It was obvious to NASA that there was a big gap of three to four years between the last Mercury flight and the first scheduled Apollo flight. There would therefore be no experience in the US in understanding the problems of orbital maneuvering, rendezvous, docking, lifting reentry, and extravehicular activity before the Apollo flights, which required all of these to be successfully accomplished to complete the lunar landing mission.

Gemini began as Mercury Mark II to fill this gap. The concept was to enlarge the Mercury capsule's basic design to accommodate two crew, provide it with orbital maneuvering capability, use existing boosters to launch it and an existing upper rocket stage as a docking target. While it superficially resembled Mercury, it was an entirely new spacecraft, although it had the same type of construction. It consisted of two sections; a manned capsule (reentry module) and an adapter section (consisting of retro and equipment modules). The former had the same truncated cone shape as Mercury, however, there was no escape tower; instead, the two astronauts had ejection seats. The forward end of the capsule was a cylinder designed for docking with another spacecraft. It contained the reentry parachute and had the attitude control thrusters for the capsule. Four solid propellant retrorockets were attached to the heat shield as they were with Mercury. The adapter section contained all equipment not required inside the manned capsule. Thus, it held the fuel cells for generating electricity and drinking water, oxygen tanks, attitude control thrusters, orbital maneuvering engines, propellant tanks for the thrusters, and reactant tanks for fuel cells. There also was room for experiment stowage.

Between March 1965 and November 1966 ten manned Gemini spacecraft were orbited. These missions gained experience in extended mission times; perfected and practiced orbital maneuvering, rendezvous, and docking techniques; and trained astronauts in extravehicular activity. By the end of the program, Gemini crews had totalled 40 days 9 hr 51 min in space; completed 604 Earth orbits; had performed five EVAs totalling 6 hr 2 min plus four stand-up EVAs totalling 6 hr 21 min; set a then endurance record of 13 days 18 hr 35 min; and set a then altitude record of 1,369 km.

GEMINI SPECIFICATIONS

First flight: 8-Apr-1964; first manned flight 23-Mar-1965 (Gemini 3)
Last flight: 11-Nov-1966 (Gemini 12)
Number of flights: 13 total; 10 manned
Principal uses: manned earth orbit rendezvous, docking, EVA tests
Unit cost: $13.00 million
Crew size: 2
Overall length: 5.7 m
Maximum diameter: 3.05 m
Habitable volume: 2.55 m³
Launch mass: 3,851 kg
Propellant mass: 455 kg total
RCS total impulse: 1,168 kNs
Primary engine thrust: 710 N
Main engine propellant: NTO/MMH
Total spacecraft delta v: 323 m/s
Power: fuel cells/batteries; 155.0 kWh total

(Weights are not typical for every mission)

REENTRY MODULE
Crew size: 2
Length: 3.4 m
Maximum diameter: 2.3 m
Habitable volume: 2.55 m³
Total mass: 1,982 kg
(structure 638 kg; heat shield 144 kg; reaction control system 133 kg; recovery equipment 98 kg; navigation equipment 63 kg; telemetry equipment 51 kg; electrical equipment 126 kg; communications systems 26 kg; crew seats & provisions 426 kg; crew mass 144 kg; misc contingency 100 kg; propellant 33 kg)
Reaction control system
    thrusters: duplicate sets of 8 x 111 N thrusters
    propellant: NTO/MMH
    specific impulse: 283 s
    total impulse: 90 kNs
L/D hypersonic: 0.16
Power: batteries; 4.0 kWh, 180.0 Ah
Environment: pure oxygen at 340 mbar
Landing system: similar to Mercury, except no landing bag. Main parachute 25.6 m diameter ringsail. Original landing mode called for deployment of a inflated Rogallo wing with a piloted landing on skids at Edwards Dry Lake. The wing could not be made to deploy reliably before flights began, so the capsule made a parachute-borne water landing

RETRO MODULE
Length: 0.9 m
Maximum diameter: 2.6 m
Total mass: 591 kg
(structure 160 kg; reaction control system 200 kg; maneuvering system 131 kg)
Propellant mass: 100 kg
Reaction control system
    thrusters: 6 x 400 N + 2 x 400 N deceleration thrusters
    propellant: NTO/MMH
    specific impulse: 273 s
    total impulse: see equipment module
Retro-rockets
    thrust: 4 x 11.12 kN
    propellant: solid
    specific impulse: 255 s
    delta v: 101 m/s

EQUIPMENT MODULE
Length: 1.4 m
Maximum diameter: 3.05 m
Total mass: 1,278 kg
(structure 250 kg; reaction control system 60 kg; telemetry equipment 40 kg; electrical equipment 294 kg; misc contingency 75 kg; environmental control system 117 kg; maneuvering system 120 kg)
Propellant mass: 322 kg
Reaction control system
    thrusters: 8 x 111 N
    propellant: NTO/MMH
    specific impulse: 288 s
    total impulse: 1,078 kNs
Maneuver system
    thrust: 710 N
    propellant: NTO/MMH
    specific impulse: 273 s
    delta v: 222 m/s
Power: fuel cells; 151.0 kWh total, 2.16 kW average