On May 4, 1961, at 7:08 am, Victor Prather, along with
Cdr. Malcolm Ross, ascended in
Strato-Lab High V from the flight deck of the
USS Antietam (CV-36) to an altitude of to test the Navy's
Mark IV full-pressure suit. The balloon, built by
Winzen Research Inc. of
Minneapolis, Minnesota, was constructed of polyethylene plastic only thick. The balloon envelope was the largest that had ever been successfully launched, expanding to in diameter when fully inflated. Beneath the balloon hung a large parachute and then the gondola. To control temperature, the gondola was protected by special
venetian blinds, but otherwise open to space. Balloon, parachute, gondola, and a trailing antenna made a craft close to tall. The primary objective of the flight was to test the Mark IV full-pressure suit. The suit was manufactured by
B. F. Goodrich of neoprene and weighed only . The Mark IV suit overcame problems of weight, bulk, ventilation, air and water tightness, mobility, temperature control, and survival capabilities so well that
NASA selected a modified version for use by the
Project Mercury astronauts. Malcolm Ross and Victor Prather were exposed to temperatures as low as when they passed at 8:10 am. Strato-Lab V reached maximum elevation of at 9:47 am, where the temperature was and the
air pressure was . At that altitude without a space suit, a person would lose consciousness in seconds. The May 4 flight was the most severe test of the Mark IV suit that was ever conducted. The flight lasted 9 hours 54 minutes and covered a horizontal distance of . As they descended, the balloonists opened their face masks when they reached an elevation where they could breathe. Strato-Lab V landed at 4:02 pm in the
Gulf of Mexico. The mission plan was to use a boat to retrieve the balloonists in the event that the gondola landed in the water instead of on the flight deck of the carrier. This had been rehearsed. However, without orders to do so, the crew in a hovering
helicopter lowered a hook. Commander Ross invited Prather to go first, but he declined. Ross stepped into the hook contrary to proper procedure and slipped partially out of it, but he was able to recover without falling completely into the water. A few minutes later, when a hook was lowered to retrieve Prather, he stood on a float attached to the gondola and grasped the rescue line. When he stepped into the hook, the trailing foot pushed the gondola away, and he fell backwards into the water. The helicopter crew assumed that the flight suit was watertight, which it would have been if the face plate was still closed, and did not effect an immediate rescue. Because the face plate was open, Prather's flight suit flooded, and he drowned before Navy divers could rescue him. ==After the flight==