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At the end of the second orbit, an indicator in the capsule suggested that the all-important heat shield was loose. Without that firewall, there was nothing standing between the astronaut and the 3,000-degree Fahrenheit temperatures—almost as hot as the surface of the Sun—that would build up around the capsule as it passed back through the atmosphere. From Mission Control came an executive decision: at the end of the third orbit, after the retrorockets were to be fired, Glenn was to keep the rocket pack attached to the craft rather than jettisoning it as was standard procedure. The retropack, it was hoped, would keep the potentially loose heat shield in place. At four hours and thirty-three minutes into the flight, the retrorockets fired. John Glenn adjusted the capsule to the correct reentry position and prepared himself for the worst. As the spaceship decelerated and pulled out of its orbit, heading down, down, down, it passed through several minutes of communications blackout. There was nothing the Mission Control engineers could do, other than offer silent prayers, until the capsule came back into contact. Fourteen minutes after retrofire, Glenn’s voice suddenly reappeared, sounding shockingly calm for a man who just minutes before was preparing himself to die in a flying funeral pyre. Victory was nearly in hand! He continued his descent, with the computer predicting a perfect landing. When he finally splashed down, he was off by forty miles, only because of an incorrect estimate in the capsule’s reentry weight. Otherwise, both computers, electronic and human, had performed like a dream. Twenty-one minutes after landing, the USS Noa scooped the astronaut out of the water.
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Margot Lee Shetterly (Hidden Figures: The American Dream and the Untold Story of the Black Women Mathematicians Who Helped Win the Space Race)