One minute into the flight, Apollo 8 reached the speed of sound—767 miles per hour—and an altitude of about 24,000 feet. At that point, the roar and crackle generated by the interaction between engine exhaust and air could not move fast enough to catch up to the spacecraft, and the cabin grew quiet, the hum of its instrument panel the only noise Borman could detect. To him, it now sounded as if he was flying an unpowered glider.
