Mercury-Atlas 8 (MA-8) was the fifth United States Space Mission, part of NASA's Mercury Programme. Astronaut Jr., orbited the Earth six times in the Sigma 7 spacecraft on October 3, 1962, in a nine-hour flight focused mainly on technical evaluation rather than on scientific experimentation. This was the longest U.S. manned orbital flight yet achieved in the Space Race, though well behind the several-day record set by the Soviet Vostok 3 earlier in the year. It confirmed the Mercury spacecraft's durability ahead of the one-day Mercury Atlas 9 mission that followed in 1963.

Planning began for the third U.S. orbital mission in February 1962, aiming for a six-or-seven-orbit flight to build on the previous three-orbit missions. NASA officially announced the mission on June 27, and the flight plan was finalized in late July. The mission focused on engineering tests rather than on scientific experimentation. The mission finally launched on the morning of October 3, having been delayed two weeks because of problems with the Atlas booster. A series of minor booster problems during launch and a faulty temperature controller in Schirra's pressure suit were the only technical problems noted during the flight. The spacecraft orbited in both automated and passive flight modes for prolonged periods while the pilot monitored it and carried out some minor scientific experiments. After six orbits, the capsule landed in the Pacific Ocean half a mile from the recovery carrier, and was hoisted aboard for Schirra to disembark.

The scientific results of the mission were mixed. The astronaut returned healthy after nine hours of confinement in a low gravity environment. Observation of the Earth's surface proved unproductive, however, because of heavy cloud cover and bad photographic exposures. The public and political reaction was muted compared with that of earlier missions, as the Cuban missile crisis quickly eclipsed the Space Race in the news. The mission was a technical success: all the engineering objectives were completed without significant malfunctions, and the spacecraft used even less fuel than expected. This confirmed the capabilities of the Mercury spacecraft and allowed NASA to plan with confidence for a day-long flight, MA-9, which had been an early goal of the Mercury program.