The Gemini (also called the Gemini capsule or the Gemini spacecraft) was a conical, manned spacecraft for two astronauts used by the U.S. space program from 1965 to 1966.

It was designed for and used throughout Project Gemini, a NASA program that worked to learn if humans could live for extended periods of time in space. Gemini spacecraft were just over 18 feet long and 10 feet wide, and they weighed up to 8,350 pounds.

Every Gemini spacecraft was launched from the Cape Kennedy (now Cape Canaveral) Air Force Station in Florida using a Gemini-Titan II rocket as the launch vehicle. The Titan II was a modified intercontinental ballistic missile with an impressive amount of power—the Titan II’s first stage had an engine thrust of 474,000 pounds and the second stage a thrust of 100,000 pounds.

Thanks to the Gemini spacecraft, Project Gemini was an immensely successful program, making the Apollo moon program possible.

The Gemini spacecraft’s purpose was to house two astronauts during a long-duration crewed spaceflight (at least eight days, and up to two weeks) in order to determine if it was possible to put humans on the moon.

Project Gemini came immediately after Project Mercury, which was the first NASA program to send an astronaut into Earth’s orbit. The goal of the Gemini program was to build upon the knowledge of Project Mercury by successfully simulating the situations that would occur during a successful moon landing.

Using the Gemini spacecraft, NASA was able to learn:

• The effects on astronauts of long-duration flights. While Project Mercury sent astronauts into space, the longest spaceflight was just over one day long. NASA knew that if they wanted to send astronauts on a lunar mission (a roundtrip of at least six days), they would need to conduct several extended-duration missions. Project Gemini proved that astronauts could survive up to two weeks in space—showing NASA that a crewed lunar landing would be possible.
• How to conduct spacewalks. The Mercury flights had launched astronauts into Earth’s orbit, but none of the astronauts left the spacecraft in what NASA calls a “spacewalk” or an “extra-vehicular activity (EVA).” Project Gemini included the first American spacewalk, by Ed White in 1965, and proved that astronauts could survive outside the Gemini craft in their spacesuits, and even perform maintenance on the crafts. Learn more about spacewalks here.
• How to connect spacecraft together in space. NASA knew that in order to land on the moon, astronauts would need to be able to connect two spacecraft together during a spacewalk. Project Gemini conducted several missions to practice this essential skill using the Gemini spacecraft and a docking station (called a Gemini-Agena Target Vehicle).

Project Gemini officially began in 1961, and NASA hired Canadian engineer Jim Chamberlin to design a new spacecraft for the program. Chamberlin heavily modeled the Gemini spacecraft off of the Mercury capsule, with two key innovations:

• Larger size. While the Mercury spacecraft only needed to house one astronaut during short spaceflights, the Gemini spacecraft needed room for two for up to two weeks. That meant more cabin space, but also more resources to support both astronauts during the longer spaceflights.
• Component-based construction. The Gemini spacecraft was built in five components rather than one large piece, so that any tests and replacements of any piece would not interfere with any other component.

The Gemini spacecraft were built by McDonnell Aircraft Corporation, who also built the Mercury spacecraft.

The Gemini spacecraft was used in twelve spaceflights—two unmanned test flights and ten manned flights—collectively called the Gemini missions:

• Gemini 1. This was the first test flight of the Gemini spacecraft, and was unmanned.
• Gemini 2. This was the second test flight, an unmanned flight done to test the heat shield on the craft.
• Gemini 3. Gemini 3 was the first crewed flight of the Gemini spacecraft, and the mission completed three full orbits of the Earth.
• Gemini 4. Gemini IV included the first American EVA (or spacewalk), a 22-minute exercise by Ed White.
• Gemini 5. Gemini V mission was a week-long flight that included tests of several future functions, including generating electrical power from fuel cells and using a new navigation and guidance system.
• Gemini 6. This mission, manned by Walter Schirra and Thomas Stafford, was the first “orbital rendezvous” test, in which two Gemini spacecraft arrived in the same low-Earth orbit and flew very close to one another.
• Gemini 7. This mission included the longest human spaceflight of any Gemini mission—a period of two weeks.
• Gemini 8. This was the first mission during which astronauts were able to successfully dock the Gemini with another space vehicle, an Agena Target Vehicle. A thruster malfunction during this mission caused the astronauts to make the first emergency landing of a crewed NASA spaceflight.
• Gemini 9. This mission was another test of docking the Gemini spacecraft with an Agena Target Vehicle, but a small malfunction made the docking impossible.
• Gemini 10. This mission tested the Agena Target Vehicle’s propulsion systems, and the crew members were also able to rendezvous with the vehicle from the Gemini 8 mission, which had suffered a near-fatal malfunction.
• Gemini 11. This mission reached the highest altitude on record (739.2 nautical miles) using the Agena Target Vehicle’s propulsion systems.
• Gemini 12. This was the final mission of the Gemini spacecraft, manned by James Lovell and Buzz Aldrin. During this mission, Aldrin set the record for longest spacewalk (5 hours and 30 minutes).

The Gemini spacecraft consisted of five main components, in order to allow each component to be tested and replaced independently of the other sections:

1. The rendezvous and recovery station (R&R). The R&R was the “nose” of the spacecraft, and it included a radar for rendezvous, a drogue parachute (a parachute designed to slow down an aircraft), and a main parachute. This section, combined with the RCS and the cabin section, is also called the reentry module.

2. The reentry control system (RCS). The RCS included oxidizer tanks, fuel tanks, and 16 thrusters. This section, combined with the R&R and the cabin section, is also called the reentry module.

3. The cabin section. The cabin section was where the two-astronaut crew (the pilot and the command pilot) sat and piloted the spacecraft. It included ejection seats, hatches with viewing windows, and a heat shield to protect the crew during reentry This section, combined with the R&R and the RCS section, is also called the reentry module.

4. The retrograde section. The retrograde section included four solid-fuel “de-orbit” motors (motors used to help the craft change orbit), along with six thrusters to help control and slow the craft. This section, combined with the equipment section, is also called the adapter section.

5. The equipment section. The equipment section included ten additional thrusters, used either for more attitude control (pitch, yaw, and roll) over the craft or for forward thrust. The equipment section was also used as a storage space for mission-specific electrical equipment such as battery containers, fuel cells, or oxygen tanks. This section, combined with the retrograde section, is also called the adapter section.