Forty-five years ago today, when Apollo 11's astronauts awoke on the day they were to enter lunar orbit, the moon’s gravity was pulling them faster and faster to their target.
Neil Armstrong, Mike Collins and Buzz Aldrin were flying through the moon's huge shadow, and the view was testing their nerves.
"Houston, the view of the moon we have now is really spectacular," Neil told Mission Control. "It fills about three-quarters of the hatch window, and of course we can see the entire circumference, even though part of it is in complete shadow and part of it's in Earthshine. It's a view worth the price of the trip."
NBC News' Jay Barbree, author of 'Neil Armstrong: A Life of Flight,' provides a day-by-day account of Apollo 11.
In Mission Control, every monitoring console was in the green. Apollo 11 was right on course, with just 10 minutes remaining before Neil, Mike and Buzz were to fly behind the moon, where their radios would be blocked. CapCom told them, “Eleven, this is Houston. You are Go for LOI."
LOI. Lunar Orbit Insertion. That was the vital maneuver needed to reach the moon's surface, and Apollo 11's crew was prepared for any contingency. The astronauts would be out of contact with Mission Control, and just before they lost signal, CapCom radioed, "Apollo 11, this is Houston. All your systems are looking good going around the corner, and we’ll see you on the other side, over."
"Roger," Neil assured them as Apollo 11 vanished.
Behind the moon it was as if Armstrong, Collins and Aldrin didn't exist. They could not communicate with Mission Control. No signals in, none out. The mission now existed only in their small world inside a spacecraft.
They were in their seats, with the Apollo command and service modules docked to the lunar lander, moving backward 300 miles above the moon — a moon they would not see again until they turned around, following the 6-minute, 2-second burn that was needed to brake their speed from 5,000 to 3,000 miles per hour. If the burn was successful, lunar gravity would secure them in an orbit ranging between 61 by 169.2 miles.
The crucial rocket firing was set to begin 7 minutes and 45 seconds into their far-side pass. Neil and Mike and Buzz checked their settings — once, then again, and then a third time to make sure they did it right the first time. It had to be perfect. Just one digit in the computer out of place could send them into a lunar mountain, or turn them and send them into an orbit around the sun.
But that wasn’t going to happen. Apollo 11’s crew made sure of that. When they reached the mark, they felt the gentle ignition and heard the satisfying rumble of the SPS propulsion system burning. The numbers told them the rocket had fired and was running — burning smoothly and evenly for what seemed an eternity to Neil and Mike and Buzz.
They stared into the blackness, unable to see the moon — only sensing it — as their SPS propulsion system slowed their speed, moving them to within 61 miles of the surface. Their only worry was that the rocket burn might last too long, crashing them into the moon that was ever so near.
For 6 minutes and 2 seconds they rode Apollo 11 to its slower speed. When it was over, finally over, it was a splendid and epochal moment: Seventy-five hours and 55 minutes after ridding itself of its shackles on the launch pad, the Apollo 11 spacecraft locked itself in lunar orbit.
No one on Earth knew this had happened. In Mission Control, this was a time of cliff-hanging suspense — a time to count the minutes and seconds that had to pass before Apollo 11 emerged from the lunar far side to signal success.
On Apollo 11, the celebration was already under way. The numbers were perfect. The astronauts had turned their spacecraft around and were looking down at the moon, excitedly pointing out one spectacular feature after another. When they came around the lunar limb, and Mission Control could hear them at the instant they should have ... it was Earth's turn to celebrate.
- Part 1: How Neil Armstrong Got Ready for the Moon
- Part 2: Memories of Apollo 11's Launch Burn Brightly
- Part 3: Lighter Moments Between Earth and the Moon
- Part 4: Apollo 11's Crew Feels the Pull of the Moon
- Read an Excerpt from 'Neil Armstrong: A Life of Flight'
This is Part 5 of an eight-part series retracing the Apollo 11 mission to the moon, day by day. The account is based on material from Jay Barbree's New York Times best seller, "Neil Armstrong: A Life of Flight." Check back with NBCNews.com on Sunday for the story of the lunar landing.
Barbree will discuss the Apollo legacy on "Virtually Speaking Science," an hourlong talk show that airs on Blog Talk Radio and in the Exploratorium's Second Life virtual auditorium. The show airs Monday at 8 p.m. ET.