Two vehicles and 238,900 miles: three days there and three days back. Twenty-one hours on the surface of the Moon for two astronauts in the lunar lander, while the service module circled the heavenly body in a parking orbit. Katherine knew better than anyone that if the trajectory of the parked service module was even slightly off, when the astronauts ended their lunar exploration and piloted their space buggy back up from the Moon’s surface, the two vehicles might not meet up. The command service module was the astronauts’ bus—their only bus—back to Earth: the lander would ferry the
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