Mercury Atlas 8 (MA-8, also Sigma 7) was the third manned orbital flight of the Mercury program. The pilot was Walter M. Schirra, Jr. The objectives of MA-8 were to: (1) evaluate the performance of the man-spacecraft system in a six-pass orbital mission; (2) evaluate the effects of an extended orbital space flight on the astronaut and to compare this analysis with those of previous missions and astronaut-simulator programs; (3) obtain additional astronaut evaluation of the operational suitability of the spacecraft and support systems for manned orbital flights; (4) evaluate the performance of spacecraft systems replaced or modified as a result of previous three-pass orbital missions; and, (5) evaluate the performance of and exercise further the Mercury Worldwide Network and mission support forces and establish their suitability for extended manned flight.
Originally scheduled for launch in early September, the mission was postponed twice to provide additional time for flight preparation. The launch was the first to be relayed live (via the Telstar satellite) to television audiences in Western Europe.
Two major modifications were made to the spacecraft to eliminate difficulties encountered during the previous two flights. The first was an alteration of the reaction control system to disarm the high-thrust jets and to permit use of the low-thrust jets only in manual operation (to conserve fuel). The second was the addition of two high-frequency antennas (mounted on the retro package) to assist and maintain spacecraft and ground communications throughout the flight.
Astronaut Schirra called his mission a "textbook flight", the only difficulty having been attaining the correct temperature adjustment on his pressure suit.
A considerable amount of attitude drift time was built into the MA-8 timeline to study fuel conservation methods. This included 18 minutes during the third orbit and 118 minutes during the fourth and fifth orbits. The result was that 78% of the fuel supply remaining in both the automatic and manual tanks at the start of reentry. The pilot was therefore able to use the automatic mode for reentry.
Four experiments were conducted as a part of the MA-8 flight. One was a light visibility experiment, similar to those conducted on the two previous missions. The second was a nuclear radiation experiment, in which radiation-sensitive emulsions were used to study the flux and composition of galactice cosmic rays. A third was an investigation, in which the ablation of various materials due to heating during reentry was measured. The final experiment used a 70 mm Hasselblad camera with various filters to gather imagery for assembling a catalog of Earth photography for comparison with similar images obtained by other satellite programs.
During the flight, the spacecraft attained a maximum velocity slightly higher than previous flights (28,092 km/hour) and an altitude of about 281 km. The capsule reentered after completing six orbits, landing 440 km northeast of Midway Island in the Pacific Ocean, about 8.2 km from the prime recovery ship, USS Kearsarge. The duration of the flight was 9 hours 13 minutes and 11 seconds during which Schirra travelled over 230,000 km.