John Glenn was not popular with the other astronauts. Glenn was the backup pilot for Shepard and Grissom on the first two manned Project Mercury flights, the sub-orbital missions Mercury-Redstone 3 and Mercury-Redstone 4. Glenn was selected for Mercury-Atlas 6, NASA's first manned orbital flight, with Carpenter as his backup. Putting a man in orbit would achieve one of Project Mercury's most important goals. Shepard and Grissom had named their spacecraft "Freedom 7" and "Liberty Bell 7". The numeral 7 had originally been the production number of Shepard's spacecraft, but had come to represent the Mercury 7. Glenn named his spacecraft, number
John Glenn delays during the countdown due to equipment malfunctions and improvements and the weather. During Glenn's first orbit, a failure of the automatic-control system was detected. This forced Glenn to operate in manual mode for the second and third orbits, and for re-entry. Later in the flight, telemetry indicated that the heat shield had loosened. If this reading had been accurate, Glenn and his spacecraft would have burned up on re-entry. After a lengthy discussion on how to deal with this problem, ground controllers decided that leaving the retrorocket pack in place might help keep the loose heat shield in place.