In May 1961, soon after the first Project Mercury flight, President John F. Kennedy declared that the United States would put an astronaut on the Moon before the end of the decade. The Space Race with the Soviets had just become a race to the Moon, and the United States was determined to win it.

At the conclusion of Project Mercury, however, the United States had amassed only two days and six hours in space. It soon became evident that an intermediate step was needed before an attempt to go to the Moon could be made. Thus, Project Gemini was conceived. On December 7, 1961, the National Aeronautics and Space Administration (NASA) announced a plan to extend the piloted spaceflight program by developing a two-person spacecraft. On January 3, 1962, NASA officially named the program Gemini, for the third constellation of the Zodiac and its twin stars, Castor and Pollux.

Gemini was planned to perfect the techniques needed for a lunar mission. Its primary purpose was to demonstrate space rendezvous and docking techniques that would be used during the later Apollo flights to the Moon, when the lunar lander would separate from the command module in orbit around the Moon, then meet up with it again after the astronauts left the lunar surface. Gemini also sought to extend astronauts' stays in space to two weeks, longer than even the Apollo missions would require.

During Gemini, ten piloted missions lifted off from Cape Canaveral, Florida, in less than 20 months. The Manned Spacecraft Centre (renamed the Johnson Space Centre in 1973) near Houston, Texas, took over the role of Mission Control, and ground operations became smooth and efficient, due in part to the extremely short launch windows. For instance, Gemini XI had only a two-second “window” when it could launch, due to the need to rendezvous with a target already in orbit. Meanwhile, the program introduced 16 new astronauts to the space environment.

The Gemini spacecraft (originally called Mercury Mark II) was an improvement on the Mercury capsule. It was 19 feet long (5.8 meters), 10 feet (three meters) in diameter, and weighed about 8,400 pounds (3,810 kilograms)—twice the weight of Mercury. But it had only 50 percent more cabin space for twice as many people, and was extremely cramped. Ejection seats replaced Mercury's escape rocket, and more storage space was added for the longer Gemini flights. The long-duration missions also used fuel cells instead of batteries for generating electrical power. An adapter module fitted to the rear of the capsule (and jettisoned before re-entry) carried on-board oxygen, fuel, and other consumable supplies. Engineering changes, such as systems that could be removed and replaced easily, simplified maintenance. Since spacewalks were an essential part of these missions, the spacesuit became a crucial piece of equipment, providing the only protection for the astronaut from the deadly environment of space.

The Titan II rocket, more powerful than the rockets used for the Mercury missions, placed the larger spacecraft into orbit. The targets for rendezvous operations were un-crewed Agena upper stages, which were launched ahead of the Gemini.

Unlike Mercury, which had been able to change only its orientation while in flight, Gemini needed to the capability to move forward, backward and sideways in its orbital path, and even change orbits to rendezvous with other spacecraft. The complexity of rendezvous demanded two people on board, and more piloting than had been possible with Mercury. It also required the first onboard computers to calculate complicated rendezvous manoeuvres.

The first two Gemini flights were unpiloted. Gemini 1 made 64 orbits and confirmed that the Titan II launch vehicle and the spacecraft were compatible. Gemini 2 tested all the spacecraft systems in a suborbital flight.

 
Astronauts Grissom and Young inside their Gemini spacecraft before launch.

The first piloted flight was Gemini III—nicknamed Molly Brown. Only the Gemini III was nicknamed; pilot Virgil Grissom named it after the Broadway musical The Unsinkable Molly Brown. His spacecraft during Project Mercury had sunk at the end of his flight. The Molly Brown, piloted by Grissom and John Young, made three orbits in its almost five-hour flight on March 23, 1965, in which the astronauts were able to alter their oval orbit to a more circular one and to execute other orbital changes using the thrusters. Their attempt at a precision landing, however, was unsuccessful, as were several future flights, and the spacecraft splashed down more than 50 miles (80 kilometres) from its target.

Gemini IV, a four-day flight piloted by James McDivvitt and Edward White, was launched on June 3, 1965. The highlight of this flight was the first extravehicular activity (EVA) by an American; White's 22-minute "spacewalk." Using a handheld manoeuvring unit, he “swam through space” while attached to his lifeline tether, moving at nearly 18,000 miles per hour (29,000 kilometres per hour). The mission also attempted to rendezvous with the second stage of the Titan launch vehicle but was unsuccessful. They later learned that a spacecraft trying to catch up with another needed to drop to a lower, faster orbit first before rising again.

