The Mercury 7 astronauts with model of Mercury-Atlas.
With the success of Mercury-Atlas 6 and John Glenn orbiting the Earth in his capsule Freedom 7, NASA continued preparations for the next mission. However, there was an unexpected change in mission assignments. DUring the continuous medical examinations of the astronauts, NASA doctors detected an "erratic heart rate" in astronaut Deke Slayton (second from left front in picture above). The Air Force medical board and the assigned civilian cardiologists recommended that he be disqualified for assignment as an astronaut. In his place would go astronaut Scott Carpenter.
Scott Carpenter in the "White Room" inspecting the bulkhead of his capsule, which he nicknamed "Aurora 7."
Scott Carpenter was a Lt. Commander in the U.S. Navy. After WW2 he learned aeronautical engineering, and during the Korean War he flew anti-submarine patrols in Navy patrol planes. He became a test pilot for the Navy, and eventually was selected as an astronaut. He served as backup pilot for John Glenn on the MA-6 mission. Now, he was moved to be the prime pilot for MA-7.
Astronaut Deke Slayton in Mercury spacesuit during tests.
Donald "Deke" Slayton was trained by the US Air Force as a B-25 bomber pilot and flew 56 combat missions in Europe during World War 2. Later he flew the A-26 attack bomber in the Pacific against the Japanese for 7 missions before the war ended. After the war, he studied aeronautical engineering and became a test pilot, eventually flying such famous planes as the F-101, F-102, F-105, and F-106 jet fighters. No doubt he was deeply disappointed by the medical decision to cut him from assignments. In fact, there were many pilots and astronauts who sided with Deke against this decision but could not change NASA management's decision.
With his assignment changed, Deke decided to stay in the program for the time being and continue helping to test and prepare missions for the Mercury Program.