The Mercury spacecraft could hold a single passenger, in a sitting position with their back to the heat shield. Near their left hand was a handle to launch the escape system. The astronaut would wear a spacesuit with an oxygen supply and a cooling system.
The Mercury missions tested not only the spacecraft, but also the spacesuits. They used pressure suits, which would protect the astronaut from the extremely low air pressure in space. The suits also monitored the astronaut's heart rate, blood pressure, and temperature. This medical information would be sent down to Earth during the flight. During the mission, the astronaut would drink water and eat food pellets.
The spacecraft design was changed three times during the missions. The shape of the heat shield was improved, and the escape system was changed to a different design. The Gemini and Apollo missions which followed were successful thanks to the Mercury missions.