The Gemini V mission, piloted by L. Gordon Cooper, Jr. and Charles Conrad, Jr., lasted almost eight days and completed 120 orbits in August 1965. It demonstrated the first use of fuel cells for electrical power, although they did not supply enough power and some experiments and a planned rendezvous had to be cancelled.

 
This photograph of the Gemini 7 spacecraft was taken from the hatch window of the Gemini 6 spacecraft during rendezvous and station keeping manoeuvres at an altitude of approximately 160 miles on December 15, 1965.

The Gemini VI flight, scheduled for launch on October 25, 1965, was scrubbed when the Agena target spacecraft with which the crew had planned to rendezvous and dock exploded during its launch. Consequently, Gemini VII, piloted by Frank Borman and James Lovell, Jr., was launched first, on December 4, 1965, to become the rendezvous target for Gemini VI. When Gemini VI was launched on December 15, piloted by Walter Schirra, Jr. and Thomas Stafford, the two spacecraft rendezvoused and flew in formation for five hours. Gemini VII remained aloft for 14 days to study the effects of long-duration flight. The 330 hours in space had no long-term harmful effects on the crew, but the flight turned into somewhat of an endurance test for the two pilots, confined in their hot, cramped quarters.

 
Astronaut Edward H. White II, pilot for the Gemini 4 spaceflight, floats in zero gravity of space. The extravehicular activity was performed during the third revolution of the Gemini 4 spacecraft. White is attached to the spacecraft by a 25-ft. umbilical line and a 23-ft. tether line, both wrapped in gold tape to form one cord. In his right hand White carries a and-Held Self-Maneuvering Unit (HHSMU). The visor of his helmet is gold plated to protect him from the unfiltered rays of the sun.

Gemini VIII, launched on March 16, 1966, was piloted by Neil A. Armstrong and David R. Scott, who carried out the first docking and undocking with another spacecraft. The flight lasted only 10 hours, 41 minutes because of a malfunctioning thruster that caused the spacecraft to spin uncontrollably at a rate of about one revolution per second. This caused the crew to execute the first emergency landing of a piloted U.S. spaceflight mission.

Thomas P. Stafford and Eugene A. Cernan became the first backup crew to fly a spaceflight mission after the original crew of Elliott See and Charles Bassett died in a plane crash four months before the flight. Gemini IX was launched on June 3, 1966, and flew almost four days. A docking had been scheduled, but the crew discovered when they rendezvoused with the target that its protective shroud was still attached, making docking impossible. Plans to test an Astronaut Manoeuvring Unit as part of a spacewalk was also cancelled when Cernan's faceplate became fogged, limiting his visibility.

Gemini X, a three-day flight launched on July 18, 1966 with John Young and Michael Collins as pilots, demonstrated the first double docking, first docking with an Agena target vehicle, and later with another Agena still floating from an earlier mission. Collins “walked” in space for 39 minutes at the end of a tether. This mission accomplished all of Gemini's objectives: rendezvous, docking, manoeuvring, and a spacewalk during which work was performed.

 
Areas of Sudan and Egypt as seen from Gemini 11 spacecraft.

On Gemini XI, launched on September 12, 1966, the crew matched orbits with an Agena test vehicle only 85 minutes after launch and docked several times. This three-day flight, piloted by Charles Conrad, Jr. and Richard Gordon, Jr., reached the highest Earth orbit to date—739.2 miles (1,189.3 kilometres). This flight ended with the first totally automatic, computer-controlled re-entry, which brought the spacecraft down only about 2.8 miles (4.5 kilometres) from its recovery ship.

Launched on November 1, 1966, Gemini XII, crewed by Edwin “Buzz” Aldrin and James Lovell, Jr., was the final flight in the series. By the time of this four-day flight, the program still had not demonstrated that an astronaut could work easily and efficiently outside the spacecraft. This flight used new, improved restraints for the astronauts to hook to outside the spacecraft, and underwater training, which became a staple of all future spacewalk simulations, was added as a training technique for working in space. Aldrin totaled a record five-and-a-half hours in space.

 
Agena Target Docking vehicle seen from Gemini 8 spacecraft.