Project Mercury was America's first human spaceflight program and the first major undertaking of the newly created National Aeronautics and Space Administration (NASA). It proved that human spaceflight was possible.

 A human spaceflight program had been envisioned even before NASA was formally established. The U.S. military and the National Advisory Committee for Aeronautics (NACA), NASA's predecessor, had debated the best approach to such a program soon after the Soviet launch of Sputnik I on October 4, 1957. NACA engineer H. Julian Allen, an expert on problems relating to the exceedingly great heat generated by vehicles travelling at high speeds, proposed using a missile to launch a blunt-shaped spacecraft into orbit. The blunt shape would dissipate heat and allow for safe re-entry into the atmosphere. Another NACA engineer, Maxime Faget, suggested using retrorockets that would slow the vehicle's speed and cause it to leave orbit, re-enter the atmosphere, and parachute to an ocean landing. By July 29, 1958, when President Dwight Eisenhower signed the National Aeronautics and Space Act that created NASA, a plan to send humans into space was in place. Its objectives were to place astronauts in space, test their reactions, and return them safely to Earth.

 
The chimpanzee Ham was the live test subject for the Mercury Redstone 2 flight on January 31, 1961. Here the 17-kg (37-pound) primate is fitted into a special biopack couch prior to flight. The 680-kilometer (420 statute miles) suborbital mission was a significant accomplishment in the American route to human spaceflight.

NASA was established on October 1, 1958, and one week later, NASA formally created Project Mercury. On December 17, exactly 55 years after the Wright brothers' first flight, NASA's first administrator, T. Keith Glennan, announced Project Mercury to the public.

NASA promptly selected the astronaut crew. Applicants had to be under 40 years of age, shorter than 5 feet 11 inches (1.8 meters), in excellent physical shape, and have a test pilot background with at least 1,500 hours in the air.  From an initial pool of 508 candidates, NASA winnowed the number down, finally subjecting 32 to an exhaustive and exhausting array of physical and psychological tests before selecting the “Mercury Seven.” Scott Carpenter, L. Gordon Cooper, Jr., John H. Glenn, Jr., Virgil I. "Gus" Grissom, Walter M. Schirra, Jr., Alan B. Shepard, Jr., and Donald K. "Deke" Slayton were introduced to the public at a Washington, D.C. press conference on April 9, 1959, and became instant celebrities and heroes. As the authors of This New Ocean, a history of Project Mercury, said: “The first seven American astronauts were an admirable group of individuals chosen to sit at the apex of a pyramid of human effort.…[They became a team of personalities as well as a crew of pilots. They were lionized by laymen and adored by youth as heroes before their courage was truly tested.”

The greatest challenge to Mercury engineers was to devise a vehicle that would protect a human from the temperature extremes, vacuum, and newly discovered radiation of space. Further, there was the need to keep an astronaut cool during the burning, high-speed re-entry through the atmosphere.

The spacecraft that was designed was cone-shaped with a cylinder on top. It was 6.8 feet (2 meters) long, 6.2 feet (2 meters) in diameter, and had a 19.2-foot, (5.8-meter) escape tower with a solid-rocket motor fastened to the cylinder. In a launch emergency, the rocket would fire and lift the capsule from an explosion and parachute it into the ocean. With a volume of only 428.5 cubic feet (12 cubic meters), there was barely enough room for its pilot, who sat in a custom-designed couch facing a panel with 120 controls, 55 electrical switches, 30 fuses, and 35 mechanical levers. The cabin's atmospheric pressure was one-third of that on Earth and contained pure oxygen.

The blunt end of the capsule, which would enter the atmosphere first, was covered with an ablative heat shield to protect it from the 3000˚ F (1649˚ C) heat of re-entry into the atmosphere. This shield would burn off and dissipate the heat during re-entry and descent. Just before the spacecraft impacted with Earth, the heat shield would detach from the base of the capsule and release a balloon that would inflate to cushion the landing. Parachutes would further slow the descent.

The capsule's total height, including its tower and the rockets attached to the heat shield, was about 26 feet (8 meters). At launch it weighed about 17,500 pounds (7,900 kg).

The program used two different launch vehicles: Redstone rockets, designed by the rocket team of Wernher von Braun in Huntsville, Alabama, were used for the suborbital flights. Atlas-D launch vehicles, modified ballistic missiles, launched the orbital flights. The Atlas had such a thin skin (to save weight) that it would have collapsed like a bag but for its internal pressure.

