Scientific American 201907

540

600 feet

520

400

330

West Crater

Planned landing zone

The onboard

LPD (Landing

Point Desig­

nator) is steering

the lander

to ward a boulder

field north of

West Crater.

Armstrong assumes

attitude control of

the lander from the

onboard computer.

The Eagle approaches

the boulder field.

West Crater

Armstrong repositions (pitches

forward) the Eagle to maintain

ground speed and fly past the

boulder field.

The lunar lander flies over

the West Crater boulder field.

58 Scientific American, July 2019

THE FINAL MINUTES

The carefully choreographed mission hit a few snags as the

Eagle descended for landing. This map, created using the

same 3-D-modeling techniques as were used for the map on

the preceding pages, shows the lander’s altitude during the

nail-biting moments in which Armstrong navigated past

the intended landing spot—which turned out to be littered

with dangerous boulders—and found a new target literally

on the fly.

The trouble began at an altitude of about 33,000 feet,

when a warning light labeled “1202 program alarm” began

flashing on the Eagle’ s dashboard. “What is it?” Armstrong

asked Aldrin as the light blinked and a bell rang at irregular

intervals. Neither recognized the alarm from any of their

flight simulations. Eventually mission controllers radioed

that it could be safely ignored, but trying to determine its

cause wasted precious time.

As the lander’s fuel reserves dwindled, the Eagle became

increasingly difficult to maneuver. When the propellant in

its tank sank below 50 percent, the fuel started sloshing

wildly, jerking the vehicle in all directions. The issue also

caused the lander’s low-fuel-level alarm to go off between

30 to 45 seconds early, making it seem like the astronauts

had less time than they did to safely reach the ground.

Finally, with the Eagle at around 2,000 feet, Armstrong

looked out of his window to examine the proposed landing

site. (He should have done this several minutes earlier, but

as he said later in a debriefing, “our attention was directed

toward clearing the program alarms, keeping the machine

flying, and assuring ourselves that control was adequate to

continue without requiring an abort. Most of the attention

was directed inside the cockpit during this time.”) He didn’t

like what he saw. As he described it in the debriefing, the

landing site was a “large rocky crater surrounded with the

large boulder field with very large rocks covering a high

percentage of the surface.”

Running out of fuel and time, Armstrong took over steering

the spacecraft from the onboard computer at around 540

feet. Just under the wire, he guided the Eagle past the boulder

field to a safe landing on relatively flat ground.

Edward Bell is a contributing art director at

Scientific American and an animator specializing

in planetary science. He is author of the award-

winning iPad book Journey to the Exoplanets.