Shepard was the oldest U. S. astronaut when he made his trip aboard Apollo 14. He is the only astronaut from Project Mercury (the original Mercury Seven astronauts) to reach the Moon. Another of the original seven, Gordon Cooper, had (as Apollo 10's backup commander) tentatively been scheduled to command the mission, but according to author Andrew Chaikin, his casual attitude toward training, along with problems with NASA hierarchy (reaching all the way back to the Mercury-Atlas 9 flight), resulted in his removal.