GEMINI was not pure pioneering like MERCURY, nor did it have the excitement of APOLLO. But its success was critical to Kennedy's goal of reaching the Moon "by decade's end."
Thel program was announced to the public on January 3, 1962, after APOLLO already was well underway. GEMINI's primary purpose was to demonstrate space rendezvous and docking - techniques that would be used during APOLLO, 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.
It was during the GEMINI program that space flight became routine. Ten manned missions left e launch pads of Cape CAnaveral in less than 20 months, and the Manned Spacecraft Center outside Houston (later Johnson Space Center) took over the role of Mission Control. Ground operations became smooth and efficient, due in part to flettingly short launch windows - the GEMINI XI "window" opened for only 2 seconds - dictated by the need to rendezvous with targets already in orbirt. Meanwhile, sixteen new astronauts chalked up experience in space.
The GEMINI spacecraft was an improvement on MERCURY (it was originally called MERCURY MARK II) in both size and capability. GEMINI weighed more than 3,628.72 kilograms - twice the weight of MERCURY - but ironically seemed more cramped, having only 50 percent more cabin space for twice as many people. Ejection seats replaced MERCURY's escape rocket, and more storage space was added for the longer GEMINI flights. The long duration missions also required fuel cells instead of batteries for generating electrical power.
Unlike MERCURY, which had only been able to change its orientation in space, GEMINI needed real maneuvering capability to rendezvous with another spacecraft. GEMINI would have to move forward, backward and sideways in its orbital path, even change orbits. The complexity of revdezvous demanded two people on board, and more piloting than had been possible wityh MERCURY. It also required the first on-board computers to calculate complicated rendezvous maneuvers.
GEMINI rode into orbit on a TITAN 2 launch vehicle. The target for rendezvous operations was an unmanned AGENA upper stage, which was launched ahead of the GEMINI. After meeting up in orbit, the nose of the GEMINI capsule then fit into a docking collar on the AGENA.
To avoid long delays between flights, GEMINI spacecraft were made more serviceable, with subsystems that could be removed and replaced easily. An adapter module fitted to the rear of the capsule (and jettisioned before reentry) carried on-board oxygen, fuel and other consumable supplies.
GEMINI gave U.S. astronauts their first real experience with living and working in space. They had to learn to sleep and keep house on long flights in crowded quarters, both of which were difficult. GEMINI astronauts also made the first forays outside their spacecraft, which required a new spacesuit design. Space walks pved more difficult than expected - following Ed White's successful solo on GEMINI IV, it wasn't until the final GEMINI flight that another extravehicular activity (EVA) went as smoothly as planned.
By GEMINI's end, an important new capability - orbital rendezvous and docking - had become routine, and space doctors had gained confidence that humans could live, work and stay healthy in space for days or even weeks at a time. GEMINI also completed a long list of on-board science experiments, including studies of the space environment and Earth photography. Above all, the program added nearly 1,000 hours of valuable space-flight experience in the years between MERCURY and APOLLO, which by 1966 was nearing flight readiness. Five days before the launch of the last GEMINI, Lunar Orbiter 2 had been sent to the Moon, already scouting out APOLLO landing sites.