It was the tenth crewed American spaceflight (including two X-15 flights at altitudes exceeding 100 kilometers (62 mi; 54 nmi)).

The highlight of the mission was the first space walk by an American, during which White floated free outside the spacecraft, tethered to it, for approximately 23 minutes.

Subsequent Gemini flights would be longer, to prove endurance exceeding the time required to fly to the Moon and back.

NASA moved up the spacewalk from the original schedule, to demonstrate that the US was gaining on the early lead taken by the Soviets in what was known as the Space Race.

"[7] A third objective was for Gemini 4 to attempt the first space rendezvous, flying in formation with the spent second stage of its Titan II launch vehicle.

For the first time an international audience, from 12 European nations, could watch the lift-off on live television via the Early Bird satellite.

At liftoff, two roll transients caused by misalignment of the Titan first-stage engines occurred; these were quickly corrected by the autopilot.

The fuel top-off umbilical failed to detach and was pulled loose when the booster had climbed about 27 feet (8.2 meters).

This was unsuccessful for a number of reasons: After expending almost half his thruster fuel, McDivitt finally gave up, in order to concentrate on the more important EVA objective.

[10] Originally planned for the second revolution, the astronauts postponed the EVA until the third after McDivitt decided that White, following the stress of the launch and the failed rendezvous, looked tired and hot.

[10] Tied to a tether, White floated out of the spacecraft, using a Hand-Held Maneuvering Unit (informally called a "zip gun") which expelled pressurized oxygen to provide thrust for controlling his travel.

White was running up against two factors which constrained the time for his EVA: loss of signal from the Bermuda tracking station and crossing the solar terminator.

The flight controllers were becoming increasingly frustrated with their inability to remind White of the time constraint, because they didn't want the first EVA to be performed in darkness or out of communication with Earth.

[12] They powered down the spacecraft's maneuvering system, intending to drift for the next two-and-a-half days to conserve the remaining fuel.

They also intended to sleep alternate four-hour periods, but this turned out to be extremely difficult with the constant radio communications and the small cabin, about the size of the front seats of a compact car.

These also showed White wearing an Omega Speedmaster chronograph watch on his spacesuit sleeve, one of two makes which had been approved by NASA for space use following extensive tests.

IBM were unable to duplicate the failure on the ground but they installed a manual override switch on subsequent Gemini missions.

Neither of the crew encountered any problems, however, contrary to NASA doctors' concerns about their landing upright – as opposed to on their backs, as in Mercury – after four days in space.

Since McDivitt and White were prohibited from naming their spacecraft, they decided to put the American flag on their suits, the first astronauts to do so, although Soviet crews wore the Cyrillic "СССР" on their spacesuit helmets.

The spacecraft is on display at the Steven F. Udvar-Hazy Center, Chantilly, Virginia This article incorporates public domain material from websites or documents of the National Aeronautics and Space Administration.