Eighteen thrusters (small rockets) operated manually by the astronaut controlled the spacecraft's attitude (the way it was pointing). Three retrorockets would fire the capsule out of its orbit and begin its return to Earth.

Seven suborbital and four orbital test flights preceded the piloted flights. On one test flight, in January 1961, a surrogate “passenger,” a chimpanzee named Ham, was put into a suborbital flight that reached about 157 miles (253 kilometres) altitude. Unfortunate Ham experienced much greater acceleration and gravitational forces during the launch than expected. Further, a leaky valve resulted in a severe drop in cabin pressure. When the spacecraft finally splashed down 130 miles (209 kilometres) from its target area, the capsule began to leak water. Ham survived, and the flight was judged successful.

The first piloted Mercury flight lifted off on May 5, 1961, with Alan Shepard behind the controls of the Freedom 7. (Each astronaut named his own spacecraft, and a “7” was added to honor the team.) Reaching a speed of 5,146 miles per hour (8,282 kilometres per hour) and an altitude of about 116 miles (187 kilometres), Shepard became the first American in space. He descended safely as the Freedom 7 parachuted into the Atlantic Ocean at the end of his flight. Although his suborbital flight lasted only 15 minutes 22 seconds, it proved that an astronaut could survive and work comfortably in space, and demonstrated to the 45 million Americans watching it on TV that the United States had joined the spaceflight business.

 
Attempted recovery of the Mercury capsule at the end of the second Mercury mission, July 27, 1961.

In July 1961, Virgil Grissom's flight in the Liberty Bell 7 resembled Shepard's until splashdown when its emergency escape hatch unexpectedly blew off. The capsule flooded and Grissom had to exit quickly as water poured in. His spacesuit became waterlogged, and there were a few tense moments before the helicopter picked him up. His capsule, however, sank, where it remained at the bottom of the sea until it was found and retrieved in 1999. It has since been on a “tour” of museums around the United States.

 
View of earth taken by Astronaut John Glenn during his MA-6 spaceflight, February 20, 1962.

John Glenn, Jr. was the first American to make an orbital flight, travelling three times around the Earth in his Friendship 7, on February 20, 1962. He was the first American to see a sunrise and sunset from space and the first photographer in orbit. The only anxious moments of his flight came before and during re-entry, when a signal received on the ground indicated (erroneously, as it turned out) that the capsule's heat shield had come loose. At one point, Glenn thought his shield was burning up and breaking away. He ran out of fuel trying to stop the capsule's bucking motion as it descended through the atmosphere, but splashed down safely 40 miles (64 kilometres) short of his target. Glenn returned to Earth a national hero, having achieved Project Mercury's primary goal.

 
Astronaut John Glenn is fitted for his space suit prior to lift-off.

Scott Carpenter's flight on the Aurora 7 in May 1962 was much like Glenn's. He splashed down 250 miles (400 kilometres) away from his target, and it took about three hours for the rescue crew to locate his capsule.

Walter “Wally” Schirra became the first orbiting television star as he beamed a telecast back to Earth from his Sigma 7 spacecraft that October. He set a record: orbiting six times in a mission that lasted nine hours and 13 minutes.

The final flight in the series, in May 1963, lasted a record 34 hours/19 minutes, and circled the Earth 22-1/2 times. The pilot, Gordon Cooper in his Faith 7, released the first satellite from a spacecraft-a six-inch (152-millimeter) sphere with a beacon for testing the astronaut's ability to track objects visually in space. His mission was so successful that NASA decided to cancel the final scheduled flight.

 
Mercury program astronaut group portrait. Front row, left to right, are Walter M. Schirra, Jr., Donald K. Slayton, John H. Glenn, Jr., and M. Scott Carpenter. Back row, left to right, are Alan B. Shepard, Jr., Virgil I. Grissom and L. Gordon Cooper

Taken together, the time in space for the six piloted flights had totalled two days and six hours. By the time of the last flight, in May 1963, the U.S. space program was looking ahead to its new goal, announced by President John F. Kennedy only three weeks after Shepard's suborbital flight, of reaching the Moon, and by 1963, only 500 of the 2,500 people working at NASA's Manned Spacecraft Centre in Houston were still working on Mercury-the remainder was already busy on Gemini and Apollo.

But Mercury had taken the critical first step. It had demonstrated that humans could survive in space, a spacecraft could be designed to launch them into orbit, and that the crew could return safely to Earth.