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The Gemini program was the United States' second human spaceflight program, an interim step designed to bridge the technological gulf between the early Mercury flights and the Apollo lunar-landing program. The National Aeronautics and Space Administration (NASA) announced plans for Gemini on December 7, 1961, two months before John Glenn's historic Mercury mission. Like Mercury, the Gemini spacecraft was built by the McDonnell Aircraft Corporation, but unlike its predecessor, Gemini carried a two-person crew. This inspired NASA to name the program after the third constellation of the zodiac, which featured the twin stars Castor and Pollux. Altogether, the Gemini program involved twelve flights, including two unpiloted flight tests of equipment.

Program Objectives and Spacecraft Features

From a pilot's perspective, the Gemini spacecraft represented a major advance over Mercury in design and capability. Gemini was designed to rendezvous and dock with other orbiting vehicles and to maneuver in space. The program also aimed to test astronauts and equipment during long-duration flights as well as extravehicular activity (EVA)a requirement for later trips to the Moon. Other major objectives of the Gemini program included perfecting re-entry and landing at preselected points and gaining information about the effects of radiation and weightlessness on crew members.

Meeting these objectives meant that the new spacecraft had to be large enough to support its two-person crew5.8 meters (19 feet) long, 3 meters (9.8 feet) in diameter, and about 3,810 kilograms (8,400 pounds) in weightand have an adapter section attached to the crew cabin to house consumables, carry equipment, and provide propulsion. The onboard propulsion system, called the Orbit Attitude Maneuvering System, gave Gemini its versatile flight capability, allowing the spacecraft to be maneuvered and docked in orbit and controlled in flight. Engineering advances also simplified the maintenance of Gemini by using independent equipment modules located outside the cabin to allow easy access for engineers and technicians.

The Initial Gemini Flights

Sometimes referred to as Gemini-Titan for the spacecraft and its launch vehicle (a converted Air Force intercontinental ballistic missile), the first piloted Gemini flight, Gemini 3, rocketed into orbit in March 1965 and completed three orbits in four hours, fifty-three minutes. Although the flight was brief, the crew of Virgil "Gus" Grissom and John Young proved that orbital maneuvers were possible and partially achieved a controlled re-entry and landing.

Just over two months later, Gemini 4, the second of the piloted flights, completed sixty-two orbits in four days and two hours, with Edward White spending twenty-two minutes outside the spacecraft during the historic first American EVA. The mission, commanded by James McDivitt, successfully evaluated real-time flight planning and procedures for crew rest and work cycles, but a planned rendezvous with the Titan II's upper stage was canceled because of fuel consumption.

In Gemini 5, Gordon Cooper and Charles "Pete" Conrad tested a prototype fuel cell that became a vital element in future spaceflights. During the mission, problems with the fuel cell precluded rendezvous with a radar evaluation "pod," but the astronauts were able to put the spacecraft through a series of orbit changes, aiming at a hypothetical target. Cooper and Conrad splashed into the Atlantic on August 29, 1965. They had flown 120 orbits of Earth in eight days, carrying out sixteen experiments and proving that a round-trip voyage to the Moon was within the physical capability of trained astronauts.

Rendezvous and Docking Operations

Having demonstrated the feasibility of a lunar trip, project Gemini prepared for the next step: rendezvous with an Agena target vehicle. The first rendezvous attempt was slated for Gemini 6 with Walter Schirra and Thomas Stafford in the cockpit, but a propulsion failure of the Agena forced the mission to be rescheduled. In its place, Gemini 7 was launched on December 4, 1965. Aboard Gemini 7, Frank Borman and James Lovell completed 206 orbits in thirteen days, eighteen hours, establishing an endurance record for human spaceflight that would stand for years. While in orbit, Gemini 7 served as a passive docking target for Gemini 6, which had finally launched on December 15, 1965, carrying Schirra and Stafford. The two spacecraft approached to within 6 meters (20 feet) of each other and flew in formation for nearly five and a half hours.

On the next flight, Neil Armstrong and David Scott successfully docked with an Agena target vehicle six and a half hours after liftoff, but the flight of Gemini 8 was cut short because of problems with Gemini's control system. The crew was forced to undock after thirty minutes and had to regain control of their spacecraft by using the re-entry control system, which prompted an early landing in the Pacific on March 16, 1966. Two months later, however, Thomas Stafford and Eugene Cernan refined rendezvous techniques in Gemini 9, including a simulation of lunar module rendezvous using a backup-docking target lashed together from spare parts. Cernan also performed a two-hour EVA, though his visor became fogged, and he was unable to test a maneuvering unit.

