I'm John Batchel with Bob Zimmerman. His book is Genesis, The Story of Apollo 8, the first man flight to another world.

Two and a half hours after the TV broadcast reading from Genesis is the trans-Earth injection.

What is that, Bob?

This is where we get to the lack of redundancy on the SPS engine.

This is the moment Susan Borman dreaded

the most. They're in lunar orbit. They've got to fire the SPS engine on the service module

to speed up, leave lunar orbit, and head back to Earth. There is no redundancy on the SPS

engine. If it doesn't work, they're stuck in lunar orbit and they're going to die there.

Just imagine science fiction of the ultimate. They would be run out of air. And of course,

they knew that if that was going to happen, they could just open the hatch and die quickly.

But even way, they would die. So it had to work. And it was like going into lunar orbit as well.

The burn of that engine would occur on the backside when the orbit was on the back side of the moon, which means no communications with the Earth. And so everyone knew they've been in lunar orbit for 20 hours. They're going around the moon, and it's near midnight on Christmas Eve, and they're circling around behind the moon, they disappear,

and they're going to be out of touch for an hour. And sometime during that period, we know

they're going to fire the engine. Do the astronauts know about the firing? Can they hear it, Bob?

Remember, there's no air in space, but the engine is attached to their spacecraft so when they fire

the engine they hear the vibration but it's not like a roar it's more like a buzz because it's coming

through the body of the the spacecraft there's no atmosphere outside so you don't hear a roar so it's like a buzz

a vibration that they can hear on their instrument panel is there a metric that tells them that it's fired correctly?

Yeah, when the engine, they set their computer using those numbers that were dictated to

them to fire at the right time.

Bowman had his hand on the button so that he could, if the engine didn't fire, he'd press

it himself immediately.

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