Gemini-Centaur Credit - © Mark Wade

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).