Gemini 10 and 11 provided additional rendezvous and EVA experience. In July 1966 John Young and Michael Collins piloted Gemini 10 to a rendezvous with two Agena target vehicles on separate occasions and used the Agena propulsion system to boost Gemini 10 to a Gemini altitude record of 760 kilometers (471 miles). In addition, during a ninety-minute EVA, Collins used a handheld maneuvering unit to float over to an undocked Agena. Gemini 11, commanded by Charles "Pete" Conrad, was launched in September 1966 and reached a Gemini altitude record of 1,190 kilometers (738 miles) using the Agena's propulsion system after a first-orbit rendezvous and docking. During the mission, Richard "Dick" Gordon completed several EVAs and tethered the Gemini and Agena spacecraft together with a 30-meter (98-foot) line to test whether two spacecraft could be stabilized in a gravity gradient .

The last flight in the series, Gemini 12, spent almost four days in orbit practicing rendezvous and docking operations and performing several EVAs. James Lovell and Edwin "Buzz" Aldrin docked with the Agena on the third orbit, largely by visual means, and Aldrin set an EVA record of five and a half hours for a single space walk. Gemini 12 also performed tethered operations with its Agena target vehicle, but docking maneuvers were canceled because of a propulsion anomaly during the target vehicle's insertion into orbit. The splashdown of Gemini 12 on November 15, 1966, marked the operational end of the Gemini program.

As a prelude to Apollo, NASA needed to perfect rendezvous and docking techniques in orbit, learn how to make precision landings, and gain experience with large propulsion systems in space. Astronauts also needed to prove they could conduct EVAs and endure long-duration missions. Over eighteen months, the ten piloted flights of Gemini met all of these goals and many other objectives to provide a solid foundation for the Apollo voyages to the Moon.

see also Apollo (volume 3); Astronauts, Types of (volume 3); Capsules (volume 3); History of Humans in Space (volume 3); Mercury Program (volume 3); Mission Control (volume 3); Rendezvous (volume 3); Shepard, Alan (volume 3); Space Walks (volume 3); Young, John (volume 3); Zero Gravity (volume 3).

John F. Kross


Hacker, Barton C., and James M. Grimwood. On the Shoulders of Titans: A History of Project Gemini. Washington, DC: National Aeronautics and Space Administration, 1977.

Lewis, Richard S. Appointment on the Moon. New York: Viking Press, 1968.

Shelton, William R. Man's Conquest of Space. Washington, DC: National Geographic Society, 1975.

Yenne, Bill. The Encyclopedia of U.S. Spacecraft. New York: Exeter Books, 1988.

Internet Resources

Project Gemini: Program Overview. National Aeronautics and Space Administration.<>.

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Gem·i·ni / ˈjeməˌnī; -ˌnē/ 1. Astron. a northern constellation (the Twins), said to represent the mythological twins Castor and Pollux, whose names are given to its two brightest stars. See Dioscuri. ∎  [as genitive] (Gem·i·no·rum / ˌjeməˈnôrəm/ ) used with a preceding letter or numeral to designate a star in this constellation: the star Eta Geminorum. 2. Astrol. the third sign of the zodiac, which the sun enters about May 21. ∎  (a Gemini) (pl. -nis ) a person born when the sun is in this sign. 3. a series of twelve manned orbiting space missions, launched by the U.S. in the 1960s in preparation for the Apollo program. DERIVATIVES: Gem·i·ni·an / -ˌnīən/ n. & adj. (in sense 2).

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Geminiblini, cine, Finney, finny, Ginny, guinea, hinny, mini, Minnie, ninny, pinny, Pliny, shinny, skinny, spinney, tinny, whinny •kidney, Sidney, Sydney •chimney •jitney, Whitney •Disney •aborigine, polygeny, polygyny •androgyny, homogeny, misogyny, progeny •Gemini •niminy-piminy, Rimini •dominie, hominy, Melpomene •ignominy • Panini • larceny • telecine •satiny • destiny • mountainy •mutiny, scrutiny •briny, Heine, liny, piny, shiny, spiny, tiny, whiny •sunshiny •Bonnie, bonny, Connie, johnny, Lonnie, Ronnie, Suwannee •Rodney •Cockney, Procne •Romney • Novotný • Grozny •brawny, corny, horny, lawny, mulligatawny, scrawny, tawny, thorny •Orkney • Courtney •brownie, browny, downy, townie •decani • Iceni • Gemini • Anno Domini • termini • acini •personae, tostone •Brunei • alumni • goldeneye

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Gemini a northern constellation (the Twins), said to represent the twins Castor and Pollux, the Dioscuri, whose names are given to its two brightest stars. Also, in astrological thought, the third sign of the zodiac, which the sun enters about 21 May.

Gemini was also the name of a series of twelve manned American orbiting spacecraft, launched in the 1960s in preparation for the Apollo programme.

Gemini comes from Latin, and means ‘twins’.

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Gemini (the Twins) Northern constellation situated on the ecliptic between Taurus and Cancer; the third sign of the zodiac. The brightest star is Beta Geminorum (Pollux).

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Gemini the twins Castor and Pollux. XIV. — L., pl. of geminus double, twin.