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Project Mercury was the United States' first human spaceflight program. It ran from 1958 to 1963, and was responsible for putting the first American in space, Alan Shepard, on May 5, 1961. Gus Grissom was one of the original seven Mercury astronauts selected for the program, and he flew the second Mercury mission, Mercury-Redstone 4, on July 21, 1961.