This is the 21st in an exclusive series of 50 articles, one published each day until July 20, exploring the 50th anniversary of the first-ever Moon landing. You can check out 50 Days to the Moon here every day .

Apollo 8 was the triumphant Christmas Eve flight of the Apollo capsule, with crew members Frank Borman, Jim Lovell, and Bill Anders. It flew to the Moon, orbited 10 times, the astronauts read to the world from Genesis during a live TV broadcast, and the capsule returned—all virtually flawlessly.

Apollo 8 was an improvised mission: it flew without a lunar module, because they were behind schedule, and there wasn’t one ready for space. The goal was, quite literally, to get America to the Moon before the Russians could fly there.

One of Apollo 8’s key missions was to test the accuracy and capability of the spacecraft’s flight computers and instruments. Could the newly created computers navigate with absolute precision to the Moon, and then bring the astronauts back to Earth with that same precision? Could the first global tracking network follow its path and communicate with it without any interruption?

Headed for the Moon, with the scientists, engineers, and programmers at MIT who designed the computers and the navigation equipment listening in real time in Cambridge, Massachusetts, the Apollo computer performed flawlessly. Apollo 8’s capsule entered lunar orbit less than a mile off the calculated trajectory, out of 240,000 miles. The navigation was so perfect that when the coordinates were called off on the squawk box from Mission Control, the room in Cambridge erupted in cheers.

Coming home, the computer’s ability to fly the spacecraft was just as perfect. In fact, the computer performed so well, it caused a problem—and provoked a cautionary memo.

Bill Tindall was a senior NASA official in Houston, a good-humored genius of space navigation and mission planning. Part of his job was to make sure the spaceships got to exactly where NASA wanted them to go.