When the history of human space flight in the 20^th century is finally, fully written, it will not be the triumphant lunar landing of Apollo 11 or the survival drama of Apollo 13, that emerges as the most significant of them all. It will be Apollo 8, the first lunar orbital mission — the moment human beings became a two-world species, wrenching ourselves away from the gravitational hold of Earth and sailing across the deep waters of deep space. That the mission occurred in 1968—the bloodiest year in modern history—and redeemed that year with the iconic Earthrise photo and the historic reading of Genesis from lunar orbit on Christmas Eve, was not part of its original objectives but was among its greatest achievements.

In the 1990s, I wrote the story of Apollo 13 — a book that became the inspiration for the 1995 movie. I have long wanted to return to the deep trove of Apollo lore and have at last done so with Apollo 8: The Thrilling Story of the First Mission to the Moon. This excerpt takes place in the first three hours of the mission, when the crew—space veterans Frank Borman and Jim Lovell and rookie Bill Anders—have arrived in Earth orbit and are preparing to light their engine for the maneuver known as translunar injection (TLI), blasting away from the home planet and out toward the moon. It was something that had never been tried before.

In the brief time they had before pointing the prow of their spacecraft toward the moon, Frank Borman, Jim Lovell and Bill Anders were not thinking epochal thoughts. They were, for the moment, thinking about not throwing up. Borman and Lovell were hardly strangers to space, but they had spent all of that time sealed inside a Gemini spacecraft, with its tiny 91 cubic feet of habitable volume that was not even enough to allow a man to get out of his chair.

The Apollo was different. It didn’t just have a comparatively spacious two hundred and eighteen cubic feet, it had two hundred and eighteen smart cubic feet, configured in a way that allowed for maximum mobility even with three men competing for the room. Lovell was the first to unlatch his restraint belt and pop out of his seat. He drifted down to the spacecraft’s small storage area to stow his helmet and promptly felt his head swim and his stomach turn over. Taking hold of a solid projection on the wall he held himself steady.

“Be very careful getting out of your seat,” he called back up to Borman and Anders. Anders looked quizzical, but Borman, who had talked with Lovell about the possibility of space sickness in so big a ship, knew exactly what Lovell meant.

“Look straight ahead for a while,” Borman said to Anders. Then he opened his seat belt too, floated down to join Lovell and felt the same sickly swoon. The steak and eggs and toast and jelly and fruit and juice and coffee they’d all wolfed down in the crew quarters at the Cape this morning swam before his eyes and he fought to keep them down where they belonged. Anders stayed where he was told for a few moments before unbuckling as well.

Motion sick or not, the astronauts would have a lot to do in the brief two laps they were going to make around the Earth before the ship was fit for TLI. Borman’s job on this mission was the job of any commander, which meant he was fluent in every single system aboard the spacecraft, could fly the ship alone if he had to and bore the weight of command as well.

Lovell had been tapped as navigator, and for this, the most long-distance mission in the history of space exploration, that was no small thing. The computer had all of the coordinates it would need for the flight stored on magnetic tape, and the data could be read on a screen that accommodated a full 30 characters at one time. But the mechanical brain still needed a human brain to confirm what the machine thought it knew before the main engine or even a single one of the sixteen smaller thrusters would be fired.

Anders was mission photographer, and that was far more important than merely serving as Apollo 8’s scrapbook maker. Unmanned lunar orbiters had done a good enough job of surveying the moon, but no image that was captured by a computer and beamed home as a series of digits could compare with a picture that was preserved on the halide crystals and gelatin emulsion of a piece of photographic film and then hand carried to a lab for development.

That kind of photographic clarity would be essential in choosing landing sites for future flights. Until the time to do that work came, Anders would also oversee the command module’s life support systems—the oxygen, heaters and power systems on which the astronauts’ lives would ride for the next week.

Straightaway, there was a problem for him to address. None of the three men had yet removed the yellow life vests they wore during liftoff—a safety precaution in case they had to abort en route to orbit and ditch in the ocean. Now, as Lovell floated past the base of Borman’s seat while checking on the navigation panel, the vest’s activation lever caught on a protruding strut and a loud pop and hiss sounded.

“Oh shoot!” Lovell exclaimed, as the thing began billowing up on his chest.

“What was that?” asked Borman, who had no clear line of sight to Lovell.

“My life jacket.”

Borman laughed. “No kidding?” he asked. Then, not wanting to miss the show, he maneuvered to where Lovell was and his laughter stopped.

Lovell looked comical alright, but the situation wasn’t. It would be easy to deflate and stow the vest, except that it had been inflated in the first place by a small canister of pressurized carbon dioxide. Releasing a big blast of CO₂ in a small, enclosed space in which breathable air was in limited supply was a terrible idea. The spacecraft was equipped with air scrubbers, but the scrubbers could get saturated like a used cigarette filter. Dirtying them up on the first day of the mission was not the way to begin.

