Mercury Atlas 7 (MA-7, also designated Aurora 7) was the second orbital flight of an American rocket with a human on board. The pilot was originally planned to be Donald K. Slayton but was changed to be M. Scott Carpenter after a medical examination of Slayton revealed an irregularity in his heartbeat. The objectives of MA-7 were similar to MA-6, i.e. to: (1) evaluate the performance of a man-spacecraft system in a three-orbit mission; (2) evaluate the effects of space flight on the astronaut; (3) obtain the astronaut's opinions on the operational suitability of the spacecraft systems; (4) evaluate the performance of spacecraft systems replaced or modified as a result of previous missions; and, (5) exercise and evaluate further the performance of the Mercury Worldwide Network. Originally scheduled for launch in early May, the mission was thrice postponed, once (07 May) due to checkout problems with the Atlas launch vehicle, once (17 May) to perform modifications to the altitude-sensing instrumentation in the parachute-deployment system, and finally (19 May) due to detected irregularities in the temperature control device on a heater in the Atlas flight control system. During the flight only one critical component malfunction was encountered. A random failure of the circuitry associated with the pitch horizon scanner, which provided a reference point to the attitude gyros, occurred. During the flight there was also concern about excessive fuel usage resulting from extensive use of the high-thrust controls and the inadvertant use of two control systems simultaneously. To compensate for this the spacecraft was allowed to drift in attitude for an additional 77 minutes beyond the time already built into the flight plan. Two experiments were on-board MA-7. One was a balloon, deployed and inflated to measure drag and provide visibility data. The other was a device to study the behavior of liquid in a weightless state. The balloon experiment failed when it did not properly inflate on deployment, but the liquid experiment behaved as anticipated. A curious event which occurred during Glenn's (MA-6) flight was his report of "fire flies" when he entered the sunrise portion of an orbit. Although this phenomenon was a mystery at the time, it was resolved during the flight of Mercury Atlas 7 when Scott Carpenter accidentally tapped the wall of the spacecraft with his hand, releasing many of the so-called "fire flies". The source was determined to be frost from the reaction control jets. During the flight, the spacecraft attained a maximum velocity in excess of 28,000 km/hour and an altitude of about 267 km. The capsule reentered after completing three orbits, coming down in the Atlantic Ocean some 200 km northeast of Puerto Rico at 19 degrees 29 minutes N, 64 degrees 05 minutes W, about 400 km beyond the planned impact point. The overshoot was traced to a 25 degree yaw error at the time the retrograde rockets were fired. Retrofire was also about 3 s late, accounting for about 20 miles of the overshoot. The duration of the flight was 4 hours 56 minutes and 05 seconds during which Carpenter travelled over 121,600 km. After the firing of the retrorockets, computers at NASA's Goddard Space Flight Center successfully predicted the area of splashdown and naval ships and aircraft were deployed to the new location. An Air Rescue Service SA-16 amphibian aircraft was the first to establish visual contact with the spacecraft some 39 minutes after splashdown with the USS Farragut being the first ship to reach the area. Carpenter was picked up after 2 hours and 59 minutes in the water and returned by helicopter to the aircraft carrier USS Intrepid. He experienced no adverse physical or biomedical effects due to the flight. The Mercury capsule was not retrieved until about 6 hours later when special equipment on-board the USS John R. Pierce arrived to retrieve it.