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On their second night outbound, Apollo 11's astronauts slept for 10 hours. They had settled in the environment of space — and when they awoke 45 years ago today, they were alert and ready to conduct a midcourse correction and a meticulous checkout of their lunar module, nicknamed Eagle.

The Apollo 11 train was chugging along at 2,406 miles per hour — the slowest part of the journey from Earth to the moon. Neil Armstrong, Buzz Aldrin and Mike Collins were 184,874 miles from their home planet, with the moon 73,732 miles ahead.

Mike opened Eagle’s hatch, and Neil squeezed through the 2.5-foot-wide tunnel, followed by Buzz.

NBC News' Jay Barbree, author of 'Neil Armstrong: A Life of Flight,' provides a day-by-day account of Apollo 11.

Neil was looking for a scratch or any sign of damage. Buzz, the lunar module pilot, began preparing Eagle for its separation from Columbia some 45 hours later. Their inspection found nothing, and they happily reported to Mission Control that their lander was “immaculate” and ready to go.

With five-sixths of their flight completed, Earth's gravity diminished. The moon's grip assumed dominance. Steady acceleration toward the small world required new thinking. The moon's mass was one-sixth of that of Earth's, and it could be set down between the United States' Pacific and Atlantic coastlines. It was, in every practical sense, a dead world.

The moon was airless. No atmosphere or weather. During what was supposed to be their third sleep period, Apollo 11’s crew was restless. Neil, Mike and Buzz knew what awaited them the next day. Entering lunar orbit was not a given. If their rockets did not slow their ship to the correct speed, they would loop around the moon and return to the vicinity of Earth.

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This is Part 4 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 newly published book, "Neil Armstrong: A Life of Flight." Check back with on Saturday to read about Apollo 11's entry into lunar orbit.

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 July 21 at 8 p.m. ET.