‘Eagle, you are go for powered descent’

July 12, 2019
by Mark Ollig

It was Sunday, July 20, 1969, and the mission to land the first two men on the moon was proceeding smoothly.

Inside the crew compartment of the moon-orbiting Apollo 11 lunar module, named Eagle, were astronauts Buzz Aldrin, the lunar module pilot; and Commander Neil Armstrong.

Armstrong and Aldrin were about to embark on their historic journey to the surface of the moon.

The two astronauts would be communicating with Mission Control in Houston, TX (some 244,391 miles away) using the lunar module’s high-gain S-band antenna.

Inside the command module, called Columbia, was astronaut Michael Collins, the command module pilot, who would remain in orbit above the moon.

If Mission Control loses radio communications with Eagle, Columbia will act as the go-between and relay messages between them.

Mission Control was receiving telemetry data from the Eagle, which included radar updates, fuel consumption, and other instrumentation readings.

“The Eagle has wings,” Neil Armstrong reported to Mission Control as the lunar module undocked from Columbia and began its journey.

“Eagle, Houston. If you read, you’re go for powered descent. Over,” Mission Control radioed.

There was no reply.

The Eagle was not receiving radio transmissions from Mission Control.

Aboard Columbia, Michael Collins radioed, “Eagle, this is Columbia, You’re a go for PDI (Powered Descent Initiation), and they recommend you yaw right 10 degrees and try the high gain (antenna) again.”

Eagle maneuvered right 10 degrees, and the high gain antenna was now in a position to communicate with Mission Control.

“Eagle, Houston. We read you now. You are go for PDI. Over,” radioed Capcom.

“Roger. Understand,” Aldrin replied.

An estimated 600 million people around the world watched on television as the Eagle slowly descended toward the lunar surface.

“Houston, you’re looking at our Delta-H (Constant Delta Height – orbital data)?” asked Armstrong.

“That’s affirmative,” replied Charlie Duke, an astronaut and the spacecraft communicator (CAPCOM) at Mission Control.

CAPCOM is an acronym for Capsule Communicator and originated during NASA’s Project Mercury program. The Mercury spacecraft was called a capsule.

NASA required voice communications between the spacecraft and Mission Control to pass through one person (usually another astronaut). This person is called the Capcom.

As many of us watched on our television, the mission to land two astronauts on the moon appeared to be proceeding according to the flight plan.

Then, all of a sudden, we hear Neil Armstrong call out, “Program alarm!”

“It’s a twelve-oh-two,” he quickly reported to Mission Control.

“Twelve-oh-two,” repeated Buzz Aldrin.

The 1202 alarm indicates the primary guidance system is becoming overloaded and running out of word space in the computer core sets used to execute programs by the Eagle as it maneuvered and descended toward the moon.

According to the NASA voice transcript, what followed was 16 seconds of radio silence as flight controllers determined if the Eagle could continue with the lunar landing, or if the astronauts needed to abort.

“Give us a reading on the twelve-oh-two program alarm,” Armstrong asked Mission Control.

“We’re go on that flight,” one flight controller in Mission Control reported to Apollo 11 Flight Director Gene Kranz.

Flight controllers had determined there was no loss of any critical lunar module navigational data or instrumentation systems because of the 1202 alarm.

“We’re go on that alarm?” asked Kranz, wanting to confirm what he heard.

“If it doesn’t reoccur, we will be go,” answered the flight controller.

“Roger. We got – we’re go on that alarm,” Capcom informed Armstrong and Aldrin.

“We got good data,” Aldrin confirmed.

“Roger. Stand by. You’re looking great at 8 minutes,” replied Capcom.

“We copy,” acknowledged Aldrin.

The lunar module continued to maneuver and navigate closer to the surface of the moon.

At Mission Control, each flight controller was polled for a “go” or “no go” for landing by Gene Kranz. All reported “go” for landing.

“Eagle, Houston. You are go for landing, over,” radioed Capcom.

“Roger, understand. We are go for landing,” replied Aldrin.

The Eagle, under computer control, was quickly descending and passed over its intended landing site.

At 7,000 feet above the surface and descending at a rate of 125 feet per second, Armstrong took maneuvering control of the lunar module.

He steered the spacecraft away from a large crater described by Aldrin as being “wider than a football field.”

As the Eagle descended, the astronauts searched for a level surface without any craters or boulders to set the lunar module down on.

Meanwhile, in Mission Control, another concern was being watched closely by Bob Carlton.

Carlton, the Lunar Module Guidance, Navigation, and Controls Systems engineer was monitoring the remaining fuel supply in the Eagle’s descent stage tanks.

“Okay, Bob, standing by for your [fuel] call-outs shortly,” directed Kranz.

“I think we better be quiet, flight,” replied a concerned Carlton.

“The only call-outs from now on will be for fuel,” Gene Kranz instructed the flight controllers.

“At 400 feet, down at 9 . . . forward . . . 350 feet, down at 4,” reported Aldrin as the Eagle came closer to the moon’s surface.

“Low level,” Carlton informed Kranz regarding the status of Eagle’s remaining fuel.

“Low level,” Kranz repeated to the flight controllers.

With the Eagle hovering 75 feet above the lunar surface, Carlton called out, “Sixty seconds.”

“Sixty seconds,” Kranz quickly informed Capcom.

Capcom radioed “sixty seconds” to Armstrong and Aldrin.

The lunar module’s descent stage propellant tanks had 60 seconds of fuel left to complete the landing.

They were running out of fuel and may have to abort the landing.

If the astronauts abort the landing, they will detach from the Eagle’s descent stage, fire the engine of upper ascent stage (containing the astronauts inside a pressurized crew cabin), and rendezvous with Columbia.

I learned it came very close to an aborted landing.

“We were 18 seconds from abort,” Carlton would later say in a 2009 Washington Times interview.

Armstrong and Aldrin were aware of the low fuel status, and quickly worked to find a safe landing spot for the Eagle.

“Thirty seconds,” Capcom urgently told Armstrong and Aldrin.

For the next 9 seconds, there was silence.

Hovering 40 feet above the moon’s surface, the Eagle’s descent stage engine exhaust began kicking up some lunar dust.

Carlton began to say, “Fifteen sec . . .” and abruptly stopped.

Read the conclusion in next week’s “Eagle is at Tranquility” column to be published one day before the 50th anniversary of the Apollo 11 moon landing.

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