Mercury-Atlas 6 (MA-6) was the third human spaceflight for the U.S. and part of Project Mercury. Conducted by NASA on February 20, 1962, the mission was piloted by astronaut John Glenn, who performed three orbits of the Earth, making him the first U.S. astronaut to orbit the Earth.
The Mercury spacecraft, named Friendship 7, was carried to orbit by an Atlas LV-3B launch vehicle lifting off from Launch Complex 14 at Cape Canaveral, Florida. After four hours and 56 minutes in flight the spacecraft re-entered the Earth's atmosphere, splashed down in the Atlantic Ocean and was safely taken aboard the USS Noa.
But the flight was far less than perfect.
During Glenn's orbits, Mercury Control had been monitoring the problem with "Segment 51". The Hawaiian tracking station asked Glenn to toggle the landing bag deploy switch into the automatic position. If a light came on, reentry should take place while retaining the retro pack. Given the earlier questions about the landing bag switch, Glenn realized there must be a possible problem with a loose heat shield. The test was run but no light appeared. Glenn also reported there were no bumping noises during spacecraft maneuvers.
Mercury Control was still undecided on the course of action to take. Some controllers thought the retrorocket pack should be jettisoned after retrofire, while other controllers thought the retro pack should be retained, as added assurance that the heat shield would stay in place. Flight Director Chris Kraft and Mission Director Walter C. Williams, decided to keep the retro pack in place during reentry. Walter Schirra, the California communicator at Point Arguello, relayed the instructions to Glenn: the retro pack should be retained until the spacecraft was over the Texas tracking station. After the mission was over, the "Segment 51" warning light problem was later determined to be a faulty sensor switch, meaning that the heat shield and landing bag were in fact secure during reentry.