Anders and Lovell saw the commander’s face clouding up and Borman, recognizing their discomfiture, rearranged his features to something more agreeable. “Well,” he began, “we…we can live with a little CO₂…” He knew that that was true, but he wished they didn’t have to.

It fell to Lovell, whose blunder it had been, to figure out a solution. All at once smiling, he floated over to the closest thing to a proper bathroom the spacecraft had—the urine station in the storage area, which was nothing more than a tube attached to a funnel at one end and to a small reservoir in the wall at the other. The urine would flow from the astronaut, through the tube and then be vented into space.

Now Lovell removed the funnel, opened the valve of the life jacket and let the CO₂ stream invisibly overboard the same way. The mission’s first—and, the astronauts hoped, only—problem was put away with it, and they could turn their attention to the translunar engine burn, which was fast approaching.

The maneuver would be an exceedingly precise one, accelerating Apollo 8 from its orbital velocity to an unheard of 24,200 miles per hour, a speed that would send the crew barreling across a quarter million miles of empty space to the moon. The astronauts had practiced the procedures to configure the ship for the burn so many times that it was all embedded in muscle memory by now. That meant they finished the job quickly—too quickly, actually. Downtime in space was always welcome, but downtime when you were waiting on a maneuver on which the astronauts’ lives and the future of the American space program rode was tortuous.

“You’re looking good, Apollo 8,” said Michael Collins, the Capsule Communicator, or Capcom, in Houston. “We don’t have anything for you. We are just standing by.” Collins was an astronaut as well, and was already training to fly Apollo 11 the next year—assuming Apollo 8 succeeded and allowed an Apollo 11 to happen at all.

The long, remaining minutes ticked slowly away. Collins held his tongue and listened to the chatter on Mission Control’s internal loop as the other controllers confirmed that all was as it should be. Finally, the Flight Director gave him the high sign to signal the astronauts that their ship was fit for the leave-taking that was the translunar injection burn. Collins nodded.

“Alright, Apollo 8,” he said. “You are go for TLI. Over.”

“Roger,” Borman answered uninflectedly. “We understand we are go for TLI. Over.”

Collins slumped back in his seat. What should have been one of the most thrilling moments of his life was actually one of the most unsatisfying.

Three human beings, he reflected, were about to tear themselves away from the close gravitational grip of Earth, and in three days time would surrender to the gravity of another celestial body. No living creature had done that before. There ought to be an oom-pah band, Collins thought. There ought to be fireworks. There ought to be some way to mark the moment. Instead, there was just this dull, flat scrap of language: You are go for TLI.

But the jargon was deliberate, designed to hollow out those very feelings of momentousness, because feelings could be distractions when you needed to focus only on the task at hand. Elsewhere in Mission Control, Jerry Bostick, the engineer at the flight dynamics console, allowed himself a flicker of the forbidden wonder.

“They’re leaving us,” he muttered to no one in particular. “We’ve got these guys headed out of orbit.”

In the spacecraft, those guys did what the flight plan directed, which was to settle back into their seats and buckle their restraints. “Alright gentleman,” Borman said, “let’s get set for this.”

The astronauts scanned their instruments, seeing nothing amiss in alignment or fuel pressure or anything else, and then scanned again because it was something to do and it was what their training called for.

“Apollo 8, Houston,” Collins called, at the five minute mark before TLI, “you’re looking good down here.”

“Roger, understand,” Borman answered.

The spacecraft and the ground held their silence as best they could as those 300 seconds melted away.

“Apollo 8, Houston, coming up on 20 seconds till ignition,” Collins said.

“Okay,” said Anders.

“Roger,” said Borman.

Lovell gave Anders an encouraging smile, then turned to Borman, who did not glance back. His eyes were fixed on the instrument panel clock and the ignition light next to it.

“Nine, eight, seven,” Borman announced, then counted in his head for a digit or two. “Four, three, two…” he resumed.

Then, behind them, the crew felt a rumble from far, far away—from the distant aft end of the ship—as the liquid oxygen and liquid hydrogen flowed from their separate tanks and mixed in the spacecraft’s combustion chamber. The ignition system engaged and the exhaust exploded out of the engine bell, silent in the vacuum of space, a low, vibrating rumble inside the spacecraft.

“Light on,” Borman called. “Ignition.”

“Roger, ignition,” Collins said.

A shadow of gravity nudged the crew from behind. Instruments throughout the spacecraft, tuned like seismographs to every change in velocity or orientation, twitched in reaction to the sudden acceleration. Far more precise sensors, these written into the brains and vestibular systems of the astronauts themselves, the result of years and years and years of flying, reacted too.

The spacecraft was accelerating, the astronauts were moon-bound, and like millennia of explorers before them, they knew that, depending on a thousand-thousand variables, their immediate future had now turned coldly binary: they would get where they were going or they would die in the effort.

Excerpted from APOLLO 8: The Thrilling Story of the First Mission to the Moon, published by HENRY HOLT AND COMPANY, LLC. Copyright © 2017 by Jeffrey Kluger. All rights reserved.