Project Mercury was the first human spaceflight program conducted by the National Aeronautics and Space Administration (NASA) in the United States. It aimed to put astronauts into Earth's orbit and pave the way for future manned missions to the Moon. The program consisted of several flights, each with a different number of astronauts on board.

The first manned flight of Project Mercury, called Mercury-Redstone 3 or Freedom 7, took place on May 5, 1961. This historic mission was manned by a single astronaut, Alan Shepard. Shepard became the first American to travel into space, reaching a maximum altitude of 187.5 kilometers (116.5 miles) and experiencing a total flight duration of 15 minutes and 22 seconds.

The second manned flight, Mercury-Redstone 4 or Liberty Bell 7, occurred on July 21, 1961. Virgil "Gus" Grissom was the astronaut assigned to this mission. Similar to Shepard's flight, Grissom flew alone and reached a maximum altitude of 189.9 kilometers (118.0 miles). However, a technical issue during the recovery phase caused the spacecraft to sink into the Atlantic Ocean, resulting in Grissom's rescue and the loss of the spacecraft.

Project Mercury's third manned flight, Mercury-Atlas 6 or Friendship 7, marked another significant milestone. On February 20, 1962, John Glenn became the first American to orbit Earth. Glenn completed three orbits before reentering the atmosphere and splashing down safely. This mission demonstrated the feasibility of humans living and working in space for extended periods.

Finally, the fourth and final manned flight of Project Mercury was Mercury-Atlas 9 or Faith 7. L. Gordon Cooper piloted this mission on May 15-16, 1963. Cooper orbited the Earth 22 times, spending a total of 34 hours and 19 minutes in space. His successful mission concluded the Mercury program, making way for the next phase of space exploration, Project Gemini.

In conclusion, each Project Mercury flight had a varying number of astronauts. The first two flights had one astronaut each, while the third and fourth flights had a single astronaut as well. Despite the small number of astronauts per mission, the Project Mercury program laid the foundation for future space missions and paved the way for human exploration beyond Earth's atmosphere.

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