Gemini-Centaur Credit - © Mark Wade Media Gallery

In the first Gemini project plans, it was planned that after a series of test dockings between Gemini and Agena rocket stages, Geminis would dock with Centaur stages for circumlunar flights. This was a threat to Project Apollo and was suppressed.

At its birth Gemini was known as the Mercury Mark II program. NASA was already committed to the three-man Apollo spacecraft and considered Gemini an interim spacecraft to test rendezvous, docking, and EVA techniques before Apollo was available. But NASA's James Chamberlin and McDonnell Aircraft considered Gemini as a viable competitor to Apollo for the circumlunar and lunar landing missions. Such proposals might have been welcomed by the later 'cheaper, better, faster' NASA. But in 1961, as a direct challenge to the Apollo project and Lyndon Johnson's dream of a Southern High Technology Crescent, they were anathema.

The original August 14, 1961 Mercury Mark II program plan went like this:

The Centaur would be launched atop a Titan II booster. The lunar Gemini spacecraft would have weighed 3,170 kg, an extra 270 kg over the basic rendezvous Gemini. The difference consisted of a backup inertial navigator and additional heat shielding for re-entry at 11 km/sec instead of 8 km/sec. This program was estimated to put an American around the moon for only $ 60 million more than the basic $ 356 million program. An even more aggressive alternative, a nine-flight program, was promised to cost only $ 8.5 million more than the basic program and fly around the moon in May 1964! This first attempt to fly Gemini to the moon was quickly suppressed, and a revision of the plan was issued only a week later, with all mention of lunar flights deleted.

Crew Size: 2. Maximum Diameter: 3.05 m (10.00 ft). Habitable Volume: 2.55 m3. Mass: 3,170 kg (6,980 lb). Associated Launch Vehicle: Titan 2.

• Marks, C D and Quest, R G, AAS Science and Technology Series, Vol. 21, "Low Cost Orbital Transportation: How Big A Step!", 1969.

• Advanced Gemini Spacecraft, Briefing, McDonnell Douglas, ca. 1967. Thanks to Mike Macowski for discovering and providing this material.

• Big G, Briefing, McDonnell Douglas, 20 December 1967. Thanks to Mike Macowski for discovering and providing this material.

• Hall, Edwin H, "Memorandum to Deputy Director, Gemini Program, "Circumlunar Missions"", June 29, 1965. Thanks to Scott Lowther for discovering and providing the memorandum.

• Hacker, Barton C and Crimwood, James,, On the Shoulders of Titans, Government Printing Office, 1977. The official version - NASA's history of the Gemini program. Accessed at: http://www.hq.nasa.gov/office/pao/History/SP-4203/cover.htm.

• Baker, David, The History of Manned Spaceflight, Crown, New York, 1981. The best overview of America's manned space programs up to Skylab. Information and details not available anywhere else. Unfortunately out of print and difficult to locate.