Two days later, Mission Control couldn’t see anything. The command module had looped around to the moon’s far side, out of the reach of radio communications. During this period of silence, the module slowed down and slipped into the moon’s orbit before coming back around. In this moment, as the men succumbed to the gravity of another world, Armstrong’s heart rate was 106, Aldrin’s 70, and Collins’s a cool 66.

The crew would loop around the moon a dozen more times before one spacecraft became two. Collins stayed in the Columbia module, their living quarters, while Armstrong and Aldrin clambered into the Eagle module and boosted it toward the ground. As they descended, the faint tug of the moon’s gravity greeted them. “Their arms sagged. Legs settled within their suits. Their feet pressed downward in their boots as they yielded to their down-rushing speed,” wrote Jay Barbree, a longtime space journalist and Armstrong confidant, in a biography of the astronaut, who died in 2012.

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Then an alarm blared; the Eagle’s computers were overloaded with signals. Mission Control told Armstrong to ignore it and keep going. Armstrong could see enough of the jagged terrain through his window to know that they had overshot their target by four miles. They needed a smooth, flat place to land, and the Eagle was now headed toward a crater filled with boulders instead. It was time to shut off the autopilot. Armstrong took control of the Eagle, and manually scooted the module away from the crater. The module was still airborne when the low-fuel light started flashing. (Another propulsion system would launch the astronauts back off the moon.)

Armstrong later said he wasn’t worried about the fuel. They were close enough then that if the engine cut off, the moon’s gentle gravity, one-sixth that of Earth’s, would let them coast safely down. But the descent must have been some adrenaline rush to push the lunar commander’s heart rate to 150. Armstrong’s pulse began to climb after he turned off the autopilot and took the controls in his gloved grip. The fate of the mission was, quite literally, in his hands. Tens of thousands of engineers had helped get him here, but this last bit was up to him. That kind of responsibility would quicken anyone’s pulse.

“Okay. I’m going to step off the LM now.”

Armstrong was outside the module, hovering between a ladder rung and history. As his foot pressed into the lunar regolith, his heart thumped at 125 beats per minute.

The two astronauts bounced along on the surface, setting up science experiments and collecting rocks for geologists like Gerald Schaber, who by this time was tracking the men’s movements in the grainy black-and-white footage at Mission Control. The moon made movement so easy that Armstrong and Aldrin didn’t stop to rest. “At no time during the exercise was there any heavy breathing,” according to a NASA mission report. “For the most part, the astronauts’ heartbeat was lower than expected—lower, in some cases, than that of those at Mission Control who were watching.”