Prior to the flights of astronauts in Project Mercury, the first U.S. human spaceflight program, chimpanzees were used to better understand the effects of acceleration and weightlessness. Instead of a spacesuit, these chimps had a pressurized capsule that allowed them to breathe even in case of a failure of spacecraft cabin pressure. The chimp was strapped into a retaining harness inside the capsule and had to operate a system of levers and lights to test its reactions to flight. It was rewarded with banana pellets or a drink of water, or punished with mild electrical shocks, for taking the right or wrong actions.

Chimps were launched into space twice: "Ham" on Mercury-Redstone 2 in January 1961, and "Enos" on Mercury-Atlas 5 in November 1961. In 1967 the NASA Manned Spacecraft Center in Houston transferred this capsule to the Smithsonian. It is unknown whether it was used on a mission.