apollo 11 landing. Steve Bales

As the MIT computer experts would later explain, the guidance computer normally ran at about 83 percent capacity during that phase of mission operations. Another 13 to 14 percent was lost due to interference from a communications circuit between the computer and the lander's rendezvous radar. When Aldrin asked the computer to display the altitude data, "it was just enough to trip it over," Bales said. "And for at least one second, it doesn't get everything done. That's where you get the 1202."

The descent continued. Aldrin was told not to call up any additional displays and to rely on mission control for trajectory data. More program alarms cropped up, but they did not occur so close together that they violated the somewhat subjective definition of "recurring." Bales and Garman were "GO" almost as soon as each alarm was called out.

Because of the downrange error noted during the initial descent, Eagle was taking the crew to a boulder-strewn crater. Armstrong took over manual control. He halted the descent, hovered and then flew sideways in search of a smoother landing area. Nervous flight controllers could only watch their data and listen as Aldrin called down altitude, vertical and horizontal velocity in feet per second.

As if the drama could get more intense, sloshing propellant uncovered a low-level fuel sensor indicating the tank would run dry in 60 seconds. And the lander was still well above the surface.

"Sixty seconds," Duke called out.

Busy flying the lander, Armstrong did not reply. Aldrin continued his running commentary, saying: "Lights on. Down 2 1/2 (feet per second). Forward. Forward. Good. 40 feet (altitude), down 2 1/2. Kicking up some dust. 30 feet, 2 1/2 down. Faint shadow. 4 forward. 4 forward. Drifting to the right a little. OK. Down a half.

Duke sends up another fuel warning: "Thirty seconds..."

And then, just in the nick of time, Armstrong set the lander down and shut off its engine.

"We copy you down, Eagle," Duke called.

"Houston, Tranquility Base here. The Eagle has landed," Armstrong famously replied.

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The Apollo 11 lunar lander Eagle safely on the surface of the moon after its nail-biting descent. This shot, taken by mission commander Neil Armstrong, shows Buzz Aldrin working at an experiment enclosure on the side of the lander's descent stage. MASA

Armstrong stepped onto the moon's surface six hours and 39 minutes after landing, taking a "giant leap for mankind." Aldrin followed suit a few minutes later and the two men spent two hours and 31 minutes collecting about 48 pounds of rocks and soil, deploying scientific instruments, erecting a U.S. flag and taking a congratulatory phone call from President Richard Nixon.

Twenty-one hours and 36 minutes after landing, the two men blasted off and headed back to lunar orbit where they docked with the command module Columbia for the trip home. They landed on-target with a Pacific Ocean splashdown on July 24.

Armstrong, Aldrin and Collins later received the Medal of Freedom from President Nixon, the highest honor the United States awards civilians. Nixon also gave the mission operations team a NASA group achievement award. The man the agency sent to accept the award was Steve Bales.

He would go on to serve as deputy director of mission operations at the Johnson Space Center before retiring to pursue business interests in the chemical industry.

Reflecting on the landing five decades later, Bales said, "I'm not sure 50 years later I really quite grasp it. I grasp it in a historical sense, it really gave a boost for the country when it really, really needed it. (But) I don't know, I can't even put it into context now because I don't know how it will all turn out.

"It may be a long time before we are able to go to those other places we want to go to. I hope not, but it may be. But when we finally do, it's going to be something that's incredible."

Editor's note: Portions of this story originally appeared in Astronomy Now magazine.

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